XXX. THE VIVIPAROUS MANGROVES OF FIJI
RHIZOPHORA AND BRUGUIERA
Rhizophora.—Represented by Rhizophora mucronata, Rhizophora mangle, and the Selala, a seedless intermediate form.—Their mode of association and characters.—The relation of the Selala.—Polyembryony.—The history of the plant between the fertilisation of the ovule and the detachment of the seedling.—Absence of a rest period.—Mode of detachment of the seedling. Capacity for dispersal by the currents.—Bruguiera.—The mode of dispersal.—Peculiar method of fertilisation.—Length of period between fertilisation and the detachment of the seedling.—Mode of detachment of the seedling.—Summary.
BETWEEN 1897 and 1899 I made numerous observations on the Fijian species of Rhizophora and Bruguiera (mostly around the coasts of Vanua Levu and in the Rewa delta); and these were supplemented in the early part of 1904 by observations on the first-named genus in Ecuador. I did not make any collections in Fiji until Prof. Schimper asked me to obtain specimens; and a fair-sized collection containing specimens dried, and preserved in spirit, was sent to him. His illness and death shortly followed, and I lost the advantage of his great experience in these matters. In a letter written to me in 1898 he expressed the hope that I would publish my notes on the mangroves of Fiji. Years have since passed by, and as I read again his words of encouragement I take up once more the interrupted task.
RHIZOPHORA
Of the three species of this genus, two of them, Rhizophora mucronata and R. conjugata, are Asiatic and are unknown in America; whilst the third, R. mangle, was until recently regarded as peculiar to the American and West African regions.
When Mr. Hemsley wrote the Report on the Botany of the Challenger Expedition he remarked (iii, 149) that the American Rhizophora (R. mangle) appeared to be restricted to that region, and he questioned its existence in the Pacific Islands as indicated by Jouan for New Caledonia. The same view was taken by Prof. Schimper in his work on the Indo-Malayan strand-flora published in 1891. There was, in fact, much to support this view, since Dr. Seemann, one of the most accomplished botanists who have explored the Pacific, describes only the Asiatic Rhizophora (R. mucronata) in Fiji, and nothing is said of any other species collected by the United States Exploring Expedition under Wilkes in Fiji and Samoa.
However, in a paper on the flora of Tonga, read before the Linnean Society in 1893, Mr. Hemsley includes the American mangrove, Rhizophora mangle, amongst the collections made there by Mr. Lister; and he refers to its occurrence also in Stewart Island (I suppose in the Solomon Group), but he suggests that it was accidentally introduced with ballast in both these localities. In 1897 I found a species of Rhizophora, to all appearances identical with the American species, in great abundance in the Rewa delta in Fiji. Subsequently the same mangrove came under my notice as the prevailing species in Vanua Levu in the same group; and on sending photographs of a branchlet in flower and fruit and of the germinating fruit to Prof. Schimper he expressed the opinion that they belonged to the typical Rhizophora mangle.
There are four typical mangroves in Fiji, namely (1) Bruguiera rheedii, the “Dongo” proper of the natives; (2) Rhizophora mangle, usually known as “Tiri-wai,” that is to say, the Tiri of the river, or rather of the estuary; (3) Rhizophora mucronata, the “Tiri-tambua” of the Fijian, signifying the Whale’s Tooth Tiri in allusion to the form of its fruit; and (4) a seedless form intermediate between the two species of Rhizophora, which the Fijians designate “Selala,” a name signifying “the tree with empty flowers.”
Bruguiera rheedii and Rhizophora mucronata were alone recorded by Dr. Seemann and his predecessors; but he significantly refers to the natives speaking of four mangroves. Mr. Horne, who spent twelve months in the group some years later, also overlooked the American Rhizophora; but it is apparent that both these botanists were naturally more interested in the vegetation of the inland regions than of the coast swamps, and we have before observed that they failed to record Scirpodendron costatum, a giant-sedge very common and conspicuous in the swamps. It is not easy to understand Dr. Seemann’s remark that “mangroves are restricted to but few parts of the larger islands.” Horne, who was in the islands eighteen years afterwards, makes frequent allusion to them. The natives whom I questioned closely on this subject scouted the idea that any of the four mangroves above named were recent arrivals. The coasts, as they said, had always been extensively fringed by mangroves; and the reader has only to refer to my remarks in the second chapter of my volume on the geology of Vanua Levu to convince himself that mangrove swamps of considerable extent existed in the time of Commodore Wilkes (1840).
_The Relative Abundance and Mode of Association of the three Fijian forms of Rhizophora._
Stated in their order of frequency, we have first Rhizophora mangle, the American species, then Rhizophora mucronata, the Asiatic species, and lastly the Selala. The first is equally at home at the sea-border and on the banks of brackish estuaries. The second is, as a rule, more exclusively at home on the sea-coasts; and the same may be said for the Selala. Usually all three kinds occur in the lower part of an estuary; but as we ascend the river and the water freshens, the Asiatic Rhizophora and the Selala disappear, and the American plant is alone found in the higher reaches, where the density of the water ranges according to the state of the tide between 1·000 and 1·010. I examined the distribution of these three forms of Rhizophora in numerous estuaries of Vanua Levu, as well as in the Rewa estuary in Viti Levu; and it was ascertained that in all cases they followed the rule above indicated. When the estuary receives but few streams and the water is mostly salt, the three Rhizophoras may extend miles inland; but when it contains a large body of fresh-water, Rhizophora mangle may be the only form observed from the mouth of the river to the head of the estuary, and it may monopolise the adjacent coasts. On the other hand, Rhizophora mucronata may occupy almost exclusively a long extent of coast; or the Selala may prevail in certain localities, as on parts of the Mathuata coast of Vanua Levu.
The manner of association of these three Rhizophoras is of interest in connection with the origin of the seedless Selala. They very rarely occur mingled together, but grow gregariously in contiguous colonies; and not uncommonly all three may occur on the same line of coast within a distance of a few hundred yards. The colonies pass into each other without a break, and there is no fixed rule of association. Whilst on the south side of Vanua Levu the Selala is generally associated with the American Rhizophora, on the north side it is usually in touch with the Asiatic species. In other localities all three occur in contiguous colonies. The Selala colony may be exposed on the line of a river-bank or along the sea-coast, or it may lie in the heart of an extensive mangrove tract. The most extensive mangrove region in Fiji, that of the Rewa delta, is in great part occupied by Rhizophora mangle; but all three forms grow together in the eastern part of the delta; and here, strangely enough, as at Daku, the Selala may grow sporadically, and all three may grow mixed together with their branches intercrossing.
_The Characters of the Selala or Seedless Rhizophora compared with those of the American Mangrove (R. mangle) and the Asiatic Mangrove (R. mucronata)._
The three kinds of Rhizophora, when seen at the same time along a tract of coast, may be readily distinguished by the different shades of green of their foliage, that of Selala being dark green, that of Rhizophora mucronata light green, and that of Rhizophora mangle intermediate in shade. The Selala is usually the tallest of the three, and attains a height of from 20 to 30 feet or even 40 feet and over, the aerial roots dropped from the higher branches giving it a characteristic aspect. Rhizophora mangle is generally the shortest, and at the coast is from 10 to 12 feet high; but where the mangrove vegetation is most luxuriant, as in the great swamps in the interior of the Rewa delta, it forms tall trees as much as 40 feet in height, displaying the aerial roots hanging from the higher branches. Rhizophora mucronata is, as a rule, intermediate in height, and is distinguished by its stout, reddish trunk and reddish aërial roots.
The trunks of Selala are often in an inclined position and supported entirely by the trestle-like aërial roots, the lower end raised some 5 or 6 feet above the ground with the rest of the trunk inclined upwards. They then look like gigantic walking-stick insects. The same habit may be sometimes observed with the larger trees of Rhizophora mucronata, and in fact all three may present at times the same habit of growth. The taller trees of Rhizophora mangle may resemble the Selala in habit, and the smaller trees of the Selala may approach the habit of Rhizophora mangle.
The distinctive characters of the Selala are given in the table opposite. It will be there seen that this form is intermediate between the other two species as regards the form and size of the petioles and peduncles; the size of the bracts and bracteoles; the colour, form, and size of the flowers; and in the length of the style. Its leaves are smaller than in the case of the other two species, but pointed and semi-aristate like those of Rhizophora mucronata. There are, however, two varieties of the Selala, both with larger foliage than that belonging to the prevailing type of the tree, and from 10 to 15 feet in height. In one the flowers are more numerous, each flowering stem branching four or five times and bearing at least twenty-four flowers, the first branch being trichotomous and the rest dichotomous. In the other, which is the prevailing form on the Mathuata coast, there is a nearer approach to Rhizophora mucronata in the rounding of the peduncles and in the length of the style. Then, again, there are divergent varieties of Rhizophora mangle which in the larger bracts and bracteoles and in the greater size, form, and paler hue of the flowers come nearer to the Selala. Taking all the characters together, the Selala, though intermediate between the Asiatic and the American species, comes in the most critical diagnostic points, as in the inflorescence, in the individual flowers, and in the form of the apex of the leaf, nearest to Rhizophora mucronata, the Asiatic species.
The seedless character of the Selala is well known to most Fijians of the coast districts, the native name signifying empty (lala) flowers (se). Now and then they aver that it produces fruit, but the numerous offers of rewards in money never resulted in their bringing me the fruits. During my residence of two years in the group I examined the Selala trees in a great number of localities and never succeeded in finding them in fruit.
With all three kinds the anthers burst in the bud before it begins to open, and we may ask why the process of self-fertilisation, which is effectual with the other two kinds, produces no result with the Selala. In all three cases the flower-buds and expanded flowers hang downwards, and the expanded flowers retain their parts for the first twenty-four hours, the pollen being caught in quantity on the hairy edges of the petals. During the next day the withering stamens fall out, and on the following day the petals fall too. With the Selala, the style soon begins to blacken and wither, and in a few days the flower becomes detached and drops off. With Rhizophora mucronata and Rhizophora mangle, the style preserves its healthy condition, and shortly evidences of fertilisation appear in the altered shape of the ovary. It is apparent, therefore, that in the case of the Selala fertilisation has not occurred, although the mechanical processes connected with it have been carried out. The cause of this is not far to seek.
+-----------------+--------------------------------------------------------+ | | FIJI | | CHARACTERS +------------------+------------------+------------------+ | | RHIZOPHORA | SELALA (a | RHIZOPHORA | | | MUCRONATA. | seedless form). | MANGLE. | +-----------------+------------------+------------------+------------------+ |Height of tree | | | | | in feet | 12-20. | 20-40. | 9-12. | | | | | | |Colour of foliage|Pale green. |Dark green. |Intermediate | | | | |shade. | | | | | | |Base of leaf |Tapering. |Sub-rounded. |Tapering. | | | | | | |Apex of leaf |Acute, and |Acute, and |Very obtuse, | | |terminating |terminating in |with no | | |in a twisted |a twisted point |twisted point. | | |point a line |less than a line | | | |(2·5 mm.) long. |(2·5 mm.) long. | | | | | | | |Leaf-stalk |Rounded, 1 - |Rather flattened |Length as in | | (petiole) |1-2/10 inch, |horizontally, |Selala, but | | |(25-30 mm.) long, |5-8/10th inch |flattening | | |about as long as |(12-20mm.) long, |very marked. | | |the peduncle. |shorter than the | | | | |peduncle. | | | | | | | |Inflorescence |Branching |Branching usually |Usually branching | | |(dichotomous) two |twice, but |only once | | |or three times |sometimes three |(trichotomous) and| | |with four to eight|times; first |bearing only three| | |flowers. |branching |flowers; but | | | |trichotomous, rest|sometimes | | | |dichotomous; six |branching again | | | |to twelve flowers;|(dichotomous) and | | | |in one variety, |bearing then six | | | |flowers as many as|flowers. | | | |twenty-four. | | | | | | | |Peduncle (lowest}|Rounded. |Flattened above. |{Flattening more | | flower-stalk) }| | |{marked than with | | | | |{Selala. | | | | | | |Pedicels |As stout as the |More slender than |As in Selala. | | |peduncle, and |the peduncle, and | | | |rounded. |angular. | | | | | | | |Bracts and |Large, 1-1/2 line |Small, 2/3 line |Very small or | | bracteoles |(4 mm.). |(2 mm.). |absent. | | | | | | |Calyx |Very pale yellow, |As in R. |Pale or bright | | |or dirty white, |mucronata. |green, angular at | | |rounded at base | |base in the bud, | | |in the bud, lobes | |lobes 3-1/2 lines | | |4-1/2 - 5 lines | |(8 mm.). | | |(11-12 mm.). | | | | | | | | |Length of style |1-1/2 lines |1 line (2·5 mm.). |1/2 line | | |(4 mm.). | |(1·5 mm.). | | | | | | |Fruit |Ovoid and usually |No fruits |Conical, somewhat | | |symmetrical, with |produced. |curved, and thus | | |large persistent | |not symmetrical; | | |bracteoles at | |bracteoles at base| | |base. | |very small or | | |(Hypocotyl 16 | |absent. | | |inches.) | |(Hypocotyl 9 or | | | | |10 inches.) | | | | | | |Colour of trunk, | | | | | rootstock and | | | | | roots |Reddish. | — | — | +-----------------+------------------+------------------+------------------+
+-----------------+-------------------------------------+ | | ECUADOR. | | CHARACTERS. +------------------+------------------+ | |RHIZOPHORA MANGLE |RHIZOPHORA MANGLE | | | (Mangle chico). | (Mangle grande). | +-----------------+------------------+------------------+ |Height of tree in| 10-15. | 50-80 and more. | | feet | | | | | | | |Colour of foliage|Pale green. |Dark green. | | | | | |Base of leaf |Tapering or |Tapering. | | |sub-rounded. | | | | | | |Apex of leaf |Very obtuse, with |Very obtuse, with | | |no twisted point. |no twisted point. | | | | | |Leaf-stalk |Flattened above |Flat above with a | | (petiole) |and below, with no|median groove, | | |median groove, |1 inch (25 mm.) | | |1/2 inch (12 mm.) |long, two-thirds | | |long; not half as |the length of the | | |long as the |peduncle. | | |peduncle. | | | | | | |Inflorescence |As described under|Branching at least| | |R. mangle of Fiji.|three times, | | | |sometimes four or | | | |five times, | | | |trichotomous or | | | |dichotomous, | | | |twelve to | | | |forty-eight | | | |flowers. | | | | | |Peduncle (lowest}| | | | flower-stalk) }|}Sub-angular. |Rounded. | | | | | |Pedicels |More slender than |More slender than | | |the peduncle, and |the peduncle, and | | |rounded. |angular. | | | | | |Bracts and |Scarcely |Well developed, | | bracteoles |developed, 1/2 |1 line (2·5 mm.). | | |line (1 mm.). | | | | | | |Calyx |As with R. mangle |As with R. | | |of Fiji. |mucronata and | | | |Selala of Fiji, | | | |but lobes 4 lines | | | |(10 mm.). | | | | | |Length of style |Less than a line |1-1/2 lines | | |(2·5 mm.). |(4 mm.). | | | | | |Fruit |As in R. mangle of|Conical, not | | |Fiji. |symmetrical, and | | |(Hypocotyl 9 or |somewhat curved; | | |10 inches.) |large persistent | | | |bracteoles at base| | | |as in R. | | | |mucronata. | | | |(Hypocotyl 12 to | | | |15 inches.) | | | | | |Colour of trunk, | | | | rootstock and | | | | roots | — | — | +-----------------+------------------+------------------+
Although the ovaries of the Selala contain four ovules, which in size and appearance do not differ from those of Rhizophora mangle and R. mucronata, its pollen when compared with that of the other two forms presents a remarkable difference. The pollen of these three mangroves was examined in five localities far apart from each other, and in all the same results were obtained. The pollen-grains of the Selala are much smaller than those of the other two, and differ much from them in form. They are irregularly oval in outline, and have a shrunken look beside the spherical symmetrical grains of the two species with which they are compared. They are from one-fourth to one-third the size of those of Rhizophora mucronata, and from one-third to one-half the size of those of Rhizophora mangle.
There is much to support the view that the Selala is a cross between the other two species, its intermediate characters and its seedless condition being especially indicative of such a derivation; but there are several difficulties in accepting this explanation.
(1) The circumstance of the anthers bursting in the flower-bud would considerably lessen the chances of cross-fertilisation; but this objection is not insurmountable, since numerous insects, such as flies, ants, and small coleoptera, visit the newly opened flowers, and they might sometimes produce a result. When I made this suggestion to Prof. Schimper he replied that insect-pollination was quite possible after the expansion of the flowers.
(2) If, as seems highly probable, the pollen of Selala is impotent and the ovules fertilisable, then its seedless condition implies not only an incapacity for self-fertilisation, but also for cross-fertilisation; and if Selala with its impotent pollen does not admit of cross-fertilisation, this would still less be expected of Rhizophora mucronata and R. mangle where the pollen is potent and where fertilisation takes place in the bud. I endeavoured to fertilise the Selala flowers with the pollen of the two other species; but there were no results, the flowers falling off in a few days. It may here be remarked that on one Selala tree I found a solitary flower with an enlarged ovary, as if through fertilisation.
(3) It is not easy to explain the gregarious growth of the Selala if it is a seedless hybrid. The colonies could not be renovated by mere intercrossing, especially in places where, as on the north coast of Vanua Levu, the dense belt of mangrove is for many miles composed in mass of Selala trees, with a few trees of the Asiatic and American Rhizophoras growing on the outskirts.
It is obvious that in order to clear the way for considering this problem the means of renovating the Selala colonies should be inquired into. In the first place, whilst seedlings occur in numbers under the trees of the other two Rhizophoras they are never to be found under the Selala trees. The mode of reproduction of the Selala is evidently vegetative, and the question arises as to what mode of vegetative reproduction occurs. The Selala trunks, as already observed, are often inclined, the trunks being supported on trestle-like aërial roots. These trunks send out branches which in their turn drop aërial roots; and when the decay of the parent trunk takes place, the branches are able to live independently. The primary branches in due time send out secondary branches which also let fall aërial roots; and thus the process is repeated indefinitely, the result being a maze of semi-prone trunks, branches, and aërial roots. The first stage of the process ends with the death of the parent trunk, and the primary branch, supported by its own aërial roots, is often all that the observer can distinguish in the centre of a colony. This is evidently the mode by which the Selala colonies are renovated in their interior. One sometimes observes in the midst of one of these colonies extensive bare mud-flats 100 to 500 yards across from which apparently the trees have died off en masse. The natives assert that when part of a Selala tract is cleared the trees never grow again.
But pari passu with this process of vegetative reproduction of the Selala, by which the mass of the colony is preserved and renovated, there is evidently some other process of reproduction in operation amongst the trees of Rhizophora mangle and R. mucronata at the edge of the colony, as a result of which Selala seedlings are produced. Whilst no seedlings are to be observed striking into the mud under the Selala trees, numbers occur, as before observed, under the trees of the other two species. Those under the trees of R. mangle possess in nearly all the cases the distinctive leaf-characters of that mangrove, and would be recognised at once as belonging to that species. On the other hand, those beneath the trees of R. mucronata are of two kinds, some of them being readily recognised by their foliage as of the Selala type others, again, being typical seedlings of R. mucronata. Only those seedlings, or “keimlings” as we might term them, were noted that had dropped plumb from the branches above.
Such were the results of my investigations on Vanua Levu. My field of inquiry was then shifted to the Rewa delta, where, with the assistance of the Daku natives, who, like most Fijians, display a keen interest in matters relating to their plants, I spent a few days in investigating the origin of the Selala trees that grow sporadically in that locality. On pulling up some of the young trees we found that the original radicular or hypocotyledonary portion of the keimling could be still distinguished. My zealous native friends also pointed out to me that though the leaves in form and colour were those of the Selala, the rootstock was reddish like that of R. mucronata, and not white as with R. mangle. The natives averred that the Selala trees are produced in the first place from fruits of R. mucronata. When young, they said, they are Tiri-tambuas (R. mucronata), but when old, Selalas. Yet although R. mucronata may be now regarded as the source of the Selala trees, and my Vanua Levu observations pointed unmistakably in this direction, it could not be definitely settled whether this was the result of a cross with the male element of R. mangle or whether the Tiri-tambua (R. mucronata), in producing two types of seedlings, one fertile with the parent characters and the other seedless of the Selala type, brought about the same end. On the whole I am inclined to the view that the Asiatic Rhizophora presents us in the dimorphism of its seedlings the true explanation.
This inference is supported by the behaviour of Rhizophora mangle on the coast of Ecuador, a subject which is discussed in Chapter XXXII, and I have given the results of my observations on the Ecuadorian Rhizophoras side by side with those on the Fijian trees in the table before given. There are two very distinct forms of the American Rhizophora (R. mangle) in the swamps of Ecuador. There is the low coast tree, the “Mangle chico” of the Ecuadorians, ten to fifteen feet in average height, which grows on the sea-front of the swamps and has all the general appearance and the more conspicuous characters of the American Rhizophora in Fiji. There is also a tall tree, 60, 80, or even 100 feet high, that forms the great mass of the mangrove swamps. In its inflorescence, in the dark green colour of its foliage, and in other characters, it comes near the Fijian Selala; but it differs in fruiting abundantly. This is locally termed the “Mangle grande,” and its true relation to the Fijian Selala appears to be as follows. Whilst both as regards the flowers approach the Asiatic Rhizophora (R. mucronata), the Fijian Selala resembles the Asiatic tree also in its foliage, whilst the “Mangle grande” or the Ecuadorian Selala more resembles the typical American tree (R. mangle) in its leaves and also in its seedlings. Here in the Ecuadorian swamps there can be no question of crossing, since both, according to Baron von Eggers, belong to one species. Therefore I am inclined to the opinion that whilst the Asiatic Rhizophora displays dimorphism in Fiji, the American Rhizophora displays dimorphism in Ecuador. The reversion on the part of the “Mangle grande” of Ecuador to some of the characters of the Asiatic plant is remarkable, and points to the greater antiquity of the Asiatic R. mucronata as compared with the American R. mangle.
This accords with the opinion expressed by Schimper in his work on the Indo-Malayan strand flora that the American Rhizophora is either a degenerated descendant of the Asiatic R. mucronata or a sister form derived from a common ancestor. America, as we have seen, possesses only one of the three species of Rhizophora, and this is the only representative that it owns of the four Asiatic genera (Rhizophora, Kandelia, Ceriops, Bruguiera) that constitute the tribe Rhizophoreæ. The rule prevailing with current-dispersed plants that America is a distributor and not a recipient evidently does not apply to the Rhizophoreæ; and to explain their distribution we must go back to some epoch very remote from the present. That Fiji derived its representatives of Rhizophora mangle from America by the agency of the currents I do not for a moment admit. The restriction of the species and indeed also of the genus to the Western Pacific is very significant. It is far more likely that, as I have pointed out in the case of Lindenia (see page 396), the American Rhizophora was once widely distributed over the tropics of the Old and New Worlds, and that it is now on the “down grade” towards extinction. Its survival in the Western Pacific could thus be explained without our being obliged to suppose that the seedlings or keimlings have been carried uninjured across the Pacific Ocean, an ocean voyage for which, as shown in a later page, they are not well fitted.
_The Occasional Occurrence of more than one Seed in the Fruits of Rhizophora mucronata and Rhizophora mangle (Polyembryony)._
The bilocular ovary contains four ovules, one of which only as a rule becomes a seed. But it is incorrect to say that the fruits are always one-seeded, since two or even three seeds are occasionally produced, and they may all germinate. In November, 1897, I noted eight hundred fruits of Rhizophora mangle germinating on the trees in one of the creeks of the Rewa delta. Out of this number eight fruits had two germinating seeds and one had three, the protruding radicles being in all stages of growth. Just two years afterwards I counted eight hundred more fruits in the same locality, and then observed seven with two germinating seeds and none with three, the radicles protruding in all cases. On another occasion at Wailevu in Savu-Savu Bay I counted four hundred, and none had more than a single radicle protruding. The results appear to vary with the locality, but in the Rewa creek the proportion of fruits in which more than one seed germinated was fairly constant at dates two years apart, namely, about one per cent. Occasionally, however, in particular localities a greater proportion may be noticed. Thus near Daku in the Rewa delta I found that the proportion was between two and three per cent. for the same species (R. mangle), those with three germinating seeds being about half per cent.
The case of more than one seed germinating in the fruits of Rhizophora mucronata never came under my observation; but in one locality, where I examined a considerable number of fruits near the stage of germination, between ten and fifteen per cent. showed two seeds approaching maturity.
Warming thoroughly investigated the polyembryony of Rhizophora more than twenty years ago, seemingly from materials brought to him from the West Indies (Engler’s Botanische Jahrbücher, band iv., 1883). With the usual German thoroughness he deals with the work of earlier observers, and goes back to Piso in the middle of the 17th century. Of the four ovules, he remarks, three usually abort, and only in rare cases are two seeds developed. He quotes Baron von Eggers to the effect that only in three per thousand cases was more than one seedling observed protruding from a germinating fruit. These remarks evidently all apply to the American species. I do not find any reference in my notes to polyembryony in Ecuador, and evidently its occurrence is not so frequent there as in Fiji.
It is frequently apparent in the cases where more than one seed germinates in a fruit that on account of the difference in the length of the protruding seedlings germination does not always begin at the same time. Thus in Fiji the difference in the length varied between one and three inches, an amount representing at least from ten to twenty days’ growth, as will be subsequently pointed out. Warming in one of his figures gives a fruit where an interval of some months seems to be indicated, since one of the seedlings has fallen out and the other is protruding less than an inch. By cutting across a fruit containing two seeds one may sometimes observe one seed quiescent and the other beginning to germinate. The significance of this occasional interval between the germination of seeds in the same fruit will be referred to in a later page.
_The Seasons of Flowering and Fruiting of the Species of Rhizophora in Fiji._
The Selala flowers all the year. With the two American and Asiatic species there are considerable variations between different localities. Generally speaking, they flower and fruit all the year through; but the flowers are usually less abundant in the warm season from December to February, and the germinating fruits which are to be observed on the trees every month of the year are more numerous in that season.
_The History of the Reproductive Process in Rhizophora from the Fertilisation of the Ovule to the Falling of the Plantlet or Seedling from the Tree._
I devoted great attention to this subject in the instance of Rhizophora mangle, being desirous of determining two points, in the first place as to whether there was any period of rest between the maturation and germination of the seed, and in the second place as to the period that elapsed between the commencement of germination and the fall of the seedling.
The principal change in the ovary for the first three or four weeks after fertilisation is shown in its increased breadth. The increase in height is but slight during this period; and in fact after thirty days the ovary only added 2 millimetres to its original height of 3 millimetres. After this the growth of the fruit proceeds until the tip of the radicle pierces its summit, the fruit being then about eleven lines (2·8 cm.) long. From the date of fertilisation to the time the radicle pierces the top of the fruit a period of about fifteen weeks elapses. (The fruit, it should be here remarked, continues to grow in length and breadth after the radicle has protruded, attaining a length of thirteen or fourteen lines [3·5 cm.] when the seedling or “keimling” is ready to fall.)
By referring to the table below it will be observed that there is no period of rest in the growth of the fruit up to the date of the protrusion of the radicle. It will now be shown that there is normally no pause between the epoch of the maturation of the seed and the beginning of germination, or, in other words, that from the time of the fertilisation of the ovule to the onset of germination there is no cessation in the process of growth of the embryo. That period of dormant vitality which almost all seeds pass through forms no normal feature in the life-history of this species of Rhizophora.
RHIZOPHORA MANGLE AND R. MUCRONATA.
+-------------------------------------------+ | RHIZOPHORA MANGLE. | +-------------------------------------------+ | Growth of fruit in height. | +---------------------+---------------------+ | Lines or tenths of | Number of days | |an inch (millimetres | since | | in brackets). | fertilisation. | +---------------------+---------------------+ | | | | 2 (5) | 30 | | 3 (7·5) | 42 | | 4 (10) | 50 | | 5 (12·5) | 61 | | 6 (15) | 67 | | 7 (17·5) | 74 | | 8 (20) | 83 | | 9 (22·5) | 92 | | 10 (25) | 100 | | 11 (28) | 105 | | | { Protrusion| | | 107{ of the | | | { hypocotyl | | | | Growth of the protruding hypocotyl. | | | | 10 (25) | 127 | | 20 (50·5) | 141 | | 30 (76) | 151 | | 40 (101·5) | 160 | | 50 (127) | 167 | | 60 (152) | 175 | | 70 (177·5) | 185 | | 80 (203) | 202 | | 90 (228) | 222 | | | {Fall of | | | 229{the | | | {seedling | +---------------------+---------------------+
Explanation of the Table.
We have here shown the period between fertilisation and the fall of the seedling from the tree.
This period divides itself into two parts, the first being concerned with the continuous growth of the fruit and of the inclosed embryo until the tip of the hypocotyl appears through the apex of the fruit, the second being indicated by the growth of the protruding hypocotyl until the fall of the seedling.
The height of the fruit is measured from the base of the calyx-lobes, and the length of the hypocotyl at first from the apex of the fruit and afterwards from the edge of the protruding neck of the cotyledonary body. The height of the ovary at the time of fertilisation is about 3 millimetres; and from that time onward it is to be regarded as a fruit.
[To face page 452.
FIGURES ILLUSTRATING THE DEVELOPMENT OF THE SEED AND THE GERMINATING PROCESS OF RHIZOPHORA AND BRUGUIERA
(Natural size. Drawn for convenience of description in the erect position.)
1. Rhizophora mucronata Fruit 3-1/3 lines (8-9 mm.) high, six to seven weeks after fertilisation. The micropyle is but slightly dilated, and is occupied by a small plug of endosperm.
2. Rhizophora mucronata Seed of fruit represented in Fig. 1.
3. Rhizophora mucronata Fruit 5 lines (12 mm.) high, eight to nine weeks after fertilisation. Germination is about to begin. A large plug of endosperm now protrudes through the dilated micropyle, but still covers the lengthening hypocotyl.
{Seed of fruit represented in Fig. 3. In 4.} Rhizophora mucronata {Fig. 4 the plug of endosperm is shown on the 5.} {upper end of the seed; whilst in Fig. 5 it {has been removed, exposing the tip of the {hypocotyl.
6. Rhizophora mucronata Fruit 7-1/2 lines (18 mm.) high, eleven to twelve weeks after fertilisation.
7. Rhizophora mucronata Fruit, seventeen or eighteen weeks after fertilisation.
8. Rhizophora mucronata Full-grown fruit with upper portion of seedling just before detachment from the tree. The long tapering plumule is here inclosed in the cotyledonary body. The * indicates the point of detachment of the seedling.
9. Rhizophora mucronata The cotyledonary body of Fig. 8.
9A.} {Illustrating different stages in the 9B.}Rhizophora mucronata {development of the plumule and of the neck of 9C.} {the cotyledonary body resulting finally in 9D.} {the expulsion of the plumular end of the {seedling from the fruit cavity as in Fig. 8. {(See page 458.)
10. Rhizophora mucronata Fruit with two seeds.
FIGURES ILLUSTRATING THE DEVELOPMENT OF THE SEED AND THE GERMINATING PROCESS OF RHIZOPHORA AND BRUGUIERA—(continued)
(Natural size. Drawn for convenience of description in the erect position.)
11. Rhizophora mangle Fruit, six weeks after fertilisation.
12. Rhizophora mangle Seed with plug of endosperm, as shown in Fig. 11.
13. Rhizophora mangle Fruit, eight weeks after fertilisation. The tip of the hypocotyl is now piercing the plug.
14. Rhizophora mangle Embryo (enlarged) shown in Fig. 13.
15. Rhizophora mangle Fruit, ten weeks after fertilisation. The growing hypocotyl has now pierced the plug.
16. Rhizophora mangle Embryo shown in Fig. 15.
17. Rhizophora mangle Fruit, nearly sixteen weeks after fertilisation.
18. Rhizophora mangle Full-grown fruit, just before the detachment of the seedling from the tree. The long tapering plumule is inclosed in the tube of the cotyledonary body. The point of detachment of the seedling is indicated by *.
19. Rhizophora mangle The cotyledonary body of Fig. 18.
20. Rhizophora mangle Fruit with two seedlings in different stages of growth (given in the first plate).
21. Bruguiera Rheedii Fruit, about four weeks after fertilisation. (The shaded portion is the calyx-tube or cup, in the midst of which rises the style.)
22. Bruguiera Rheedii Germinating seed.
23. Bruguiera Rheedii Germinating fruit, about eight weeks after fertilisation.
24. Bruguiera Rheedii Germinating fruit, about ten weeks after fertilisation. Here the growing hypocotyl, carrying the style with it, has pushed upwards the lining membrane of the floor of the calyx-tube, which has ruptured and forms a cap on its extremity.
25. Bruguiera Rheedii Germinating fruit, thirteen or fourteen weeks after fertilisation.
26. Bruguiera Rheedii Fruit with full-grown seedling just before its detachment from the tree.
[To face page 453.
RHIZOPHORA MUCRONATA.
_Growth of the first seven inches of the hypocotyl after it protrudes from the fruit._
10 lines (25 mm.) after 26 days 20 lines (50·5 mm.) after 41 days 30 lines (76 mm.) after 51 days 40 lines (101·5 mm.) after 61 days 50 lines (127 mm.) after 70 days 60 lines (152 mm.) after 78 days 70 lines (177·5 mm.) after 86 days
In my description of the germinating process of Rhizophora mangle from this particular standpoint I adopt the general views of Prof. Schimper, the observations being my own, the phraseology employed being his. It would be out of place here to deal with the biological significance of a process to which observers like Warming, Goebel, Karsten, Schimper and Haberlandt have applied their greater talents as well as their greater experience. I investigated the subject carefully from my own standpoint of inquiry, and whilst the reader will find in my rough sketches of the various stages of the process a little aid in following the argument, he is referred for detailed treatment of the subject to the memoirs of the above-named botanists as well as to those of yet more recent investigators.
After fertilisation, according to Prof. Schimper (Ind. Mal. Strandflora), the embryo-sac is filled with endosperm, which subsequently protrudes and forms a plug completely closing the micropyle (see my figures). As my observations showed, the seed during the first eight weeks after fertilisation increases continuously in size, and the plug of endosperm, at first inconspicuous, becomes of considerable size, the seed attaining a length of seven millimetres. The embryo meanwhile grows rapidly, and at the end of this period of eight weeks the radicular tip or the point of the hypocotyl begins to protrude from the micopyle, still covered by the plug of endosperm, the fruit being between four and five lines (10-12 mm.) in length (figures 11-14). In another week, when the fruit has grown another line in length, the tip of the radicle is on the eve of piercing the plug, and this may be termed the commencement of germination, nine weeks after the act of fertilisation. The next stage, after an interval of one and a half weeks, is illustrated in figure 15; and after a period of about fifteen weeks from the date of fertilisation the tip of the radicle pierces the top of the fruit. As shown in the figures, the fruit grows in length throughout the process.
The question as to whether the matured seed passes through a stage of quiescence before it germinates finds its answer in the statement that only nine weeks elapse between fertilisation and germination. It may, however, be urged that the maturation of the seed could be accomplished in a few weeks, and that after this a period of dormant vitality might follow. This objection can be at once disposed of and the whole matter placed beyond reasonable doubt by making, as I did, a large number of vertical sections of the fruit in all its stages. It will then be perceived that there is a fairly constant relation in all stages of growth between the seed and the fruit, whether maturating or germinating. Since the growth of the fruit is continuous (see Table) up to the time of the protrusion of the tip of the hypocotyl through its coats, it follows that there can be no appreciable pause between the completion of maturation and the commencement of germination of the seed. In other words, both fruit and seed preserve the same relation during the process, and the absence of any period of rest is to be inferred from the uninterrupted growth of the fruit.
We will take, to illustrate this point, a fruit between four and five lines long in the stage that immediately precedes germination (see figure 11). The fruit proceeds with its growth, and the seed, we will suppose, remains quiescent for a month. At the end of that time (see Table) the fruit would be eight lines long, and the seed, of course, would be unchanged. This condition of things never presented itself to me. Fruits eight lines long were always far advanced in germination (see figure 15). If the seed passes through an interval of rest before germination, it must be of a very short duration and practically nil.
This absence of any period of rest between the final maturation of the seed and its fertilisation had already been assumed by Prof. Schimper. Writing to me on July 14, 1898, when my observations were in progress, he says:—“I am ready to assume, according to my own experience, that there is continuous development until the falling off of the embryo. More accurate observations on the subject would be interesting, and would not present any great difficulties.” At the end of the same month he wrote the preface to his great work on Plant-Geography; and he expresses himself decidedly on this point. Speaking of Rhizophora mucronata (English edition, p. 396), he says that “the fruit ... soon after the completion of its growth is pierced at its summit by the green hypocotyl, as the embryo does not undergo any period of rest, but continues to develop without interruption.”
Though the rest-period is normally non-existent with the seeds of Rhizophora, it has already been observed that it is indicated in rare cases and under exceptional conditions. Thus I have already remarked that in Fiji about one per cent. of the germinating fruits of the American species exhibit more than one seed. These seeds usually begin to germinate about the same time, but in a few cases, say, one in ten, a marked difference in the length of the protruding hypocotyls points to the fact that one of the seeds began to germinate some weeks after the other. We at times also meet with fruits which when cut across display two seeds, of which only one is beginning to germinate. Such cases indicative of a pause between the maturation of the seed and the beginning of germination would be very rare. With Rhizophora mangle, probably one in a thousand fruits would be a generous estimate.
In passing it may be remarked that the same stages occur with Rhizophora mucronata in the development of the seed and in the subsequent germinating process. When the fruit is three lines long the micropyle is but slightly dilated (see figures 1 and 2). When it is four lines long the endosperm begins to escape from the gaping micropyle and forms a projecting plug. The growth of the embryo now becomes rapid, the endosperm escapes in greater quantity, and by the time the fruit is five lines long the tip of the radicle is on a level with the micropyle, although still covered by the plug (see figures 4, 5). After this, germination begins; and when the fruit is six lines in length the radicle is in the act of penetrating the plug. Ultimately the tip of the radicle pierces the top of the fruit when this last is nine or ten lines long. As shown in the figures there is continuous growth of the fruit during the maturation and germination of the seed, until, in fact, the plantlet drops into the water. With reference to the stage when germination begins, it should be remarked that the formation of the large plug of endosperm outside the micropyle does not necessarily indicate the beginning of germination. Germination is in progress only when the hypocotyl or radicle begins to lengthen and is on the point of piercing the plug of endosperm that fills up the gaping micropyle. This is well shown in this species in the case of fruits with two seeds. Both seeds may have large plugs of endosperm, and yet only one may show indications of germination in the lengthening hypocotyl.
We must now return to the subject of the growth of the hanging seedling of Rhizophora mangle. We have already remarked that, as shown in the Table, about fifteen weeks (107 days) is the average time elapsing between the fertilisation of the ovule and the protrusion of the tip of the radicle through the top of the fruit. A further period of seventeen and a half weeks (122 days) is occupied by the growth of the seedling on the tree, at the end of which period it drops into the water or mud according to the state of the tide. This gives a total period of nearly thirty-three weeks (229 days) as the duration of the time between fertilisation and the fall of the seedling. This may be divided, as has been already implied, in the following manner:—
(1) Period between fertilisation and germination. 9 weeks.
(2) Period between the commencement of germination 6-1/2 and the protrusion of the tip of the radicle weeks. through the top of the fruit.
(3) Period occupied by the growth of the hypocotyl 17-1/2 outside the fruit, and terminating in the fall weeks. of the seedling from the tree.
——
Total 33 weeks.
This represents the average of numerous observations, the deviations being from two to three weeks on either side. In the latter part of its growth, the lower end of the hypocotyl becomes thickened or club-like, and during the last week or ten days the increase in length is arrested altogether.
My observations on the growth of the seedling on the tree of Rhizophora mucronata were comparatively few; but, as shown in the Table on page 453 they give nearly the same rate of growth. Taking the average length attained by the hypocotyl on the tree at sixteen inches, and employing as well the data supplied by Rhizophora mangle, a period of 26-1/2 weeks would elapse from the time the hypocotyl pierces the top of the fruit until the plantlet falls from the tree. If we then add, as in the case of the other species, 15-1/2 weeks for the preceding period between fertilisation and the protrusion of the hypocotyl, we get a total of 42 weeks for the whole period from fertilisation to the fall of the seedling. In the extreme cases where a length of almost two feet is attained on the tree, the period would somewhat exceed twelve months; and in those rare instances in other regions, when, according to Schimper, the seedling is a metre in length, probably eighteen months would be required. The period for Rhizophora mucronata is thus considerably longer than for R. mangle, which is sufficiently indicated by the difference in the average length of their hypocotyls on the tree in Fiji, that for R. mucronata being sixteen inches, and that for R. mangle nine or ten inches.
The only other observations that have come under my notice relating to this subject are those made by Jacquin on Rhizophora mangle in the West Indies in the middle of the eighteenth century. The results are literally quoted by Warming; but I have referred to the original account in the work of Jacquin, entitled Selectarum Stirpium Americanarum Historia, Vindobonæ, 1763. According to this observer the seedling falls from the tree in the twelfth month from the fecundation of the flower. This happened in my observations on the same species in Fiji in the eighth or ninth month. Jacquin states that the tip of the radicle protrudes from the fruit in the third month, whilst my results give it as taking place in the fourth month. The difference in the length of the total period, it may be remarked, would be to a great extent determined by the varying length acquired by the seedling before it drops from the tree. In ordinary conditions it averages about ten or eleven inches, and the hypocotyl itself attains a length of nine or ten inches on the tree, both in Fiji and Ecuador; but in sheltered localities it may attain a length half as long again. I have already pointed out in the case of the fruits of Rhizophora mucronata that a year and more would be sometimes required, and the same remark would apply to unusually long fruits of R. mangle. Local conditions would often produce varying results, both in the rate of growth of the hanging seedling and in the duration of the period of its attachment to the tree; but it is probable that nine or ten months would represent for the genus the average length of the period between fertilisation of the ovule and the detachment of the seedling from the parent tree.
_The mode of separation of the seedlings of Rhizophora mangle and Rhizophora mucronata_
This is a process of expulsion almost akin to parturition, and is brought about by the outward growth of the neck of the cotyledonary body. There is much that is of great interest in this subject; and I may add that Haberlandt, in a memoir published in the Annales du Jardin Botanique de Buitenzorg for 1894, gives the results of an elaborate study of the viviparous process in this and other genera of mangroves. The same analogy seems also to have presented itself to him, but only in connection with the means employed in some of the genera, as with Bruguiera, for conveying nourishment to the growing embryo. He remarks that he was involuntarily reminded by these structures of the chorion-tufts and lobes in the placenta of mammals, and that such structures in the mammal are functionally nothing more than true haustoria as found in the viviparous mangroves.
When studying the germination of the American and Asiatic Rhizophoras in Fiji, I observed that the neck of the cotyledonary body did not begin to form, nor the inclosed plumular bud to show signs of differentiation, until the hypocotyl had protruded about 4-1/2 inches with R. mangle, and between 6 and 7 inches with R. mucronata. The neck of the cotyledonary body then proceeds to grow in length, pushing before it the plumular end of the embryo-seedling, which it surrounds as a sheath. This operation continues until the hypocotyl has acquired a length of about seven inches with R. mangle, and about nine inches with R. mucronata, when the neck begins to protrude outside the fruit. The cotyledonary neck proceeds with its growth, and by the time the seedling is ready to fall from the tree it protrudes about an inch from the fruit-shell, having carried the growing plumular bud with it. The plumular end of the seedling has been now more or less expelled from the fruit-cavity, and the connection between the suspended seedling and the fruit now alone depends on a slight bond between the base of the plumule and the inner margin of the cotyledonary neck, as indicated by a cross in the figures given in the plate. The union is soon broken and the seedling falls.
Whether there is anything more than an analogy between the expulsion of a Rhizophora seedling and the birth of a mammal seems most unlikely; but the process is at all events a very remarkable one.
The means of dispersal of the genus Rhizophora
My experiments and observations were for the most part made on the Asiatic and American species in Fiji; but I enjoyed the opportunity of confirming some important points on the coast of Ecuador. We can only look to the currents for the explanation of the capacity of the genus to cross tracts of ocean; but, given this capacity, there is much that is difficult to understand in the distribution of the genus and of a species like Rhizophora mangle; and it is probable that we shall have to look behind the means of dispersal to a distant age in the distribution of shore-plants of the mangrove type.
When Schimper published his work on the Indo-Malayan strand flora in 1891, but little was known of the duration of the floating capacity of Rhizophora seedlings (p. 166). In giving the results of my investigations I am merely describing the agencies of dispersal at present in operation. Such agencies have their limitations, and we may, perhaps, be thus able to explain why Rhizophora is restricted in the Pacific islands to the archipelagoes of the Western Pacific; but many serious objections would at once present themselves if we regarded the occurrence of the genus in America, as well as in Asia and Africa, as a matter depending on capacities and means of dispersal.
The fruits of Rhizophora, as they display themselves before the protrusion of the germinating seed, have no buoyancy, and the germinating fruits until the hypocotyl has protruded for some inches (6 inches in the case of R. mangle) also sink in sea-water. With a further increase in the length of the hypocotyl, the germinating fruit acquires buoyancy; and when the seedling, usually 10 or 11 inches in length, becomes detached from the fruit on the tree and falls into the sea, it floats readily in 95 per cent. of the cases. Such seedlings occur very commonly in the floating drift of the estuaries and out at sea both in Fiji and in Ecuador.
Out of five seedlings of the Asiatic species, Rhizophora mucronata, that had fallen naturally from the tree, three were afloat and healthy after eighty-seven days’ immersion in sea-water. Out of twenty seedlings of the American mangrove, Rhizophora mangle, sixteen floated after ninety days and four were afloat and healthy after one hundred and twenty days, the greater number sinking during the fourth month. These results indicate considerable powers of buoyancy, and go to show that extensive tracts of ocean could be traversed by the floating seedling.
It should, however, be observed that not all the full-sized seedlings float. With Rhizophora mangle about 5 per cent. sink in sea-water and from 20 to 50 per cent. sink in fresh-water; whilst with R. mucronata the proportion of non-buoyant seedlings is rather greater. There would thus appear to be a rather nice adjustment of the specific weight of the seedlings to the density of sea-water. Generally speaking, they may be seen floating vertically or steeply inclined in the fresh-water of estuaries and horizontally in the sea. With the buoyant seedlings of Rhizophora mucronata, as a rule, about 90 per cent. float horizontally in sea-water, and about 70 per cent. float vertically or steeply inclined in fresh-water. The same general rule applies to R. mangle, whether in the rivers and seas of Fiji or in those of Ecuador. In those cases where the seedling drops prematurely on account either of storms and floods or of the depredations of a grub that frequently attacks the fruit, this rule would not apply. One may frequently notice in Fiji after heavy weather that seedlings detached prematurely, and often carrying the fruit, are floating in numbers horizontally in the rivers. In a few days, as a rule, the fruit-case becomes detached and sinks.
It may be remarked that the horizontal position is much better adapted for the safety of the seedling in transport than the vertical position. In the last case the plumule, which protrudes above the water, would be unable, as indicated in my experiments, to withstand the scorching rays of the sun in a smooth sea; whereas in the horizontal position, which the seedlings assume in sea-water, the plumule is more or less completely submerged, and the risk of withering in the sun is very much less. The Rhizophora seedlings would certainly have little chance of crossing in safety a large tract of sea, if they floated, as they do in river-water, with the plumule exposed above the surface. It is not unlikely that the comparatively restricted area occupied by Rhizophora conjugata may be due to the attitude its seedlings assume when floating in sea-water.
The stranded seedlings of Rhizophora readily establish themselves for a while in very different situations; and it is by no means necessary that they should be washed ashore on a muddy coast. When half-buried amongst the heap of vegetable drift piled up on a sandy beach they are frequently to be found striking into the sand and showing their first leaves. Here they ultimately perish in the great majority of cases; but when protected long enough to reach the moist sand four or five inches below, they may give rise to a little mangrove colony. When caught in a fissure in the bare reef-flats these plantlets are sometimes able to establish themselves. Rhizophora seedlings would, however, require a coast prepared by them by the work of ages before they could form extensive swamps. It is, therefore, not surprising that Prof. Penzig found no evidence of mangrove-settlements on the shores of Krakatoa fourteen years after the eruption.
Yet suited as Rhizophora seedlings are for crossing tracts of sea, I regard them as quite unfitted for being transported by the currents unharmed across an ocean. The plumular bud is insufficiently protected for such a long voyage of many months, and perhaps of years. Though the horizontal position of the seedling would secure the plumule against being scorched in the sun, it increases considerably the risk of injury from direct impact.
As bearing on their capacity for dispersal in other fashions, it may be remarked that Rhizophora seedlings can withstand long drying. Five which had been kept dry for nine weeks, after having been found stranded on a beach, were planted in the mud of a mangrove-swamp. In a fortnight two of them were developing the first leaves and throwing out roots. As long as they are protected by a covering of vegetable débris and sand, the stranded seedlings might retain their vitality for months.
BRUGUIERA RHEEDII (Blume)
This species is reduced in Hooker’s Flora of British India to Bruguiera gymnorhiza (Lam.), and thus viewed it has a very wide range in the Old World, corresponding very much to that of Rhizophora mucronata, namely, tropical East Africa, tropical East Asia to the Liukiu Islands, the Indian Archipelago, New Guinea, tropical Australia, and Western Polynesia, as in New Caledonia, Fiji, Tonga, and Samoa. There are four or five species of the genus, but all are confined to the Eastern Hemisphere, none occurring in America.
As with the species of Rhizophora, this plant is indebted for its present dispersal to the floating seedling, which, however, often falls from the tree whilst still attached to the fruit, but is generally freed in a day or two. The seedlings float for a long time in sea-water. I kept one of them afloat for 117 days, when it was quite sound and healthy. They appear to be better fitted than the species of Rhizophora for the “rough-and-tumble” of ocean transport, since the plumule is much less prominent, projecting only one line (2·5 mm.) or less, whilst with the two Fijian species of Rhizophora the plumule measures from seven to twelve lines (18 to 30 mm.). In the latter part of the year they are to be found in abundance in the floating drift of rivers, and there they readily develop the first leaves and roots. They are also frequent in the sea off the coasts, and they are stranded in large numbers on the beaches, where they readily strike into the sand when partially buried amongst the vegetable drift.
The empty flowers and the germinated fruits containing the cotyledons are very common in floating drift. They look much alike, but the flowers are much smaller and possess the long style, whilst the fruits contain the cotyledons at the bottom of the seed-cavity.
As with Rhizophora, there is a rather curious adjustment of the buoyancy of the seedling to the density of sea-water. About 75 per cent. of those afloat in the fresh-water of rivers assume the vertical position, the plumular end protruding between two and five lines (5 to 12 mm.) above the surface, while the remainder float horizontally or nearly so. In sea-water about 50 per cent. float either vertically or steeply inclined, and the other half float horizontally.
With regard to the times of flowering and fruiting, it may be remarked that the trees are mostly in flower during the hot months from November to February, and that the fruiting is in active operation in the latter half of March. The floating seedlings occur in abundance in the river-drift at the end of the year, a circumstance which corresponds with the fact that a period of six months passes between the fertilisation of the ovule and the fall of the seedling into the water.
Fertilisation, or, more correctly speaking, the discharge of the pollen, takes place after the opening of the flower, and not before, as in the case of the species of Rhizophora. The flower-bud is at first erect, but subsequently it begins to bend downwards, and ultimately it hangs more or less vertically. The provision to secure fertilisation under these circumstances is rather curious. Without some such contrivance as is below described, the pollen would merely fall out of the flower. Each petal has its sides rolled or folded inwards so as to completely inclose two stamens. In the bud the folded petals are white and flexible, but as the flower expands they redden and become dry and elastic, and are only prevented from flying open with a spring by the interlocking of the hairy tips of their lobes. Whilst the folded petals are becoming stiff and elastic during the opening of the flower, the inclosed stamens are at the same time preparing themselves for their function. The anthers are dehiscing and the filaments are acquiring elasticity. All is now ready, and a slight shake or a touch puts the mechanism into action. The petals unfold themselves with a spring, and the stamens thus suddenly exposed and released fly forward, and a little shower of pollen is thrown towards the centre of the flower. This process is accomplished in ordinary fine weather during the first twenty-four or thirty-six hours after the expansion of the flower. When the opening occurs in the early morning, half of the stamens will be found released in the evening and the rest on the following day. During the next day or two the petals and the stamens fall out of the flower. In wet weather, the petals never acquire elasticity, and in consequence do not unfold. In this case pollenisation is never effected, and the folded petals soon fall to the ground, carrying the stamens within them. Cross-fertilisation would be much more likely to occur with species of Bruguiera (if, as is probable, the same process of pollenisation is usually followed) than with species of Rhizophora, since the stamens are securely inclosed in the petals for some hours after the expansion of the flower.
Nearly eight weeks pass between the date of fertilisation and the commencement of germination. This is somewhat similar to the period given for Rhizophora mangle, namely, nine weeks, and it obviously leaves little or no time for any stage of quiescence or dormant vitality in the case of the seed. The changes which the fruit undergoes in this interval are a considerable increase in girth and a thickening of the calycine walls, together with a contraction of the mouth of the tube. However, I found no method sufficiently accurate for recording the rate of increase of the fruit.
It is known that germination is in progress when the end of the hypocotyl begins to lift up the lining membrane at the bottom of the calycine tube (see Figs. 21 to 26). The floor of the tube begins to bulge up, but since this cannot be well seen at first, a better index is afforded in the elevation of the style which accompanies it. The top of the style preserves previous to this time a constant level with regard to the tips of the calycine teeth. But this does not indicate the actual beginning of germination. As shown in Fig. 21, the seed lies about two and a half lines (6 mm.) below the floor of the calycine tube, and the tip of the hypocotyl has to penetrate the intervening tissues before it can push up the lining membrane and raise the style. Judging from the subsequent rate of growth, seven or eight days at least, and perhaps as much as two weeks, are requisite for this purpose. It is not necessary to give further details here, and it may be at once stated that the average of numerous observations on the length of the interval between fertilisation and the elevation of the style was sixty-four days, the range being fifty-nine to sixty-nine. After deducting ten days for the time occupied for the radicle in reaching the floor of the calycine tube (see Figs. 22 and 23), we obtain, as already remarked, nearly eight weeks as the time elapsing between fertilisation and germination.
The radicle or hypocotyl, therefore, in the first stage of germination pierces the tissues above it and reaches the floor of the calycine tube. It does not, however, pierce the lining membrane of the tube but pushes it upward until it ruptures about 4 millimetres below the base of the style which is carried up with it. Thus a kind of cap is formed, as shown in Fig. 24, which does not fall off from the end of the hypocotyl until it has protruded rather more than an inch. The hypocotyl attains a length varying between 5 and 11 inches, the average being about 8 inches.
The whole period may be thus divided up:—
(1) Period between fertilisation and germination 7-1/2 weeks.
(2) Period between the beginning of germination and the 1-1/2 protrusion of the point of the hypocotyl at the floor of the weeks. calycine tube
(3) Period occupied in the growth of the hypocotyl 8 inches 18 weeks. outside the fruit and terminating in the fall of the seedling
——
Total 27 weeks.
The total period of twenty-seven weeks between fertilisation and the fall of the seedling is thus six weeks shorter than that estimated for Rhizophora mangle. On comparing the two tables it will be seen that the difference mainly lies in the length of the second period, namely, that between the commencement of germination and the protrusion of the hypocotyl from the fruit. With Rhizophora mangle the fruit grows considerably in length during this period of the germinating process. On the other hand with Bruguiera rheedii there is, during this period, practically no increase in the length of the fruit, and the radicle has only to penetrate the tissues, 2-1/2 lines in thickness, between the seed and the floor of the calycine tube.
In the mode of separation of the seedling there are very marked differences between this species of Bruguiera and the species of Rhizophora. With Bruguiera rheedii the four small cotyledons, which are united at the base, are, however, left behind at the bottom of the seed-cavity, when the seedling is detached. But there is no expulsion of the seedling, the connection being ultimately severed at the contracted base of the cotyledons inside the fruit. When the seedling is full-sized the nutritive supply begins to fail, and in consequence the pressure of the sides of the fruit on the inclosed plumular end of the seedling becomes slacker, the union with the cotyledons becomes weaker, and the connection of the fruit with its peduncle at the basal joint becomes slighter. Usually the fruit falls before the seedling is ready to drop out, and the connection is severed after a few days’ flotation in the water; but sometimes the union between the seedling and fruit is weaker than that between the fruit and its peduncle, and in that case the seedling falls and leaves the fruit containing the cotyledons on the tree. The whole process of separation is much simpler than with species of Rhizophora. Here it is mainly a matter of the failure of the nutritive supply, whilst with Rhizophora it is almost a process of parturition.
Haberlandt, in the memoir before quoted, describes quite a different mode of detachment in the case of Bruguiera eriopetala. Here the seedling falls normally whilst still attached to the fruit, and the separation is subsequently effected by the expansion of the mouth of the calyx-tube due to the swelling of the “endosperm-neck” from the entrance of water.
Summary
(1) There are four typical mangroves of the Rhizophoraceæ in Fiji, Bruguiera rheedii, Rhizophora mucronata (the Asiatic species), Rhizophora mangle (the American species), and the Selala, a seedless form intermediate between the two species of Rhizophora just named, but nearest to the Asiatic species.
(2) It is shown that the sterility of the Selala is connected with the impotent character of the pollen; and since the ovules appear capable of fertilisation this is held to indicate that cross fertilisation has not been in operation in producing the barren form.
(3) Good reasons are given for the belief that the Asiatic species of Rhizophora is the parent of the Selala, not as the result of a cross between the Asiatic and American species, but as connected with dimorphism, the Asiatic species producing two kinds of offspring, one of them with impotent pollen.
(4) In support of this view it is pointed out that there are two forms of Rhizophora mangle in Ecuador, one of which comes near the Fijian Selala, though producing seed. There could thus be no question of crossing, since but one species occurs there.
(5) The Selala reproduces itself in a vegetative fashion when growing, as it often does, in an inclined position. The parent trunk dies and the primary branches supported by the aërial roots, remain alive and in their turn give rise to secondary branches similarly supported.
(6) Although, as a rule, only one of the four ovules of Rhizophora becomes a seed, occasionally a fruit contains more than one seed. With R. mangle in Fiji about one per cent. of the germinating fruits displayed more than one hypocotyl.
(7) As a result of a protracted series of observations in Fiji, it was established that in the case of a seedling of average length of Rhizophora mangle a period of thirty-three weeks elapsed between the date of fertilisation of the ovule and the detachment of the seedling from the tree. In the instance of R. mucronata it was placed at forty-two weeks. A period of thirty-eight weeks, or nine to ten months, is regarded as typical for the genus.
(8) It is established that normally there is no rest-period for the seed in the case of Rhizophora, the seed at once beginning to germinate on reaching maturity. In those exceptional instances, however, where there is more than one seed, it is shown that in some cases the seeds do not begin to germinate together, and that a rest-period of at least some weeks can be at times postulated for one of the seeds.
(9) An analogy exists between the process of expulsion ending in the detachment of the seedling of Rhizophora from the fruit and the process of parturition.
(10) Experiments show that Rhizophora seedlings can float unharmed in sea-water for a period of at least three or four months. Though nine-tenths or more float in sea-water, as much as a fourth or a half sink in fresh-water. As a rule they float vertically in fresh-water and horizontally in sea-water, the horizontal position safe-guarding the plumule against the risk of being withered up by the sun in a calm sea.
(11) It is shown that in the case of Bruguiera rheedii the seedlings when detached from the tree can float unharmed in sea-water for months. In their specific weight they display a similar fine adjustment to the density of sea-water, as is above described in the case of Rhizophora.
(12) With this species of Bruguiera, fertilisation takes place not in the unopened flower, as in Rhizophora, but after the flower’s expansion; and a very singular mechanism is here described which secures the completion of the process.
(13) A period of twenty-seven weeks elapses between the fecundation of the ovule and the detachment of the seedling from the tree in the case of Bruguiera rheedii; and it is shown that there is normally little or no room for any rest-period, and that, as with Rhizophora, the seed on reaching maturity begins to germinate.
(14) Though the seedlings of Rhizophora and Bruguiera could be transported in safety a few hundred miles across the sea, it is held that they could never cross the Pacific and reproduce the plant. That the American species of Rhizophora has reached the Western Pacific from the New World is not accepted. Rather is its present distribution regarded as representing its original wide range over much of the tropical zone.
XXXI. A CHAPTER ON VIVIPARY
The significance of vivipary.—The scale of germinative capacity.—A lost habit with many inland plants.—The views of Goebel.—The shrinking in the course of ages of tropical swamp areas.—The variation in the structures concerned with vivipary.—Abnormal vivipary.—Summary.
IT was remarked in Chapter IX that the study of the germination of the floating seed carried us to the borderland of vivipary; and we may now observe that our study of the mangroves, Rhizophora and Bruguiera, in the previous chapter, has brought us into contact with vivipary in its most complete development in the tropical swamps of our age. There is a great gap between the two extremes, represented by the occasional germination of a seed in a capsule or in a berry on the plant, and by the elaborate process of vivipary exemplified by Rhizophora; but most of the intermediate stages can be illustrated by known examples of vivipary. There is, however, no pretension to deal with this subject here in anything but a cursory fashion; but it will, I venture to think, add completeness to a work in which germination on and off the plant has been such a frequent theme if I endeavour to connect together some of the various sets of facts known to us concerning germination from the standpoint of vivipary.
The principal argument here followed has been already outlined in Chapter IX, where I have remarked that it is possible to construct a scale of the germinative capacities of plants, presenting a continuous series beginning with the mangroves, where germination takes place on the tree, and ending with those numerous inland plants where seeds are liberated in an immature condition. It is suggested that vivipary was the rule under the uniform climatic conditions of early geological periods, and that with the differentiation of climates that has marked the emergence of the continents the viviparous habit has been lost over much of the globe, the mangrove-swamps alone illustrating the climatic conditions once prevailing. The rest-period of the seed is regarded as an adaptation to climatic differentiation and to seasonal variation; and even the seed-stage may be broadly regarded as the price paid for adaptation on the part of the evolutionary or determining power that lies behind plant-development. When discussing the germination of Cæsalpinia in Chapter XVII, I have shown that the contraction and induration of the seed-tests appear merely as an adaptation to climatic differentiation and to seasonal variation, and that it would be quite possible by exposing the maturing seed to very warm and moist conditions to induce germination without any rest-period, as actually occurs with Rhizophora. One would then dispense altogether with the final processes of the contraction and induration of the seed-coats, as illustrated in the Leguminosæ; and the rest-stage would appear as an adaptation to secular differentiation of climate in the later epochs of the world’s history.
The significance of occasional vivipary was long ago pointed out by Goebel in his Pflanzenbiologische Schilderungen (teil I., 117-134, Marburg, 1889), when he observed that vivipary, as displayed in the mangroves, and particularly in the Rhizophoreæ, represented the fullest expression of a habit that is only occasionally exhibited by other plants under exceptionally moist conditions. His view was that the seeds of plants living in wet places are suited in a varying degree for rapid germination, and that vivipary presents itself as the most complete development of this capacity. If I regard the views of Goebel and of Kerner aright, vivipary as normally developed in the mangrove is to be traced in a descending scale to small beginnings, the principal determining condition lying in the great difference that exists amongst plants in the readiness of the seed to germinate. In the ascending scale we would have first the detachment of the immature seed, where the embryo is often in a rudimentary state, the ripening of the seed taking place in the soil. Then would come those plants where the seeds on being detached are quite mature and are ready to germinate as soon as they fall to the ground. Then would follow the stage represented by those plants where the seeds merely begin to germinate on the plant, such as occurs more or less normally with some mangroves like Laguncularia, and abnormally with a number of plants living in drier stations. After this come those mangroves, where, as in Avicennia, germination is completed on the tree or shrub, but the seedling at once liberates itself from the parent. Last of all there is the stage of the typical mangroves, Rhizophora and Bruguiera, where the seedling remains for months growing on the tree and hangs from the branches.
Vivipary, as above stated, presents itself as a matter of small beginnings. My own view, however, is that it is a matter of small “endings”; and that if we were to commence the scale not with the immature seed lying on the soil, but with the seedling suspended from the branches of a Rhizophora tree, we should record the various epochs in the history of vivipary throughout the plant-world. From this standpoint the occasional cases of incomplete vivipary displayed outside the mangrove-swamp represent a lost habit belonging to a primeval period when the climatic conditions were uniform over most of the earth, an age almost of eternal gloom, when the air was ever saturated with aqueous vapour, and when the sun’s rays were screened off by a dense cloud-covering that enveloped the globe, an age of which the existing mangrove swamps alone afford an imperfect indication. Yet even now we can say with Schimper that “dense and frequently repeated cloudiness apparently represents the most essential climatic condition for the occurrence of mangrove in the tropics” (Plant Geography, p. 409).
But, to return to the subject immediately under consideration, if my view is correct we ought to find indications of the lost habit in the anomalous structure of the seeds of some inland plants; and, indeed, it is shown in Note 50 that this view can be taken of the singular structure of the seeds of the Myrtaceous genera, Barringtonia and Careya, and of the genera of some other orders, and can be extended by implication to several other plants possessing similar seed-structures.
With regard to the subject generally, it may be remarked that although normal vivipary is mainly restricted to the plants of a mangrove swamp, by no means all mangrove plants are typically viviparous. This habit in its most complex form is exhibited as a rule by plants with firm, somewhat fleshy, usually one-seeded, indehiscent fruits, such as we find with Rhizophora and Bruguiera; but plants with follicular fruits, such as occur with Ægiceras, may also display it in a fashion nearly as complex. Generally speaking, however, plants with hard, dry fruits, such as are owned by Excæcaria, Heritiera, and Lumnitzera, are non-viviparous, though to all appearances quite at home in a mangrove-swamp. Others again, like Carapa, Laguncularia, and Nipa, whilst displaying vivipary in a varying degree, in some cases as a general rule, in others only occasionally, exhibit no special structures connected with it. This point is well brought out by Schimper in his work on the Indo-Malayan strand-flora (p. 43), and no further mention need be made of it here.
The structures connected with vivipary vary greatly in their degree of specialisation. At the one end of the scale we have highly complex structures, such as are described in the preceding chapter. At the other end we have those cases of occasional germination on the parent plant where there is seemingly no special structure of any sort. That the complex arrangements concerned with the vivipary of Rhizophora, Bruguiera, Ægiceras, and Avicennia are adaptations is argued by Haberlandt and Schimper, both of whom devoted much attention to the study of these plants. This is seemingly indicated by the circumstance that complex structures concerned with vivipary are found in plants so divergent in their characters (the four genera above-named representing three orders, Rhizophoreæ, Myrsinaceæ, and Verbenaceæ) that they only possess their stations in common. It does not, however, follow that all mangroves that exhibit a complex form of vivipary are of the same antiquity. I should be inclined to regard those of the Rhizophoreæ as the more primitive types, whilst it is possible that plants of other orders, though ancient denizens of a mangrove-swamp, may be more recent intruders into the mangrove-formation after the differentiation of a dry-land flora.
Of particular interest in this connection are the cases of abnormal vivipary, or of “precocious germination,” that have been recorded from time to time respecting a number of plants not denizens of a mangrove swamp, none of which would appear, according to Schimper’s views, to present anything of the nature of an adaptation. Goebel mentions a number of instances, such as that of wheat-grains germinating on the stalk in a wet summer, and that of Dryobalanops camphora, the Borneo camphor-tree, when during a prolonged wet season in Java the seed germinates in the fruit on the parent tree. Amongst other examples he cites the Cacti, Epilobium, Agrostemma, and Juncus, the last case coming also under my observation in a wet season in England. One may here notice the instance of Dracæna, of which Mr. Hemsley, in April, 1902, exhibited at a meeting of the Linnean Society of London a specimen showing the seeds germinating in the berries on the plant.
Several cases of this kind came under my notice in Fiji. Pulpy fruits rather favour the precocious germination of seeds. Thus I sometimes found the seeds germinating in the Mandarin orange and in the Papaw fruit (Papaya) shortly after they had been gathered. But more interesting examples were displayed in those instances where the seed was found germinating on the plant. When the Convolvulaceæ grew in wet situations, as on the borders of a mangrove swamp, the seeds were sometimes observed germinating in the capsule. This came under my notice with Ipomœa glaberrima (Boj.) and with I. peltata, more particularly in wet weather. With some other plants, like Hibiscus diversifolius, that grow in wet places, this at times occurs. A species of Croton, employed as a support for the Vanilla plants in a plantation near Suva, displayed seeds germinating on the plant. I was informed that the seeds of the common cultivated Luffa (L. cylindrica) growing in a garden on Vanua Levu sometimes germinated in the fruit still attached to the parent. It is possible that the seeds of the parasitical genus, Myrmecodia, may occasionally germinate on the plant, since I found them germinating inside some of the small berries that had been lying forgotten within a newspaper for a fortnight.
Perhaps the most curious case of abnormal vivipary observed by me in Fiji was that concerned with the Coco-nut palm. Though not known to many residents in the island, this habit was described to me by Mr. Matthew Simpson, a planter on Vanua Levu, who told me that he had noticed nuts germinating on the tree in unusually dry seasons. Coco-nut palms displaying the nuts germinating on the tree came under my observation near Bale-bale, Savu-Savu Bay. In these cases the mature fruit, instead of falling, remains attached and dries on the stalk. In one case the seedling was about eighteen inches high. This seems to be what takes place normally according to Blume with Nipa fruticans, the swamp palm of Indo-Malaya. Goebel quotes this author to the effect that the fruits are not separated from the head before germination is so far advanced that sea-water can no longer injure the seedling. The fruits, we are told, may remain for years attached in a state of incomplete germination.
Summary
The scale of germinative capacity, that begins with the seedling hanging from the branches of a mangrove like Rhizophora and ends with the detached immature seeds of many inland plants that only germinate after lying for some time in the soil, is regarded as supplying a record of the various epochs in the history of vivipary throughout the plant-world. In the occasional cases of incomplete vivipary occurring among inland plants and in the singular structure presented by the seeds of certain genera of the Myrtaceæ and other orders we perceive indications of a lost viviparous habit belonging to a primeval period when vivipary was the exception and not the rule, an age when the same climatic conditions prevailed over much of the globe. At such a period the sun’s rays were screened off by a dense cloud-covering that enveloped the earth, and the atmosphere was ever charged with moisture. With the differentiation of climate that has marked the emergence of the continents during the secular drying of the earth, the viviparous habit has been alone retained within the confines of the mangrove-swamp, where the conditions once almost universal now survive; and as an adaptation to the differentiation of climate and to the resulting seasonal variation the rest-period of the seed has been developed.
XXXII. THE WEST COAST OF SOUTH AMERICA
The littoral floras of the West Coast of South America.—The Convolvulus soldanella zone of Southern Chile.—The plantless or desert zone of Northern Chile.—The Sesuvium zone of Peru.—The Mangrove zone of Ecuador and Colombia.—The two varieties of Rhizophora mangle, the “mangle chico” and the “mangle grande.”—The floating vegetable drift of the Guayaquil River.—The Humboldt current and the climate of the West Coast of South America.—The advance northward of the arid climatic conditions of the Peruvian sea-border.—The retreat of the mangroves.—Evidence of ancient coral reefs on the coast of Peru.—The shore plants and stranded seed-drift of the Panama Isthmus.—Summary.
MY acquaintance with the strand-flora of the west coast of South America began at Corral, the port of Valdivia, in Southern Chile in lat. 40° S., and terminated at the mouth of the Guayaquil River, in Ecuador, about 2° south of the equator. During the period December 23, 1903, to March 17, 1904, I examined the coast plants at sixteen localities in this region, which covers 38 degrees of latitude and thus measures about 2,300 miles. Travelling in a steamer to Callao that was trading on the coast I had opportunities of staying for periods ranging from half a day to a couple of days at a considerable number of places; and a week spent at Valparaiso gave me a good opportunity of examining the beaches north and south of it. At Lima I spent some weeks, and from that centre examined the shore-plants at Callao, Ancon, and Chancay to the northward. North of this I had not the same opportunities, until we passed the Peruvian and Ecuadorian boundary; but from a visit to the shore at Paita, from the general look of the country in places as we coasted along, and from information derived from other sources, I was able to obtain a fair general idea of the prevailing character of the beach plants. After my previous experience to the southward, one could fairly gauge the character of the beach-flora from the appearance of the land behind. In the Gulf of Guayaquil and in the vicinity of the city of that name I spent about three weeks in the investigation of the coast flora.
If it were not for the interposition of the great rainless deserts of Northern Chile and for the scantily vegetated, scantily watered and semi-sterile condition of almost the whole coast of Peru, the botanist would be presented with a splendid opportunity of studying the distribution of shore-plants along a meridian stretching through some fifty degrees of latitude from Patagonia to Ecuador. As it is, drought and sterility in one form and another reign over about half of this great stretch of continental coast. This is reflected in the beach-flora; and though the observer will often have his interest attracted by the wonderful climatic anomalies arising from the presence on the coast of the cold Humboldt current, to which the sea-border of North Chile owes its desolation and the coast of Peru its semi-sterility, yet for a long time he will feel as if Nature had hardly dealt fairly with him.
Along the sea-border corresponding to the deserts of North Chile there would seem to be practically no plants growing on the beaches, except here and there where some stray plant from the saline districts inland intrudes on the coast. Along the whole sea-border of Peru from Arica north to Tumbez on the borders of Ecuador, the coast-districts, though more or less rainless, receive the benefit of the drizzly garuas and sea-fogs, and the sterility of the land immediately backing the beaches is much less pronounced than with the sea-border corresponding to the deserts of Northern Chile. This difference shows itself in a peculiar type of littoral vegetation, a strand-flora that is very scanty but one where on the beaches Sesuvium prevails. North of Tumbez the mangrove-formation predominates along the sea-borders of Ecuador and Colombia to Panama, excepting on a stretch of sterile coast extending north from the Gulf of Guayaquil to the equator.
Though in one sense the botanical observer will be disappointed with the littoral floras of the west coast of South America, in another sense when he remarks the manner in which the coast-vegetation reflects the abrupt changes in the prevailing climatic conditions he will be fascinated by the interesting problems presented to him. We are accustomed to connect a tropical coast with mangroves, coral-reefs, and beaches of calcareous sand supporting a luxuriant littoral flora. Climatic conditions banish all these from the tropical west coast of South America until within four degrees of the equator, and then with startling suddenness the dominion of the mangrove begins, the neighbouring hills commence to be clothed with tropical jungle, and the climate is completely changed. Mr. John Ball, who sailed along this coast about twenty years ago, referring to this remarkable phenomenon on the borders of Peru and Ecuador, remarks that no such abrupt and complete change both in climate and vegetation is known elsewhere in the world, and he adds that few parts of the American coast better deserve careful examination (Naturalist in South America). This subject has since been discussed at length by Dr. Wolff in his “Geografia y Geologica del Ecuador,” and by Baron von Eggers in a paper to be subsequently quoted, two very competent observers, but the latter considers that the subject still requires a systematic investigation, and suggests that an observing station should be established on this coast by the combined meteorological societies of Europe. A sojourn of more than a week in the swamps at Puerto Bolivar, a few miles from Tumbez, enables me to appreciate the nature of the problem, and to throw a little light on the line of investigation required.
But to return to the general subject of the littoral floras of the west coast of South America, I may say that beginning with the island of Chiloe in lat. 42° S., this coast may be divided into four zones.
(1) The Convolvulus soldanella zone of Southern Chile, which extends as far north as Coquimbo about 30° S. lat.
(2) The Plantless or Desert zone stretching north to the vicinity of Arica in lat. 18° 30ʹ, and corresponding to the coast of Northern Chile.
(3) The Sesuvium zone, extending north from Arica to the 4th parallel of south latitude in the vicinity of Tumbez, a sea-border of semi-sterility that comprises the entire coast of Peru.
(4) The Mangrove zone, stretching from Tumbez, on the frontiers of Ecuador, to the equator and on to Central America, but interrupted at first by a strip of sterility on the coast extending from the Gulf of Guayaquil to the borders of Colombia, or, strictly speaking, to the equator.
THE CONVOLVULUS SOLDANELLA ZONE (SOUTHERN CHILE).
This zone, which answers to the coast of Southern Chile, from Chiloe as far north as Coquimbo, corresponds to watered and vegetated inland regions, in which, however, the amount of rain and the degree of fertility decreases from south to north, that is to say, as we approach the desert regions. Here we find none of the dry beaches that prevail for twenty-five degrees of latitude north of Coquimbo. When we scoop with our hands to a depth of three or four inches in the sand we find it relatively cool and more or less moist, as in an English beach. In a hot summer’s day on a Valparaiso beach we should find that the temperature of the sand at the surface (half-inch deep) was about 112° F., and at a depth of four inches about 80°. This would be above the average for the zone, which would be probably near the typical summer-temperature of an English beach, namely, 102° at the surface and 77° four inches down. This subject of beach temperature is discussed in Note 70.
Plants typical of the beaches of this zone, and evidently occurring over the length of it, are Convolvulus soldanella, Nolana (paradoxa?); Polygonum maritimum, Salsola Kali, and Selliera radicans. Nolana is a Chilian and Peruvian genus. This beach plant, which is especially abundant on the beaches near Coronel and at Bahia San Vincente, has the creeping habit of its associate, the Convolvulus. However, it possesses seeds, or rather seedvessels, of more limited buoyancy; and it is shown in Note 71 that prolonged drying is needed for effective dispersal by currents over great distances. This beach species of Nolana has narrowly escaped being a widely-spread littoral plant; whereas it is now restricted to the Chilian beach flora. Selliera radicans, a little creeping Lobeliaceous plant, growing under the shade of tall clumps of Juncus at the edge of the beach or in wet places where springs ooze out in the sand, is a very interesting species that occurs also on the other side of the Pacific in Australasia. Of the mode of dispersal of its small seeds I know nothing, as the fruits were not ripe at the time of my visit; but I would suggest that some resident botanist should investigate this important point. I found it at Corral and at Coquimbo; and Gay speaks of it as growing on wet coast places from Chiloe to Coquimbo, a range of 12-1/2° of latitude.
It is probable that all the shore-plants of this zone extend south to Chiloe in latitude 42° S.; and it is likely that some of them reach towards the Straits of Magellan. I did not find any of them within the Straits on the beaches in the vicinity of Punta Arenas, where, however, I noticed the three plants recorded by Ball, namely, Armeria maritima, var. andina; Senecio candidans, also found in the Falkland Islands; and Plantago maritima; besides a Chenopodiaceous plant not in fruit. The Plantago has no capacity for dispersal by currents, and probably none of the other plants are thus dispersed. I formed the opinion when in the Straits that the beach plants on the Pacific and Atlantic coasts of Patagonia could have but little communication by the currents, and that they are in this respect quite cut off from each other. A botanist who investigates the strand-flora of Patagonia and Tierra del Fuego in connection with the littoral plants of the opposite coasts ought, if he has not already done so, to obtain some very interesting results from the standpoint of plant-dispersal.
The northern limit of the plants of this zone near Coquimbo, in lat. 30° S., is not determined by the change in climatic conditions that goes normally with decrease in latitude, but by the vicinity of the great deserts of Northern Chile, the aridity extending to the beaches.
Amongst the other plants occurring generally in the Convolvulus soldanella zone of Southern Chile, species of Salicornia and Samolus are to be observed in wet places. On the beaches near Valparaiso and in the vicinity of Talcahuano there thrives a species of Franseria, a Composite plant possessing prickly fruits well suited for conveyance in bird’s plumage, but not adapted, as shown in Note 71, for dispersal by currents. Mesembryanthemum is a typical beach-plant at Coquimbo, and an intruder from the adjoining hill-slopes at Valparaiso. Raphanus, seemingly R. maritimus, occurs in places, but apparently only as an intruder from the cultivated districts behind the beaches. One or two species of Euphorbia are not uncommon. A few small trees or bushes of Acacia farnesiana grow typically on the beach at Coronel and in neighbouring sandy tracts at Talcahuano, though the plant, as Gay observes, has been introduced. Sophora tetraptera, found also in New Zealand, and one of the most interesting plants of the Antarctic flora, thrives as a small tree on the hill slopes overlooking the harbour of Corral, becoming bushy where in places it intrudes on the beaches, and fruiting there as freely as on the slopes above. It was by testing the buoyancy of the seeds of this plant that I was led to the discovery of its mode of dispersal by the currents (I am indebted to Mr. Holland for the specific determination of the fruits sent by me to the Kew Museum). Other shore-plants, of course, occur in this zone; but I have gone far enough to illustrate the subject. Of the numerous occasional intruders from the neighbouring inland districts, frequently Compositæ, I say nothing. The results of my observations on the floating power of the seeds and seed-vessels of some of the shore-plants of this zone are given in Note 71.
Stranded seeds and fruits that belong to the proper beach-drift are not easily found on the beaches of Southern Chile, as they are often buried in rubbish. Those most characteristic are seeds of Convolvulus soldanella and drupes of Nolana (paradoxa?), both typical beach-plants of the zone. Portions of Salsola Kali bearing mature fruits, as described in Note 17, are also frequent. Seeds of Sophora tetraptera were found on the beach of Bahia San Vincente, whither they must have been brought by the Humboldt Current from the south, as I did not observe the tree in the vicinity. On this beach, as well as at Valparaiso, the prickly fruits of Franseria were abundant in the drift, doubtless derived from the plants growing on the same beaches. In addition we get as frequent components of the beach-drift materials that mark the white man’s presence over much of the globe. Corks are widely distributed over the beaches of the world; but on no coast have I found them more numerous than on the Valparaiso beaches. Here we find Medicago fruits, the empty stones of the cherry, the plum, and the peach, empty filberts and other materials, all of which I have gathered on the shores of the Straits of Messina and on English beaches. Amongst this medley we find also Casuarina cones and fruits of Eucalyptus. Then we find special indications of the New World in the pea-nut (Arachis hypogæa) and in the abundant seeds of a huge pumpkin (Cucurbita), which is a favourite food with the Chilian indigenes. These seeds are cited as an example of futile buoyancy in Chapter XIII.
THE PLANTLESS OR DESERT ZONE (NORTHERN CHILE).
This zone of the coast, which stretches north for some 700 miles from Coquimbo to near Arica (30°-18°30ʹ S. lat.), corresponds to the great desert region of North Chile. On the beaches of Antofagasta, Tocopilla, and Iquique, which are situated in the midst of this zone, I found no plants. This rainless sea border of barren mountains, presenting to the eye of the traveller from the deck of a passing steamer nothing but rock and sand, must be one of the most desolate coasts on our globe. It is therefore not a matter for surprise that the beaches are of dry loose sand in which the hand fails to find on scooping below the surface that refreshing coolness which is the character of beaches in all latitudes where the land is vegetated and a subsoil drainage seaward exists. Under ordinary conditions the sensation of moisture in the sand a few inches down is not produced by the mere proximity of the sea. On the Antofagasta and Iquique beaches the temperature in the heat of the day of the surface half inch ranged from 120° to 130° F., whilst four inches down it was 95° to 100°, and no moisture was found by scooping five or six inches down. On the Taltal beach, which lies towards the southern end of the desert region, I noticed, besides a few plants of Suæda fruticosa, two other species of the orders Santalaceæ and Nolanaceæ, evidently intruders from the inland regions. Where the zone of extreme aridity terminates at the north between Pisagua and Arica a few bushes are to be seen on the hill-slopes behind the beaches.
Very little seed-drift came under my notice on the beaches of the desert zone. Here and there I found a few Medicago pods and some seeds of the large pumpkin above noticed, but that was all. This is due as a rule to the seed-drift being masked by an enormous amount of rubbish, mostly brought from the south by the Humboldt Current. My walk for five miles along the beaches immediately north of Antofagasta gave me an experience in the way of stranded drift such as I have never met with on the beaches of any other region. All the dead bodies of the Chilian coast to the southward seem to have been stranded in the bend of Moreno Bay, on the shore of which Antofagasta lies; and the air was tainted with decaying flesh, the past being mixed up with the present in a most unrefreshing fashion. Besides carcases of sea-lions, six feet in length, sharks, dog-fish, and fish of many sorts, some of them dried up, others in a state of putrefaction, there were dead penguins, dead pelicans, dead sea-birds of other kinds, the bodies of horses, cattle, dogs, &c., all preyed upon by the numerous vultures and skuas, and in some localities by hungry-looking dogs of large size that took no notice of me as they slunk along. The past was represented by great quantities of bones that lay bleaching on the sand, with here and there a vertebra of a whale, making in all quite a varied osteological collection. But this was not all. Carcases of all sorts were drifting towards the beach. Here a vulture, there a skua, there again a dog stood just beyond the tide-wash looking keenly seaward; and by following the direction of their gaze one could see that each had marked down a carcase slowly drifting in. Now and then they would make a dash, scarcely waiting for the new arrival to be washed up by the waves. But there was no competition, since there was enough for all.
Under such conditions my investigation into the seed-drift was out of the question; but I saw what would be considered by some as more interesting, namely, the dead of many latitudes piled up on the beach by the Humboldt Current.
THE SESUVIUM ZONE (THE PERUVIAN COAST).
This zone, which comprises the whole Peruvian sea-border from Arica in 18°30ʹ S. to the vicinity of Tumbez in about 3°30ʹ S., usually possesses in its scanty littoral flora one or two species of Sesuvium, and in some places Sesuvium alone occurs on the beach. The beaches here do not line a region of almost complete aridity, as in the coast corresponding to the great desert region of North Chile. Though here also scarcely any rain falls, the sea-border receives the benefit of the “garuas” or drizzling sea-fogs; whilst the region immediately behind the coast may either be desert or semi-sterile during much of the year, or may be scantily vegetated, or, as along the river-valleys, may display a vegetation more fitting to the latitude. The general aspect, however, of the coast of Peru is one of aridity; but there are probably few beaches where a certain amount of subsoil drainage from the land sea-ward does not exist. This is well exhibited at Ancon, north of Callao, where in the most unlikely situations water is reached by digging wells; but in spite of this the Sesuvium alone grows on the beach. The beaches examined by me in the heat of the day in February, as at Mollendo and Ancon, had much the same surface-temperature noticed in the preceding month on the beaches of North Chile, namely, 120° to 130° F., and in one place 135°; whilst at a depth of four inches the sand was rather cooler, and instead of being between 95° and 100°, as on the Antofagasta and Iquique beaches, it was here usually only about 90°. But it was only occasionally that the sand felt at all moist at a depth of five or six inches; and in this zone, therefore, only a few shore plants of a peculiar type could be expected to find a station on the beaches, excepting, of course, those localities where low marshy districts or lagoons lie behind the beach.
The beach plants of the coast of Peru as observed by me though usually scanty, presented two types according to the character of the district bordering the beach. I make no mention here of those local plants, often belonging to the Compositæ, that as at Callao and Arica descend the valleys to the beaches, or to those numerous introduced plants that accompany cultivation, such as we find at Arica. In those coast localities, as at Arica, Callao, and Chancay, where salt-water pools or brackish lagoons lie behind the beach, or where a stream or a river empties into the sea, Sesuvium portulacastrum, Heliotropium curassavicum, and a Salicornia are to be generally noticed, and, as at Callao, Batis maritima may also abound. On the Chancay coast, about 30 miles north of Callao, there lies inside the shingle-beach a large shallow lagoon of brackish water (spec. gr. 1·012) with extensive muddy marginal flats, the temperature of the water at the edge being at mid-day on Feb. 3rd, 90° F. In the water flourished Ruppia maritima, which was also exposed in dead, dry, matted masses on the bordering mud-flats. On these mud-flats grew Sesuvium portulacastrum, which near the water’s edge was associated with a small species of Salicornia, whilst further away from the water it was accompanied by Heliotropium curassavicum.
But the most typical beach-flora of the Peruvian coast is such as we find on the dry beaches skirting the base of sand-covered or barren hill-slopes such as occur at Mollendo, Ancon, and Paita. As at Ancon, sand-covered hills and plains may extend miles inland, displaying here and there lines of shifting sand-mounds or “medanos.” On such beaches we may often find only a solitary plant, a species of Sesuvium which seems to differ only in its larger flowers, its much larger leaves (2 inches long), and its stout stems, of the thickness of the little finger, from the ordinary Sesuvium portulacastrum. This seems to be the only plant that can make its home on such beaches. At Mollendo, where there are signs of desiccated pools behind the beach which are occasionally filled with sea-water, the vegetation was of an intermediate character and more abundant; and here grew Sesuvium portulacastrum, a tall Salicornia, and Suæda fruticosa; whilst the commonest plant was a prostrate Nolanaceous species with a handsome purplish flower.
Excepting with the fruits of Batis maritima, and perhaps the buoyant joints of Salicornia, scarcely any of the prevailing shore-plants of the coast of Peru possess a capacity for dispersal by currents. In this zone I rarely found any seed-drift on the beaches. Much rubbish, such as roots of bamboos, however, may be brought down by the rivers; and where the Humboldt Current strikes a bend in the coast we get a repetition, on a smaller scale, of the scenes on the Antofagasta beaches. Ancon Bay, for instance, receives much of the floating offal of the south.
THE MANGROVE ZONE (THE COASTS OF ECUADOR AND COLOMBIA)
We come now to the mangrove zone which comprises, with the remarkable exception of a long stretch of arid sea-border to the north of the Gulf of Guayaquil, the whole remaining western sea-border of South America, namely, the Ecuadorian and Colombian coasts. My own acquaintance with this region is limited to the estuary of the Guayas or the Guayaquil River and to the southern shore of the Gulf of Guayaquil; but I am able to avail myself of the researches of Baron von Eggers, which cover the entire Ecuadorian coast; and with Ecuador, therefore, I will bring this brief sketch of the littoral flora of one side of a large continent to a conclusion.
The Ecuadorian coast, lying, as Baron von Eggers observes, between the rainless and desert coasts of Peru and the “ewig grüne” coasts of Colombia, may be regarded as a transition-area presenting very varied and complicated conditions. With the cause of the remarkable contrasts exhibited by the strand-flora, not only on the coast of Ecuador, but along the whole west coast of South America through some forty-five degrees of latitude from Patagonia to Colombia, I will presently deal. Here it may be remarked in passing that the Humboldt Current has played the determining part in producing the abnormal climatic conditions to which these remarkable contrasts in the strand-flora of this coast of the continent are mainly due.
The mangrove zone, marking a more or less abrupt transition from a region of drought and semi-sterility to one of humidity and rank tropical vegetation, begins about lat. 3° 30ʹ S., that is, in the vicinity of Tumbez, or perhaps nearer the boundary-line between Ecuador and Peru in lat. 3° 20ʹ (see Note 72). Occupying the southern shore of the Gulf of Guayaquil it extends up the Guayas estuary to Guayaquil and rather beyond. But when we follow the coast of Ecuador northward from the island of Puna towards Santa Elena Point, we come upon one of the most remarkable phenomena presented on the west coast of South America. The dry region begins again and the mangroves disappear; and these conditions continue through about 2-1/2 degrees of latitude until we reach the equator, when the mangrove zone soon recommences, and, as I infer, continues northward without a break to the coast of Central America.
Dealing first with the mangrove districts of the south side of the Gulf of Guayaquil and of the Guayas or Guayaquil estuary, we may observe that probably in few localities of the globe have the forces of nature worked more in unison to produce the conditions favouring the growth of the mangrove. The reason why this particular locality has been thus favoured will be discussed later on in this chapter. I may here observe that Baron von Eggers was so struck with the exceptional features of the mangrove-growth in this region that he was inclined to look for the American centre of the genus Rhizophora, the prevailing mangrove, in the estuary of the Guayas River.
I will not enter into a detailed description of the mangrove-formation of this coast, which has indeed been given by the German botanist; but I will merely refer to the leading features such as they presented themselves to me. In the first place, reference will be made to the sea-border of the province of Eloro, where I spent nine or ten days, making Puerto Bolivar, the port of Machala, my headquarters—a locality about thirty miles east of Tumbez. Except in the Guayas estuary I have never seen such a magnificent growth of mangrove.
By following the line of light railway that runs about six kilometres inland from Puerto Bolivar to Machala, the capital of the province, we obtain a good section of the mangrove-belt, which may be thus described. The mangrove-swamp proper extends about three kilometres inland. Whilst the small variety of Rhizophora mangle (mangle chico) immediately fronts the sea, Laguncularia grows on the islets close to the seaward margin of the swamp. When we enter one of the numerous broad creeks that intersect the border of the mangrove-belt we soon find ourselves in the true mangrove forest, where prevail tall trees of Rhizophora mangle (mangle grande) that rise to a height of 70 or 80 feet or more. Gloomy as the depths of the swamp are, they acquire quite a funereal aspect, the branches of the trees being draped with pendent Tillandsias. These long, hair-like, tangled growths hang vertically from the branches of the trees and may be 20 or 30 feet in length. In the rear of the zone of tall mangroves we come upon a more open district of the swamp. The forest proper gives place to a tract occupied by small trees of Rhizophora, Laguncularia, and Avicennia, with here and there whole acres occupied only by the shrubby Salicornia peruviana which attains the height of a man.
[To face page 484.
(The main stream of the Humboldt current, as indicated by the arrows, turns off to the north-west at Cape Blanco; whilst a small branch crosses the mouth of the Gulf of Guayaquil and flows along the Ecuador coast north of Santa Elena Point.)
Here terminates the mangrove-swamp proper, and about three kilometres from the sea it passes gradually into a region of extensive bare mud-flats which are penetrated by salt-water creeks, two or three yards across and a foot or two in depth, that are bordered by low and shrubby Avicennias, the Salicornia bushes above noted, and dwarfed trees of Rhizophora mangle only four or five feet high. These flats, which are evidently only overflowed by the sea at the higher spring tides, were at the time of my visits much sun-cracked and in some parts incrusted with salt; but the mud was rather soft, and in places Sesuvium portulacastrum and Batis maritima flourished in quantity on it. These mud-flats, about two kilometres across, pass by degrees into the low-lying level district known as the Machala plains, on which the capital of the province is built. Here the soil is dryish, and, notwithstanding that it displays on its surface when exposed to the sun a white saline efflorescence, a dry jungle type of vegetation of the xerophilous character here thrives. I noticed casually the Algaroba (Prosopis), a yellow-flowered Cordia, cacti of the Opuntia and Cereus kinds, besides several small trees and shrubs often thorny.
These Machala plains, on account of the fine saline incrustation above mentioned, are of much interest, since at a distance of six kilometres from the coast they thus display on their surface the effect of sea-water infiltration, their level above the sea being only a few feet. We have seen the three stages of this infiltration landward of sea-water: first, the mangrove-swamps daily overflowed by the tide; second, the mud-flats behind them which are only overflowed by the fortnightly spring-tides; third, the vegetated plains behind all, which are sufficiently raised to be above the reach of the tides, but which are nevertheless soaked with sea-water that displays its presence in the salt left by evaporation on the surface of the soil.
But another interesting point is here raised. At the back of the mangrove-belt, in most parts of the world, we usually find a particularly rank and luxuriant vegetation where the Scitamineæ often take a leading part; whereas on the sea-border of this part of Ecuador the mangrove-swamps pass gradually into arid saliniferous plains. With this singular fact is to be associated the circumstance that we see here in operation, some four or five miles from the coast, a process by which great quantities of sea-salts are accumulating below the surface. This may possibly be concerned with the origin of the great saline deposits of Northern Chile. However this may be, there is some reason for believing—and I understand that this is the opinion of Dr. Wolff, the historio-geographer of Ecuador—that in the course of ages the tendency will be towards an extension of the dry, sterile regions of Northern Peru into Ecuador. This subject is referred to again in a later page of this chapter.
Whilst in this neighbourhood I made the ascent for some fifteen miles of the Santa Rosa River, which opens into the sea near Puerto Bolivar. It is a tidal estuary that has no proportion in size to the small river that enters it. In its lower third we passed at first between long mangrove-islands formed almost entirely, as viewed from the boat, of the tall Rhizophora trees draped with Tillandsias, and presenting really a magnificent spectacle. In the middle third we were penetrating into the rear of the mangrove-belt. The giant swamp-fern (Chrysodium aureum) abounded, and here and there we passed by a patch entirely held by the large shrubs of Salicornia peruviana. The tall Rhizophora trees were replaced by the short variety, the “mangle chico,” which ceased altogether about ten miles from the mouth of the estuary, but probably only about five miles from the nearest part of the coast. The water at the place where the Rhizophora trees ceased was evidently quite fresh during nine out of the twelve hours, being only salt in the latter part of the rising tide. Above the mangroves, in the upper third of the ascent, Hibiscus tiliaceus, with Chrysodium aureum, flourished on the banks. The shallows at the margins were occupied by a considerable variety of semi-aquatic and other plants, such as Pontederia (two species); one of the Alismaceæ, with the flower and fruit of Sagittaria and the leaves of Alisma; Typha, Polygonum, and an Amaryllid like Crinum. Plants of Pistia and Pontederia floated in the stream.
I have said enough to give a general idea of the composition of the mangrove-belt of the Ecuador littoral, and will refer but briefly to the mangroves and other river-side plants in the neighbourhood of the city of Guayaquil, some forty miles up the Guayas estuary. As I have remarked in Note 38, the water of the river off the city is usually quite fresh except at high water; but the sea has much freer access to the channels at the back of Guayaquil, where at high water the density was 1·014. In these channels are displayed the typical mangrove formation, trees of Rhizophora mangle bordering the water, whilst behind they are mingled with Avicennia tomentosa and Laguncularia. On the banks of the main river, where they are overflowed at high water, Anona paludosa was the most frequent tree, being associated with the Rhizophora, Hibiscus tiliaceus, and other trees. Above the city, Polygonum glabrum was growing in dense masses at the river’s edge, whilst Pontederia and Pistia flourished on the low muddy banks and floated in quantities in the river.
Before quitting the subject of the mangrove-formation of Ecuador, I will refer shortly to the two varieties of Rhizophora mangle that here occur. Baron von Eggers received the impression that the common type of this species, a low tree bordering the coast, did not exist in Ecuador, such a type as he says is characteristic of the West Indies and of Central America, and, I may add, also of Fiji. The species he regards as acquiring a new facies in Ecuador, where it exists as tall forest-trees, branchless for half their height, and exhibiting other divergent characters. However, I found that the common type of the species occurs normally on the coast in the vicinity of Puerto Bolivar, thirty miles east of Tumbez, a district above described.
There are two distinct forms of Rhizophora mangle exhibited in the mangrove-belt of the coasts around Puerto Bolivar. One of them, which the indigenes name “mangle chico,” is a small tree, 10 to 15 feet high, with useless timber, that immediately borders the sea, and, in fact, largely forms the margin of the swamp, not only on its seaward side, but also on the land side, where it passes into drier ground. The other, the “mangle grande,” a tall tree reaching to 60 or 80 and sometimes perhaps to 100 feet in height, composes the interior, and indeed the bulk, of the mangrove-belt, and possesses a hard and durable timber much employed in the district.
Distinct as these two types are, it is not difficult to find intermediate forms, and, in truth, in some localities they prevail. But the interesting point is that this peculiar Ecuadorian type of the species, a type that attracted the attention of the eminent German botanist, comes near the “Selala,” the mysterious seedless Rhizophora of the Fijian swamps—a subject fully discussed in Chapter XXX., where I have compared the Fijian and Ecuadorian Rhizophoras. Both the “Selala” of Fiji and the “mangle grande” of Ecuador are intermediate between the American Rhizophora mangle and the Asiatic R. mucronata, resembling the last in their inflorescence, but in other points approaching the American species. The “Selala,” however, comes nearer to the Asiatic tree, whilst the “mangle grande” comes nearer to the American tree. Unlike the Fijian tree, that of Ecuador is not sterile, but matures its fruit; and it displays no evidence of the vegetative reproduction so characteristic of the “Selala.”
Sandy beaches are not common on the mangrove-fronted shores of the south side of the Gulf of Guayaquil. However, on the seaward side of the long low mangrove island of Jambeli, on which the lighthouse is placed off Puerto Bolivar, there is a long stretch of beach of whitish, mainly non-calcareous, sand. The Coco palms behind the beach give the coast quite the aspect of a Pacific island strand. Ipomœa pes capræ flourishes on the sand nearest to the sea; and immediately behind, the beach is more or less occupied by a Cyperus 2 to 3 feet high, and by Canavalia obtusifolia. Further back grows a small Acacia tree, and behind it the yellow-flowered Cordia tree of the district; and in the rear of all lie extensive mud-flats, partly occupied by stunted bushes of Avicennia tomentosa and by Sesuvium portulacastrum, which in their turn pass into the mangrove-swamps.
On account of the enormous amount of drift of all descriptions that is carried to the sea by the Guayas or Guayaquil River, floating vegetable materials are abundant in the Gulf of Guayaquil, and are thrown up in quantity on the coasts of Ecuador. One of the most interesting spectacles at Guayaquil is presented by the floating river-drift. Huge tree-trunks and floating islets, the last-named ranging from 3 or 4 to 30 or 40 feet or more across, were, at the time of my visit in February, being carried to and fro unceasingly in front of the city by the tide, gradually making their way down the river, and ultimately reaching the open waters of the gulf. Floating plants of Pistia were in abundance; and their fate when they reached the sea must have been tragical. The islets were exceedingly interesting; they were evidently formed of materials lifted up bodily from the shallows at the margin of the river, and then carried off in the stream. They were mainly composed of two species of Pontederia and of Polygonum glabrum in the position of growth; the first often in flower. Pistia and a variety of smaller plants nestled among them, such as Salvinia, portions of Azolla, Lemna, &c.; and in one islet I noticed, oddly enough, the growing rhizome of a sensitive plant (Mimosa pudica). A great quantity of floating seeds collect amongst the roots and stems of the plants composing the islets, and here I obtained much of the smaller seed-drift.
Most frequent in the floating drift of the river at Guayaquil were the seeds of Anona paludosa, often in a germinating condition. The seeds are liberated by the decay of the floating fruit, which was also common in the drift. Amongst the larger materials were the seeds of Entada scandens and of Mucuna; the empty seeds of the vegetable-ivory palm (Phytelephas macrocarpa), the sound seed possessing no floating power; the “stones” of Spondias lutea, L., as identified by Mr. Holland, of the Kew Museum; the empty small nuts of several palms, including, apparently, Oreodoxa, &c. Amongst miscellaneous materials were small gourds, which are referred to in Note 47, and an occasional empty cacao fruit. Smaller seeds were also abundant, and included those of Hibiscus tiliaceus, Erythrina, Vigna, Ipomœa, and others. Carried into the river from the neighbouring mangrove-creeks, where they abound, there were floating seedlings of Rhizophora and Avicennia, fruits of Laguncularia often germinating, and the seeded joints of Salicornia peruviana.
There was of course, in addition, much that was strange in the floating drift of the Guayas River, which received its contributions not only from the river-side vegetation and the neighbouring mangrove-swamps, but also from the interior mountain ranges culminating in Chimborazo, the slopes of which are drained by its tributaries. I had several opportunities of meeting the drift of the Guayas River in the open waters of the Gulf of Guayaquil. Much of it is carried along the south side of the gulf; and I picked up at sea, ten to twenty miles from the mouth of the estuary, many of the things above enumerated, such as Erythrina and Mucuna seeds, seeds of Hibiscus tiliaceus, the empty vegetable-ivory seeds, the seedlings of Rhizophora and Avicennia, and the germinating fruits of Laguncularia and Salicornia peruviana. Much of these materials mingled with local drift is stranded on the long beach of Jambeli Island, thirty miles from the mouth of the estuary. Here, besides the seeds of Canavalia obtusifolia and Ipomœa pes capræ derived from the locality, I found the seedlings of Rhizophora and Avicennia, and the fruits of Laguncularia and Salicornia peruviana, that might have been in part derived from the adjacent swamps, as well as much of the drift of the Guayas River, such as the seeds of Anona paludosa, Entada scandens, Erythrina, and Mucuna, the small gourds, the same small palm-nuts, the empty seeds of Phytelephas, the “stones” of Spondias lutea, and much other material previously familiar to me, but nowhere a sign of the floating Pistias and of the flowering Pontederia islets of that estuary.
The Stretch of Dry Coast from the Vicinity of Puna Island to the Equator.— This remarkable piece of sea-border, covering nearly three degrees of latitude, and in its aridity and general character recalling the sterile sea-coast of Peru, is placed between the humid mangrove-fronted coast of the Guayas estuary and the similarly humid and mangrove-fronted coasts of Northern Ecuador and Colombia. The mangrove seems to be almost absent from this stretch of dry coast. Mr. F. P. Walker, of the Santa Elena Cable Station, tells me that some time ago a little mangrove-growth existed near the Point, but that it has disappeared; and Baron von Eggers implies the absence of mangroves from the whole coast. The first-named speaks of the dry character of the coast district from Santa Elena Point to within half a degree of the equator; and the last-named, in his description of the coast, mentions cacti and thorny plants as typical of the vegetation. Since this region represents a typical locality where the direct influence of the Humboldt current on the climate of almost the whole west coast of South America can be put to the proof, I will refer to its peculiar climatic conditions below in my discussion of the general question, and will here content myself with saying that on this dry portion of the coast of Ecuador we have reproduced, but in a less pronounced degree, the climatic conditions of the coast of Peru.
The Humboldt or Peruvian Current and the Climate of the West Coast of South America.—The question we will now briefly consider is one that is concerned with the determining causes of the singular distribution of coast-plants on the west coast of South America. The reader will have already seen that the matter is an affair of climate; but it is an affair of climate in which (although it affects forty or more degrees of latitude), latitude, in a general sense, scarcely counts. All the naturalists, from Humboldt onward, who have sojourned in this region of the globe have displayed a deep interest in this subject; and I suppose there can be no region of the globe where there are so many climatic anomalies as interesting to the meteorologist. Here, for instance, might be obtained materials for solving the irritating mystery of a London fog; and if the suggestion of Baron von Eggers, before alluded to, is carried out, and a station is established by the Meteorological Societies of Europe and America at some suitable locality like Santa Elena on the coast of Ecuador, we might obtain, among other results, another line of investigating the causes of the fogs of our metropolis, a subject about which Captain Carpenter has recently made an important preliminary inquiry.
I will assume that my readers are already acquainted with the nature of the problem to be discussed relating to the climate of the west coast of South America, and that they are familiar with the view generally held that the aridity of this extensive coast region, stretching from the thirtieth parallel of south latitude to the equator, arises from the loss by the trade-winds of all their moisture in the interior of the continent before reaching the western countries of Chile and Peru. Mr. Ball, in his book on South America, opposed this view, though from reasons only partially valid, since he instanced the Ecuador coast as being, contrary to the theory, a wet coast, whereas we know that a large stretch of it is arid and not unlike Peru. The parting of the ways in this discussion lies in the answer to the query, Why should the south-east trade carry so much moisture to the east side of South America, whilst the south-west winds, that are equally prevalent on the west coast of the continent, are drying winds which convert the sea-border into a desert, as in Northern Chile, or into a region of semi-sterility, as in the instance of Peru? Other things being equal, we should expect both sea-borders of the continent in these latitudes to be well watered. In the answer to the question why the south-east trade should be a wet wind and the south-west wind a dry one lies a fatal objection to the prevailing view.
When Professor Davis, in his article on North America in the Encyclopædia Britannica (vol. 25), observes in connection with the arid coast regions on the west side of the continent that the southerly flow of the winds along the Pacific coast gives them a drying quality, thus causing the extension to the coast in South California and in North Mexico of the arid regions of the interior, he seems to imply that these winds acquire their drying capacity in flowing from cooler to warmer latitudes. On this view all trade-winds should be drying winds, whereas the reverse would appear to be the case.
There is some condition, present on one coast of the South American continent and absent on the other, which determines why a southerly wind, blowing landward, is in the one case moist and in the other dry. According to my own view the winds of the arid coast regions of western North America cross the cool waters of the Californian current, and thus acquire their drying quality on striking a sea-border more highly heated than the winds themselves. On the tropical west coast of South America the winds also become drying winds by passing over the cold waters of the Peruvian or Humboldt current, where mists are in consequence of frequent occurrence; and on striking the more highly heated land-surface at the sea-border the moving air does not part with any more moisture until an altitude of some thousands of feet above the sea is reached, when the cloud-belt forms. On the mountains bordering the coast of the Antofagasta province, in January, the clouds gathered at an elevation of 4,000 to 5,000 feet. Perhaps the best way to contrast the east and west coasts of tropical South America in this respect would be to say that whilst the wind blows landward in both regions, the land is the condenser on the east side, and the sea, owing to the interposition of the cold Humboldt current, is the condenser on the west side.
During a fortnight spent in February at Ancon, about twenty miles north of Callao, I noticed that with the prevailing cool south-westerly wind the coast was clear, but it was misty at sea. On the few days when there were warm westerly and north-westerly breezes, the weather was thick at sea; and if this condition was pronounced, the whole coast was enveloped in mist; but more usually the coast-line was fairly clear except at the promontories, along the sides of which clouds blown in from the sea rolled in lines inland, not generally attaining an elevation over 300 or 400 feet, but sometimes reaching 900 or 1,000 feet, and gradually disappearing a mile or two inside the coast-line. These sea-born clouds thus vanished as they traversed the more highly heated land-surface; and the air-current continuing its inland course mounted the slopes of the adjacent mountain ranges of the Andes, some three or four miles from the coast, until at an altitude of some 5,000 or 6,000 feet condensation again occurred and the cloud-belt was formed at those cooler levels.
From the summit of a range rising to a height of about 2,500 feet to the north-west of Lima I had presented to me a splendid spectacle, on February 12th, in the formation of the coast-belt of clouds. The forenoon was clear, but about 2 p.m. the sea-born clouds began to roll inland, concealing the lower two-thirds of the island of San Lorenzo, which has an elevation of almost 1,400 feet, and completely covering up Callao and the low country bordering the sea, but extending only a mile or two from the coast-line. The dense cloud that covered Callao appeared, as I looked down upon it from my mountain-peak, like a billowy field of snow sparkling in the sun, with the summit of San Lorenzo standing out like some bare alpine summit from amidst the snows. Yet beneath that dazzling covering Callao lay all in gloom; whilst only six miles up the broad valley of the Rimac the city of Lima stood in a blaze of sunlight, its domes and towers reflecting back the light as I looked at the strange contrast it presented with the buried city of the coast. The mystery of a London fog seemed to lie unfolded at my feet, ready for the man who can read the signs aright.
That the mere presence of a cold current on a coast with the winds blowing off the land (as in the case of the Labrador current, which extends down the Atlantic coast of North America to Cape Hatteras and beyond) produces no sterilising effect on the vegetation of the sea-border of a continent is well brought out in the beautifully executed maps in Prof. Russell’s recent work on North America. The essential condition for producing sterility on the sea-border of a continent is not only that the waters of a cold current should wash its coasts, but that the regular winds should blow landward across its cool surface. These are what we find on the west coast of South America.
Not with the hope of adding anything new to our knowledge of the climatology of this region, but with the purpose of becoming personally acquainted with the problem involved, I paid considerable attention to this subject during the three months passed on the west coast of South America between Port Valdivia and Guayaquil. It was not until I had dropped my thermometer into the cool water of the Humboldt current and had watched the formation of the fogs on the sterile coast of Peru that the real nature of the problem presented itself. From the pages of a work like Tschudi’s Travels in Peru one acquires an excellent idea of the extraordinary climatic conditions of this region, and the same may be said of the narratives of Darwin and other travellers; but it is necessary to be brought into personal contact with these conditions before one can appreciate their significance.
As is well known, says Baron von Eggers, the Humboldt current explains the anomalous climate of the coast of Peru, and one may add of North Chile and Ecuador. The current, which represents the extension northwards of the west wind-drift of the Roaring Forties (see Dickson in Encycl. Brit., xxxi. 404; and Admiralty Current Charts of the Pacific), begins on the coast between the 33rd and 40th parallels of south latitude, according to the season. North of Valdivia, as we approach Valparaiso, in lat. 33°S., the effect of its presence is at once seen in the increasing dryness of the climate and in the alteration in the character of the vegetation. It has, however, been shown that the current needs the co-operation of the prevailing southerly and westerly winds as they blow landward over its cool waters. On the coast of Peru these moist winds often generate fog and mist as they cross the current. They reach the coast as drying winds, having a temperature much cooler than the lower coast regions; and the air-currents do not precipitate any moisture on the land until an elevation of 4,000 to 6,000 feet is attained where the cloud-belt is formed.
In order to establish this theory it is, however, necessary to show that when the Humboldt current leaves the coast normal conditions of humidity occur, to which the vegetation responds, and that when the current strikes the coast again the conditions of aridity reappear. In its course northward the current divides off Cape Blanco, the principal mass of its waters making towards the Galapagos Group, whilst the remainder, after crossing the Gulf of Guayaquil, flow along the coast of Ecuador between Santa Elena and the equator. Now, it is along this stretch of the Ecuador coast that the conditions of aridity reappear and that the climate of the Peruvian sea-border is in a modified form reproduced. In the interior of the Gulf of Guayaquil, on the other hand, where the sea-border is no longer subjected to the influence of the cold waters of the Humboldt current, the genius of the tropics, repressed through so many degrees of latitude, bursts its bonds, and presents us with a spectacle of littoral vegetation that, so far as mangrove-growth is concerned, is probably unrivalled on our globe.
This contrast is well shown in the mean annual temperatures on the opposite sides of the Gulf of Guayaquil. Baron von Eggers, quoting Dr. Wolff, states that whilst the mean for the year at Puna is about 75° F., and at Santa Elena about 73°, on the south side of the gulf at Balao it is several degrees warmer and is evidently not under 80°. The mean temperature for the second week of March during my sojourn at Puerto Bolivar, which is near the beginning of the mangrove region on the south side of the gulf, was 79°, the mean daily range being 74° to 83·5°. This stretch of dry coast reaching north from Puna to the equator is evidently regarded by Baron von Eggers and others who have studied the climatology of Ecuador as the critical area required to confirm the theory connecting the aridity of the west coast of South America with the Humboldt current. Here the sea for the greater part of the year has a temperature (according to the British Admiralty chart of surface-temperatures) of 70° to 75°; the mean temperature of the air is 73° to 75°; the rainy season, instead of covering a period of six months and over, as in the humid regions north and south of this coast, has a duration of only two or three months; the prevailing wind is south-west; whilst the direct influence of the cool waters of the current is shown in the general cloudiness that prevails during the last half of the year and in the drizzling mists that are frequent from June to October. Reference has already been made to the manner in which the vegetation on this dry coast of Ecuador responds to the arid conditions, as, for instance, in the absence of mangroves and in the prevailing character of the plants of the sea-border, cacti, thorny plants, and such like. For my information on this exceedingly interesting tract of coast, which is the test-ground of the Humboldt current theory, I am indebted to the papers of Baron von Eggers (see end of this volume) and to Mr. F. P. Walker, of the Central and South American Telegraph Company’s Station at Santa Elena, who very kindly communicated with me by letter. Some additional remarks are given in Note 73, and my own observations on the temperature of the Humboldt current from Antofagasta northward are summarised in Note 74.
Before quitting the Ecuador coast a word may be said relating to the prediction of Villavicencio that the climate of this sea-border will assimilate itself to that of the rainless coasts of Peru. This is, I believe, also the opinion expressed by Dr. Wolff in his Geografia y Geologia del Ecuador (Leipzig, 1892); and it is referred to by Mr. Webster in his article on Ecuador in the seventh volume of the Encyclopædia Britannica. There is a prevailing impression amongst the more observing residents that I met in the Ecuadorian province of Eloro, on the Peruvian border, that the country is drying up. A few pages back I have described how in the Machala district of this province the mangrove-belt passes landward into an arid region suggestive of the sea-border of Peru. This transition is startling to one who expects to find behind the mangrove-belt, as he would find in most parts of the world, a humid region where Nature revels in the rank luxuriance of plant-growth. This is, however, not always the case, since on the lee or dry sides of the large islands of Fiji the mangrove-belt is backed by extensive arid plains, for an explanation of which, as I have shown in Note 22, we have to appeal rather to the hygrometer than to the rain-gauge. This is true also of Ecuador; but whilst the reason is intelligible enough in Fiji, it only carries us a step farther back in the case of the Machala plains in Ecuador. These plains are continuous with similar districts across the Peruvian border where they reach the coast; and if the reader will refer again to my description of the section of the mangrove-belt and the plains in its rear from Puerto Bolivar to Machala, he will incline to the view that the desiccation of the sea-border of Ecuador is now in progress.
Evidence of a more direct nature could doubtless be supplied by those who have long resided on the coast of Ecuador, and in illustration I will give an extract from one of Mr. Walker’s letters dated May, 1904, from Santa Elena.—“The rainfall here might for the last ten years be put down at two showers per year. It is said that the last good rainy season was in 1891. The inhabitants say that formerly it always rained enough to make the grass grow every year, but during the eleven years I have been here there appears to be a marked falling off of the rainfall.”
It has been only possible to touch the fringe of this interesting question here; but from the standpoint of the study of the littoral flora of the west coast of South America it is of some importance. Immediately behind the epoch of the present marine molluscan fauna of this coast there lies an age when, as we learn from Philippi, the shells of Chile were more akin to those of the Atlantic and Mediterranean faunas than to those now found on the Chilian coasts. The transition is a sudden one; and amongst other explanations of this strange transformation Suess suggests the sealing up of a communication through the Panama isthmus by volcanic eruptions and the appearance of the Humboldt current (Das Antlitz der Erde, French edit. by Margerie, ii. 825). May it not be, my readers may ask, that the west coast of South America is still in the age of progressive sterility; and that before this age began Peru possessed a normal tropical strand-flora? It has been remarked in Chapter VIII. that the same species of mangroves occur on both the Atlantic and Pacific coasts of America, and that at all events their present distribution belongs to an age when the Gulf of Mexico was in communication with the Pacific Ocean. May we not, again, suppose that in that age the mangroves extended far south on the coast of Peru, just as they do now on the coast of Brazil?
Coral reefs are stated not to exist in tropical latitudes on the west coast of South America in our own day; but we might almost expect that at the close of the Tertiary period, and perhaps before the appearance of the Humboldt current, they existed with the mangroves on the coast of Peru. As bearing on the subject of a change of climate on that coast in times geologically not remote, I may allude to the circumstance, which is discussed more in detail in Note 75, that I found, sometimes in fair quantity, blocks of massive coral, long since dead, much pierced by boring shells, and in places undergoing a chemical change, at Arica (lat. 18° 25ʹ S.), at Callao (12° 3ʹ S.), and at Ancon (11° 45ʹ S.) on the coast of Peru.
These masses, which varied from a few inches to two or three feet in size, gave me the impression of having been torn off the bottom, in some cases in recent times, in others perhaps centuries ago, by the huge sea-waves that from time to time overwhelm this coast. At Ancon, where they were sufficiently abundant to be used for bordering the flower beds in the hotel garden, they were most numerous in the vicinity of a rocky spur of andesite that protruded from the beach between the tide levels and was more or less covered at high water. A few paces inland from the beach some of these coral masses, evidently stranded long ago, were undergoing that queer process of disintegration which everything calcareous seems to undergo on the beaches and plains of this almost rainless coast. Like the bones of the Incas lying bleaching on the neighbouring plains, like the sea-shells and bones of bird and beast cast up long ago on the beach, they were falling to powder where they lay, and the coral fragment lay often in the midst of its own débris. The blocks on the beach proper were for the most part still hard and compact, and the same may be said of those observed on the beaches of Callao and Arica.
The corals were quite different from those with which I was familiar in the reefs of the Pacific islands, and, bearing in mind the known distribution of coral reefs, I was a little dubious about them. Accordingly I sent some specimens to the British Museum, and Mr. Jeffrey Bell has kindly informed me that they seem to be decayed and much injured perforated examples of Porites. When powdered they effervesce in an acid, but the bulk of the material remains undissolved.
No more eloquent testimony could be afforded of the rainless climate than these corals crumbling on the Ancon plains when washed a few paces inland from the beach. They could be noticed in all stages of disintegration from the block surrounded by a little line of disintegrated material, representing the initial products of its own decay, to the crumbling mass, almost friable in the fingers, that was lying in the midst of its own dust and loose polyp-tubes, and finally to the little mound of débris that alone remained. Mr. Darwin, in his Journal of Researches (chap. xvi.), refers to a similar process of decay in the elevated shell-beds of San Lorenzo, off the coast of Callao. On the higher terraces a layer of saline powder, consisting of sulphates and muriates of lime and soda but with very little carbonate of lime, was the sole indication of the shell-beds. Dry climatic conditions at the sea-border evidently favour, as he observes, the early decay of exposed calcareous remains.
The Shore-plants and Stranded Seed-drift of the Panama Isthmus.
I spent two days at Panama and two days at Colon in examining the neighbouring beaches and estuaries of the Pacific and Atlantic coasts of the isthmus. On the Panama side the mangrove-belt was formed on the seaward border of “mangle chico” (the small prevailing type of Rhizophora mangle), Laguncularia, and Avicennia; whilst behind it passed into extensive swampy tracts occupied by the Swamp Fern (Chrysodium aureum), Hibiscus tiliaceus, and other plants. On the Colon or Atlantic side the mangrove-belt had precisely the same composition and presented the same species, Rhizophora and Avicennia usually forming the outposts on the reef-flat, whilst Laguncularia was abundant in the rear. In the estuary of the Rio Chagres, Rhizophora and Laguncularia were abundant near the mouth, and Chrysodium aureum and Hibiscus tiliaceus by the waterside higher up. Dr. Seemann, in his volume on the botany of the voyage of H.M.S. Herald, observes that the species of Laguncularia common on both the Atlantic and Pacific coasts of the Panama isthmus is L. racemosa. This species differs in the form of its fruit from the Ecuador tree. Laguncularia racemosa, Rhizophora mangle, and I may add Anona paludosa and Conocarpus erecta, are all plants of the mangrove-formation that occur not only on the Pacific and Atlantic coasts of America but also on the west coast of Africa. It is likely, I may add, that the “mangle grande,” the Ecuadorian type of Rhizophora mangle, exists in the Panama isthmus, since in the higher part of the estuary of the Chagres I found trees approaching it in characters.
Amongst the plants growing on the Panama beaches I noticed Canavalia obtusifolia, Hibiscus tiliaceus, and Ipomœa pes capræ, all of which occur also on the Atlantic side of the isthmus. The Manchineel (Hippomane mancinella), found also on the Atlantic side of the continent, grows on the Panama beaches. Its fruits, which look like crab-apples, lose their outer fleshy covering when drying on the sand. Not being familiar with this poisonous tree, I allowed some of the milky sap of the fruits to touch the skin, and suffered great pain for five or six hours. The fruit possesses an inner coat of air-bearing cork-like tissue; and the stone, if I may so term it, thus acquires great floating power. I kept some afloat in sea-water for five weeks, and no doubt they will float for months.
The seed-drift to be observed stranded on the beaches and floating in the estuaries on both sides of the isthmus is, generally speaking, the same—a circumstance of great importance in plant-distribution, since we can here see rivers bringing down the same seeds from the same “divide” to the shores of the Pacific and Atlantic oceans. In the case of a plant like Entada scandens, which grows in the interior, this is a matter of much interest, as it thus possesses here a centre of dispersal from which its seeds can be carried by the currents eastward to the West African coast and westward across the Pacific to Malaya and (given time) around the shores of the Indian Ocean to the East African coast. In describing the possible routes of dispersion from this centre I have described the distribution of the species.
I am indebted to Mr. Holland, of the Kew Museum, for the identification of some of the drift-seeds and fruits collected by me on the isthmus, those identified by him being followed by the letter H. On the beaches and floating in the estuaries on both sides of the isthmus I found Rhizophora seedlings; seeds of Entada scandens and Mucuna urens (medic.), H.; seedvessels of Spondias lutea (Linn.), H.; Prioria copaifera (Griseb.), H., with decayed seed; and the empty nuts, 1-1/2 to 2 inches in size, of more than one species of Astrocaryum, H. Although in the case of the two last-named genera the seedvessels were useless for dispersal, being evidently brought down from the interior by the rivers, they serve to illustrate the important principle that the rivers bring down the same seed-drift on both the Atlantic and Pacific coasts of Central America. Mr. Hemsley includes amongst the seed-drift stranded on the coast of Jamaica the seedvessels of Spondias (probably S. lutea) and of Astrocaryum (Bot. Chall. Exped., iv. 299, 304).
Those of Spondias lutea were found by me floating in the Guayaquil River and stranded on the beaches of Ecuador and of the Pacific and Atlantic coasts of the Panama isthmus. This is the Hog-plum, which in tropical America and the West Indies is both wild and cultivated. Its buoyant “stone” has a covering of cork-like air-bearing tissue. This is a remarkable case of non-adaptation in the matter of buoyancy. The seedvessels cut across contained sound seeds; and they are provided with the essential qualities of “long floaters.”
Summary.
(1) The strand-district of the west coast of South America is divided into four zones:—
(a) The Convolvulus soldanella zone of Southern Chile.
(b) The Desert or Plantless zone of Northern Chile.
(c) The Sesuvium zone of Peru.
(d) The Mangrove zone of Ecuador and Colombia.
(2) The mangroves do not extend south of Ecuador or, more strictly, south of Tumbez (3° 30ʹ S.).
(3) The absence of mangroves on the tropical coasts of Chile and Peru is attributed to the Humboldt current, which has so influenced the climate that it has converted the sea-border of North Chile into a desert and that of Peru into a region of semi-sterility.
(4) It is considered that this has been effected through the prevailing winds acquiring drying qualities on crossing the cold waters of the current in tropical latitudes.
(5) To establish this it is shown that when the Humboldt current leaves the coast at Cape Blanco mangroves thrive in the Gulf of Guayaquil, and that when it strikes the coast again near Santa Elena Point and courses along that seaboard to the equator we find the Peruvian conditions of semi-sterility reproduced.
(6) The probability that the arid climate of Peru is in our own time extending northward into Ecuador is pointed out; and from the presence of old coral blocks on the Peruvian beaches it is considered likely that when these corals throve the mangroves extended far down the coast of Peru.
(7) It is shown from the presence of the same species of mangroves on the Pacific and Atlantic coasts of America that there must have been, not long ago, a communication between these two oceans across Central America; but it is at the same time observed that this could not be inferred from shore-plants with buoyant seeds that, like Entada scandens, occur inland, since, although they occur on both sides of the continent, their distribution can be explained by the transport of their seeds by rivers to the Atlantic and Pacific coasts, such as we see in operation on the Panama isthmus in our own day.
(8) Stress is laid on the great development of mangroves in the Gulf of Guayaquil and in the Guayas estuary; and it is pointed out that there are in this locality two varieties of Rhizophora mangle, a large and a small variety, the first approaching in some of its characters the Asiatic species, R. mucronata, and being akin also to the seedless intermediate form of Fiji.
(9) Amongst other matters dealt with in this chapter are the floating seed-drift of the Guayas or Guayaquil River and the shore plants and stranded seed-drift of the Panama isthmus. From the locality last named we learn that rivers bring down from the interior to the Atlantic and Pacific coasts much the same seed-drift, and that from this centre littoral plants with buoyant seeds can be distributed over the whole tropical zone.
XXXIII. SEED-DISPERSAL AND GEOLOGICAL TIME
The shifting of the source of the Polynesian plants from the New to the Old World.—The floral history of Polynesia stated in terms of geological time.—The suspension of the agencies of dispersal in later periods.—Parallel differentiation in the course of ages of climate, bird, and plant.—New Zealand.—Insects and bats as agents in plant-dispersal.—The effective agency of sea-birds in other regions.—The observations of Ekstam.—The Spitzbergen controversy.—The efficacy of ducks as distributors of aquatic plants.—Summary.
IN the matter of the dispersal of seeds by birds in the tropical Pacific, there are at least two questions which my readers must have frequently put to themselves. The one would be concerned with the shifting of the source of the Polynesian plants from America to the Old World, which occurred probably near the close of the Tertiary period. The other would be connected with the suspension of the work of dispersal over a large portion of Polynesia, which has become more and more pronounced as we approach our own day.
Suggested Cause of the Shifting of the Source of the Polynesian Plants from the New to the Old World.—In previous chapters we have discussed the various epochs in the plant-stocking of these islands. There was first the age of Coniferæ, in which the islands of the Western Pacific were only concerned, an age prior to the appearance of the volcanic groups of the Tahitian and Hawaiian regions, and placed in the Secondary period. Then followed, in the Tertiary period, after the birth of Hawaii and Tahiti, and when the island-groups of the Western Pacific were mainly submerged, the general dispersion from America of the Compositæ, Lobeliaceæ, and other orders, now represented only by genera peculiar to the Hawaiian and Tahitian islands. Last of all, towards the close of the Tertiary period, when the island-groups of the Western Pacific had re-emerged, a general dispersion of Old World plants, mainly Malayan, took place over all the present archipelagoes of the tropical Pacific.
Since the climate of Hawaii must have, to a great extent, shared in the vicissitudes of the continental climates of the northern hemisphere before, during, and after the Glacial epoch, it is assumed that in the Ice Age no tropical plants reached the group, and that only the plants now represented in its mountain-flora could have then reached there. The area of active dispersion of tropical plants was pushed far south. During the Ice period, Indo-Malayan plants doubtless crowded into the equatorial region of the Western Pacific; but, cramped and confined within this limited area, they were cut off by a climatic barrier from the cool latitudes of Hawaii. As the cold ages passed away, migratory birds, confined during that period to the southern hemisphere, would extend their ranges north, sometimes reaching Hawaii, and transporting to it the seeds of New Zealand and Antarctic genera, now represented by endemic species on its mountain-slopes. The Indo-Malayan plants, with the increasing warmth in the climate of the northern hemisphere, would overrun the Pacific, set free from their prison in the south-west portion of that ocean. Dispersal, we might imagine, would be at first very active over the whole ocean.
My point is, then, that whilst the Malayan era of the plant-stocking began after the Ice Age in the northern hemisphere, the dispersion of the New Zealand and Antarctic genera over the Pacific took place during that period; whilst, as before noticed, the dispersion of the Compositæ, Lobeliaceæ, and other orders, represented now in Hawaii by endemic genera, would be pre-Glacial and well back in the Tertiary epoch. I would, therefore, suggest the following scheme, in illustration of the floral history of the tropical Pacific.
(1) The Age of Conifers of the Western Pacific during the Mesozoic period, and before the appearance of the Hawaiian and Tahitian archipelagoes.
(2) The Age of Compositæ and Lobeliaceæ, and of other genera. This is an era of American plants, and it is referred to the Tertiary period. In it only the newly-formed Hawaiian and Tahitian groups shared, the islands of the Western Pacific being largely submerged.
(3) The Age of Malayan plants, regarded as mainly post-Glacial, and subsequent, therefore, to the re-emergence of the Western Pacific islands at the close of the Tertiary period.
Dispersion then was general over the Pacific. The distribution of the New Zealand and Antarctic genera, plants that take a subordinate part in the floras of the Pacific islands, is regarded as having occurred during the glaciation of the northern hemisphere.
On the Suspension of the Agencies of Dispersal in the Tropical Pacific.—If the remark of Drake del Castillo that genera possessing only non-endemic species in the Pacific islands owe their presence in this region to existing agencies of dispersal looks something like a truism, we must remember that, assuming Nature to be uniform in her methods, it involves not merely the original co-operation of the same agencies with genera that own only peculiar species, but also the subsequent suspension of the work of these agencies.
The nature of the connection between freedom of dispersal and specific differentiation is well brought out by Beccari in contrasting the species of Ficus and the palms of Borneo; whilst out of fifty-five species of Ficus collected by him in that island, 30 per cent. were apparently peculiar, 85 per cent. of the 130 Borneo palms had not been found elsewhere. In the English edition of his Nelle Foreste di Borneo he says that “the explanation lies in the fact of the facile dissemination of the various species of Ficus through the agency of birds, an explanation which applies to all trees which produce edible fruits specially relished by animals.” He shows, also, that the same principle applies within the limits of the genus Ficus, since of those Bornean species known to him as belonging to the section Urostigma, which possesses fruits most preferred by birds (pigeons, hornbills, &c.), nearly all (fourteen out of sixteen) are found elsewhere; whilst of ten species belonging to the section Covellia, where the fruits are more or less hidden and inconspicuous, and with difficulty discovered by birds that would effectively distribute the species, four, at the most, are found elsewhere. “Such facts,” he goes on to say, “show that in tropical countries the various kinds of Ficus are, to a large extent, biologically connected with birds, which, perhaps, on their part, also owe some of their peculiarities in the shape of the bill or in the plumage to the nature and coloration of the fruits which form their food.”
Whilst Dr. Beccari as a botanist lays especial stress on the biological connection in Malaya between the plant, as illustrated by the genus Ficus, and the bird, Mr. Perkins, as a zoologist, is similarly emphatic on the biological connection in Hawaii between the bird, as illustrated by the peculiar family of the Drepanididæ, and the plant. The plants here are the arborescent Lobeliaceæ and the Freycinetias. To the flowers of the arborescent Lobeliaceæ the nectar-feeding Drepanids are particularly partial; and the development of the extreme forms of these birds, as Mr. Perkins observes in the Fauna Hawaiiensis, “is not comprehensible without a knowledge of the island flora.” Not only does he point to the modifications in the form of the bill of the bird in connection with the tubular form of the flowers; but in at least one species of these arborescent Lobeliaceæ he shows that it is dependent on the Drepanid for its fertilisation, and he inclines to the view that changes such as that of lengthening of the bill may have taken place side by side with the increasing length of the tubular flowers. In connection with the Freycinetias of Hawaii, Mr. Perkins regards the bill of the Ou, a finch-like Drepanid of the genus Psittacirostra, as “entirely formed and adapted for the purpose of picking out the component parts” of the fruiting inflorescence.
That in an isolated island-group birds and plants often “differentiate” together is a fact well known in distribution. In Hawaii, for instance, as I learn from Mr. Perkins, quite 45 per cent. of the birds are peculiar; whilst according to Dr. Hillebrand 80 per cent. of the flowering plants are confined to the group. Then, again, in the Galapagos Islands, half of the plants and two-thirds of the birds are confined to that archipelago. At the other end of the series we have the Azores, with about a tenth of its plants peculiar, and about 4 per cent. of its birds peculiar to the islands, and Iceland with no endemic plants and, as far as I can gather, few peculiar birds.
Accepting Mr. Charles Dixon’s view (The Migration of Birds, 1897) that specific differentiation does not occur along lines of migration, we must assume that the differentiation of the avifauna of an isolated group like Hawaii began with the breaking off of its regular communication through birds with the outside world. I do not consider that in the past these Pacific archipelagoes received their birds in any haphazard fashion, as, for instance, in the guise of stragglers that had lost their way. From the circumstance pointed out to me by Mr. Perkins that 25 of the 67 genera of Hawaiian birds are peculiar, we must postulate a high antiquity for the bird fauna dating far back into the Tertiary period. Mr. Perkins, who kindly supplied me with his general views of the nature of the Hawaiian fauna, tells me that it is “positively oceanic-insular and could be continental only on the supposition that everything continental had been at some time destroyed and that the group had been subsequently re-stocked as would any oceanic island.”
The view naturally presents itself that in past ages birds in the Pacific were much more uniform in their characters, and the agencies of dispersal far more active in their operations and far more general in their range than in more recent times, “It may be accepted (says Mr. Dixon) as an axiom of geographical distribution that all existing species are surviving relics of more ancient forms or ancestral types, whose dispersal in a remoter past was more continuous, and whose affinities and characteristics were therefore more homogeneous.” I assume that in past ages the differentiation of birds has largely been favoured by differentiation of climate acting through the limitation of their ranges. To these changes, plants, so often biologically connected with birds, have largely responded.
There is, of course, no difficulty in imputing to birds the capacity of reaching Hawaii in the mid-Pacific, and there are many regular migrants now (sea-birds, waders, ducks, &c.). The only difficulty is in the estimation of the time occupied in the trans-oceanic journey. According to Gätke the journey, which is 1,500 to 2,000 miles, ought to be accomplished within the limit of fifteen hours, which he regards as “the longest spell during which a bird is able to remain on the wing without taking sustenance of any kind.” As he considers that a bird might cover the 1,600 miles between Newfoundland and Ireland in nine hours (Heligoland as an Ornithological Observatory, p. 140), the Hawaiian traverse would offer to him no difficulties. It has frequently occurred to me in this connection that in ancient times, when the volcanoes of the mid-Pacific were in full activity, their light at night-time would have often given a direction to the migrating bird, and that they might have sometimes determined the line of migration across the Pacific.
It has not been possible to discuss here the capacity of pigeons to cross an ocean, a subject bearing directly on the floras of all the Pacific groups (excepting Hawaii, which possesses no indigenous Columbæ) and as concerning these islands generally presenting no difficulty. Dr. H. de Varigny, who amongst his other studies has long displayed an active interest in plant-dispersal, has directed my attention to two very important papers on the flight of pigeons in the Revue Scientifique, one by M. A. Thauziès (April 30, 1898) and the other by M. M. Dusolier (Nov. 28, 1903). That land birds, as well as swimmers and waders, cross the Atlantic is well known, and in this connection the reader might profitably consult Prof. Heilprin’s Geographical Distribution of Animals (vol. 58, Internat. Sci. Ser. 1887).
Much might be said of these matters, but it would be out of place here; and I will content myself with stating the view above indicated that the suspension of the agencies of seed-dispersal over the Pacific is probably connected with a general principle affecting the whole plant-world. The tendency in the course of ages has been towards the differentiation of climate, bird, and plant, the range of the bird being largely controlled by the climate, and the range of the plant being mainly dependent on the range of the bird.
It is evident that in some cases the plants themselves may make the endemism of a flora more pronounced by creating their own difficulties and by standing in the way of their own dispersal to outside regions. It has been shown that some of the endemic Hawaiian genera (see Note 68) have deteriorated in their capacity for dispersal by birds; and similar remarks are made with reference to the genera Sicyos (page 365) and Eugenia (page 350). Genera with stone fruits like Elæocarpus possess in the different species stones of various sizes, some of them suitable in point of size for conveyance in a bird’s body over an ocean, others so large that one could only predicate for them a limited capacity for distribution by birds over a few hundred miles of sea. One, for instance, could safely assume that species of Elæocarpus, with stones an inch and over in size, that occur in Fiji and Hawaii, are not suited for distribution over an ocean now; whilst other species found in New Zealand and Rarotonga have stones less than half this size, which are quite fitted for distribution by birds over broad tracts of ocean (page 337).
This brings us to discuss the relative difficulties presented from the dispersal-standpoint by the forest floras of Hawaii, Fiji, and New Zealand. It is with the forest floras that nearly all the difficulties of distribution lie; and I hope I shall not be considered presumptuous, or at all events too heterodox, in expressing the opinion that judging from the details given in Kirk’s Forest Flora of New Zealand those islands present no greater difficulties for the student of plant-distribution, if we exclude the Coniferæ, than either Fiji or Hawaii. Indeed, even with the Conifers included, New Zealand presents fewer problems than Fiji, and Hawaii has its own special difficulties connected with the inland species of the Leguminosæ. There is, on the other hand, no special New Zealand difficulty. It possesses the Conifers in common with Fiji; and it shares with Fiji and Hawaii genera like Elæocarpus and Sideroxylon, that take a foremost place amongst the trees of the Pacific forest flora presenting puzzling points to the student of distribution. The existence of Elæocarpus in New Zealand admits of a simpler explanation than the occurrence of the same genus in Hawaii. Pandanus in Fiji is a more difficult genus from the standpoint of dispersal than Corynocarpus in New Zealand, and in fact, than any of the non-coniferous genera of forest trees in that region.
Whilst it is likely that birds of the genus Porphyrio have, up to almost recent times, been active in distributing the seeds of New Zealand plants outside the region (see p. 296), it would seem that the fruit-pigeons, as represented by a solitary peculiar species of Carpophaga, have long since ceased to be active in this direction. It is true that Sir W. Buller gives a long list of trees, including Corynocarpus, Elæocarpus, Litsea, Olea, Podocarpus, and many others, the fruits of which are appreciated by this pigeon; but since the bird is confined to this region its efforts in plant-dispersal possess only a local interest. Mr. G. M. Thomson, indeed, has expressed the opinion (Trans. and Proc. N.Z. Instit. vol. 33) that in recent times not a single plant has been added through the agency of birds to the New Zealand flora. Besides the regular migratory birds two cuckoos only reach the region, the one from Australia and the other from Polynesia; whilst Australian birds which had managed to survive the long flight across the ocean have been met with only at times on the west coast of the North Island. From the standpoint adopted in this work we should have expected that, with the exception of current-dispersed plants, New Zealand would be out of touch with the world outside. Varied only by occasional inrushes of plants, its history, dating back to the Mesozoic age, has been one of insular isolation.
When, however, we apply the principles of plant-dispersal in the Pacific, deducible from the study of the Hawaiian flora, we learn that the stocking of New Zealand with its plants could have been carried out (with the exception of the Coniferæ and a few other genera like Fagus that are in a geological sense ancient denizens in this region) by the agencies that stocked Hawaii with its flora. New Zealand genera like Elæocarpus, Sideroxylon, Sophora, etc., that are represented in the forests of Hawaii, could not be taken to illustrate any former continental connection. If we, so-to-speak, put the New Zealand forest flora in the Hawaiian sieve, all will pass through with the exception of Fagus, the genera of the Coniferæ, and plants of similar history in high southern latitudes. This residuum belongs more to the palæobotanist than to the student of means of dispersal.
I should be inclined to think that the tropical genera of the New Zealand flora, more especially of the forest flora, reached that region during the glaciation of the northern hemisphere, when the Indo-Malayan plants were, so-to-speak, cornered in the Western Pacific. Yet it must be noted that these are, as a rule, genera that either display an indifference to the varying thermal conditions of different latitudes or are known to at times extend their range beyond the tropics. Thus Elæocarpus and Freycinetia are equally at home in the temperate rain-forests of New Zealand and in the tropical rain-forests of Polynesia and Malaya; whilst widely-spread tropical genera like Pittosporum and Peperomia, that occur in New Zealand, exhibit their power of adaptation to varying climates by extending outside the tropics in other regions and by their vertical range in the Hawaiian mountains, where they are found alike at low elevations a few hundred feet above the sea and at altitudes of 6,000 or 7,000 feet.
All these plants, however, are in a relative sense recent intruders. When the student of dispersal looks at the long list of the conifers of the New Zealand forest flora and reflects that he knows but little of their means of dispersal, and that if his acquaintance were far greater it would not avail him much, he has no choice but to take his place behind the earlier investigators of the flora, and to see in these trees evidence in favour of a remote continental period, probably referable to the mesozoic age.
A Discussion of some Means of Dispersal.—Not many authors seem to have discussed the possibility of insects as agents of seed-dispersal in the Pacific. They appear to me quite suited for transporting the spores of ferns and lycopods as well as the minute seeds of plants like the orchids and the begonias. Darwin, who allowed few possible means of dispersal to escape his notice, procured the germination, as my readers will remember, of grass seeds found in the dung of Natal locusts. When on the barren summit of Mauna Loa, I noticed that the recently dead bodies of some butterflies, that had been carried up the slopes from the forests below by the “southerly updraught,” were already attacked by bugs, parasites that must have been transported from the lower regions by some of the numerous larger insects that are blown up the slopes.
In Note 61 the occurrence of the wind-blown insects on the summit of Mauna Loa is described. That insects can be transported into the upper regions of the atmosphere by ascending air-currents was long ago remarked by Humboldt, and the subject has been discussed with his usual acumen by Whymper (Travels amongst the Great Andes of the Equator). Carried along in the higher air-currents these insects might finally be deposited at places far distant from their home. One reads occasionally extraordinary accounts of a rain of insects. A very circumstantial account was given to me when I was on Keeling Atoll of a shower of dragon-flies that fell on the islands, their remains being found in quantities in the lagoon. Dragon-flies, it is known, are often found at sea far from land, and one species has been observed nearly all over the world, including the Pacific islands. In this connection it is interesting to recall Mr. M’Lachlan’s remark in his article on the dragon-fly in the seventh volume of the Encyclopædia Britannica that some of the earliest fossil forms seem to have been washed ashore after having been drowned at sea.
Another creature that has been often ignored as a possible agent in seed-dispersal is the bat. Bats are found all over the world, including the oceanic groups, and one can scarcely doubt that they must have often transported seeds, at all events in their hair. They are found at times high up in mountainous regions, and Sir H. Johnston, in his recent work on the Uganda Protectorate, refers to the occurrence of bats at an altitude of 13,000 feet. The large frugivorous bats (Pteropidæ) are known to be very destructive feeders; but I doubt whether they swallow the seeds. Dr. Warburg, as is remarked in Chapter XXV, says that they feed on the flowers of Freycinetias, and I have already observed that they visit the flowers of Geissois ternata in Fiji (p. 394). In this fashion Dr. Warburg regards them as agents in pollenisation; and it seems to me that if, as appears likely, they are attracted by trees with large, brightly-coloured flowers, they would often aid in the dispersal of the minute seeds of trees like Metrosideros.
Until recently sea-birds, and some particular birds of passage, have been generally considered as only fitted for dispersing seeds in their plumage. That they can also transport seeds inside their bodies is shown below. Dr. R. Brown in his book entitled Our Earth and its Story, 1888, gives a general account of plant-dispersal with numerous references to the Literature on the subject. On the direct route between Scotland and Cape Farewell in Greenland snow-buntings (Plectrophanes nivalis) and other birds of passage frequently alight, as we are told, on ships when hundreds of miles from land. Dr. Brown says that when taking this voyage he examined dozens of these birds. Only in one case, however, did he find any seeds, namely, in the case of a snow-bunting which carried, attached to its plumage, an achene of, perhaps, a Ranunculus, and in its gizzard a seed like that of a Suæda. My discovery of a small, hard seed in the gizzard of a Cape-pigeon (Daption capensis) 550 miles east of Tristan da Cunha has been referred to by Mr. Hemsley in his introduction to the Botany of the “Challenger” Expedition (p. 45). On p. 188 I have mentioned the probable dispersal of the seeds of Cæsalpinia by frigate-birds and boobies; and in Note 59 reference is made to some large seeds found in the crop of the Fulmar petrel.
Gulls, when they nest at the coast, where the sea-thrift (Armeria vulgaris) and the sea-campion (Silene maritima) thrive, or inland amongst the heather-covered slopes, must often carry the seeds of these plants from place to place in their plumage (see Notes 15 and 16); but, as shown below, they can also disperse plants with fleshy fruits which at times form their food. Gulls, geese, and arctic grouse take an important part in the dispersal of seeds in the cold latitudes of the northern hemisphere; and few things are more suggestive in this way to the student of distribution than the data supplied by Ekstam, Hesselman, Sernander, and others for the region including Spitzbergen, Nova Zembla, and Arctic Norway. The history of the discussion relating to the flora and fauna of Spitzbergen reproduces in its main features the various stages in the controversy that has been waged in connection with the Pacific islands.
When Ekstam published, in 1895, the results of his observations on the plants of Nova Zembla, he observed that he possessed no data to show whether swimming and wading birds fed on berries; and he attached all importance to dispersal by winds. On subsequently visiting Spitzbergen he must have been at first inclined, therefore, to the opinion of Nathorst, who, having found only a solitary species of bird (a snow-sparrow) in that region, naturally concluded that birds had been of no importance as agents in the plant-stocking. However, Ekstam’s opportunities were greater, and he tells us that in the craws of six specimens of Lagopus hyperboreus shot in Spitzbergen in August he found represented almost 25 per cent. of the usual phanerogamic flora of that region, in the form of fruits, seeds, bulbils, flower-buds, leaf-buds, &c. This observer now also realised the importance of gulls and geese in dispersing certain types of plants in those latitudes. Species of Larus, he says, consume greedily all kinds of berries, and especially those of Empetrum nigrum, the stones of which were found uninjured in their droppings by Professor Lagerheim in Arctic Norway. Geese, as we are also informed, are hearty plant-eaters in Spitzbergen; and Ekstam found in their droppings the fruits of Oxyria digyna as well as an abundance of uninjured bulbils of Polygonum viviparum, some of which proved to be capable of growth (See Ekstam in Tromso Museums Aarshefter, vols. 18 and 20, 1895-7).
The result of Ekstam’s observations in Spitzbergen was to lead him to attach a very considerable importance in plant-dispersal to the agency of birds; and when in explanation of the Scandinavian elements in the Spitzbergen flora he had to choose between a former land connection and the agency of birds, he preferred the bird.
I have gone into some detail in this matter because the Spitzbergen controversy in some respects might have equally centred around New Zealand or some of the large continental islands of the tropical Pacific. There is at first the endeavour in the absence of precise knowledge to disregard the bird and to look for a land connection. With the increase in our acquaintance with the efficacy of bird-agency in seed distribution there is the abandonment of such a view. In both localities, however, there are the same counter-indications of the insect faunas, and the same considerations are raised by the absence or presence of larger animals in the regions concerned. The principal difference lies in the frozen sea, and yet, strangely enough, it does not seem to affect the problem much. It would indeed appear that the questions raised by the floras and faunas of the Pacific islands are not peculiarly Pacific in their character; and it is probable that the difficulties here presented are repeated in one form or other in the case of large islands over all the globe.
On the efficacy of Ducks and other Waterfowl in the Distribution of Aquatic Plants.—It is highly probable that aquatic plants, like the beach plants distributed by the currents and the ferns and lycopods distributed mainly by the winds, have changed much less in the course of ages than the plants of the inland forest. This in all three cases is chiefly due to the uninterrupted freedom of communication by means of the dispersing agency.
Wild ducks and their kind are active agents in the distribution of the seeds of aquatic plants; but it is curious that the early experiments of Caspary went far to discredit them in this respect. As quoted by Dr. Schenck in his Die Biologie der Wassergewächse, 1886, he fed tame ducks with the seeds of water-lilies and found that in a short time they thoroughly digested the seeds. Those familiar with the seeds of our British species of Nuphar and Nymphæa will not be surprised at such a result; but, unfortunately, the inference drawn from this experiment has been by some extended to aquatic plants in general. Since the seeds or seed-vessels of some aquatic or semi-aquatic plants of the genera Potamogeton, Sparganium, &c., appeared to me to be quite fitted for conveyance without injury in a duck’s body, I made several years ago a number of observations on this subject, the results of which were published in Science Gossip for September, 1894.
Out of 13 wild ducks obtained in the London market and stated to have been sent from Norfolk and Holland, eleven contained in their stomach and intestines 828 seeds, which I thus classed:—
295 seeds of Sparganium in 8 birds 41 seeds of Potamogeton in 3 birds 270 seeds of Cyperaceæ in 5 birds 222 not identified
In the case of four birds the germinating capacity of the seeds was tested, and in three cases very successfully. The seeds of Potamogeton, Sparganium, and of the Cyperaceæ germinated readily in water, but few of them failing, the process beginning in a few days or a few weeks. At that time I was conducting an extended series of observations on the postponement of germination of the seeds of aquatic plants, the results of which were published in the Proceedings of the Royal Physical Society of Edinburgh for 1897. It was there shown that the seeds of these plants often postpone their germination to the second and even to the third spring. It thus happened that, whilst seeds obtained from the stomach and intestines of the wild duck germinated in a few days or weeks, I had to wait often a year and more for such a result with seeds in their ordinary condition. This was well brought out in another experiment made on a domestic duck, which I have described on page 369. That wild ducks are to be regarded in the light of “flying germinators” is thus very evident.
Summary.
(1) In explanation of the shifting of the source of the Polynesian plants from the New to the Old World, it is suggested that during the glaciation of the northern hemisphere the Indo-Malayan plants entering this region were “cornered” in the tropical Western Pacific, and were only set free after the cold period had passed away, when they overran Polynesia.
(2) Whilst the age of the Conifers is placed in the Mesozoic period, that of the Compositæ is accredited to the Tertiary period, and the era of Malayan immigration followed the glacial epoch.
(3) The suspension to a great extent of the agencies of plant dispersal in the Pacific in later times is connected with a general principle affecting the whole plant-world. With the secular drying up of the globe the differentiation of climate, bird, and plant have gone on together, the range of the bird being mainly controlled by the climate and the range of the plant being largely dependent on the bird.
(4) Accepting Hawaii as entirely insular in its history, it is pointed out that the principles deducible from the study of its flora can be applied to the forest-flora of New Zealand, with the exception of the Conifers and some genera that are ancient denizens of Antarctic latitudes, and indicate a remote continental age dating back to the Mesozoic period. It is suggested that the Indo-Malayan element in its flora arrived there during the glaciation of the northern hemisphere.
(5) Insects and bats have probably been effective agents in seed-dispersal in the Pacific, and it is shown that sea-birds carry seeds in their stomach and intestines as well as in their feathers.
(6) It is shown that birds of the grouse family, gulls, and geese are active seed-dispersers in cold northern latitudes, and that the discussion of their influence in stocking Spitzbergen with its plants reproduces many of the points of the controversy concerning the floras of the continental islands of the South Pacific.
(7) The results of experiments and observations are cited to establish the efficacy of ducks in distributing the seeds of aquatic plants, the seeds ejected in their droppings germinating in a few days or weeks, whilst those remaining in the pond or river often do not germinate for a year or more.
XXXIV. GENERAL ARGUMENT AND CONCLUSION
THE problems concerned in the study of the floras of the Pacific islands from the standpoint of dispersal are here approached through the buoyant quality of the seed and fruit; and it is shown when dividing the plants into two groups, those with buoyant and those with non-buoyant seeds or fruits, that there has been at work through the ages a great sorting process, by which the plants belonging to the group first named have been mostly gathered at the coast. Its operation may be also observed within the limits of a genus, where the species possessing seeds or fruits that float is stationed at the coast, whilst the species with seeds or fruits that sink makes its home inland.
When the principle here involved is applied to the British flora, it presents itself as part of a much wider principle, by which plants endowed with buoyant seeds and fruits have been stationed at the water-side, whether on a river-bank, or beside a lake or pond, or on a sea-beach. The broader principle proves in its turn to belong to a far larger scheme, in which the fitness or unfitness of a plant to live in a physiologically dry station appears as the primary determining quality, the xerophyte (the plant of the dry station), provided with buoyant seed or fruit, finding its way to the coast, and the hygrophyte (the plant growing under more moist conditions), that is similarly endowed, establishing itself by the side of the river, or the lake, or the pond.
When dealing with the general character and composition of the strand-plants of the tropical Pacific, it is shown that in Fiji the beach-plants often assert their primary xerophilous habit or fitness for occupying any dry station by extending into the inland plains on the dry sides of the islands. The Fijian shore-plants are divided into three formations, those of the beach, those of the mangrove-swamp, and those of intermediate stations on the borders of the swamps. The great majority of the Fijian shore-plants are dispersed by the currents. The Tahitian Islands, which are representative of Eastern Polynesia, lack the mangroves and most of the plants that grow at the margin of a mangrove-swamp; and their strand-flora is mainly composed of plants of the beach, such as are dispersed by the currents far and wide in tropical regions. The Hawaiian strand-flora is very meagre in its character, lacking not only the plants of the mangrove and intermediate formations, but almost all the large-fruited beach-trees of the South Pacific. Since Hawaii possesses but few current-dispersed shore-plants that are not found in the New World, reasons are given for the inference that such shore-plants were originally brought by the currents from America, and not from the South Pacific.
We are led on various grounds to the conclusion that tropical shore-plants distributed by currents belong to two great regions, the American including the west coast of Africa, and the Asiatic, or Old World Region, which includes the African east coast. It is held that America is so placed with regard to the currents, that it is a distributor, and not a recipient of tropical shore-plants dispersed by that agency. From this it follows that all cosmopolitan tropical beach-plants that are dispersed by the currents have their homes in America.
The results of observation and experiment are given to show that there is no direct relation between the specific weight of seeds and fruits and the density of sea-water. Yet, although the floating or sinking of a seed or fruit is but an accidental attribute, it has had indirectly a far-reaching influence not only on plant-distribution, but on plant-development. In accordance with this want of relation between the specific weight of seeds and fruits and the density of sea-water, the great variety of structures concerned with buoyancy are regarded in the main, after a detailed examination of their character, as not arising from adaptation. Rather, it is urged, is buoyancy connected with structures that now serve a purpose for which they were not originally developed. Nature, it is held, has never concerned herself directly with providing means of dispersal of any sort.
In the discussion of the relation between the littoral and inland Pacific floras, it is shown, as a result of the examination of those genera possessing both shore and inland species, that they have been on the whole developed on independent lines. Two special difficulties in explaining the modes of dispersal of plants of the Pacific islands here come into prominence. There is the Hawaiian difficulty, where with genera containing both shore and inland species only the last are found in Hawaii; and although the shore-plants are known to be dispersed by the current, the inland plants display little or no capacity for this or any other mode of dispersal. Here belong the Leguminous genera Canavalia, Erythrina, Mezoneuron, and Sophora, and the Apocynaceous genus Ochrosia; and it is assumed that the inland Hawaiian species are derived from a current-dispersed shore-plant that has since disappeared from the group. The Fijian difficulty is displayed in those genera where both coast and inland species occur in the islands, but no known existing means of dispersal across an ocean can be postulated for the inland plants, though the shore species are distributed by the currents. Of such genera Pandanus is the best example, and it is pointed out that this genus presents the same difficulty in the Mascarene Islands, in which case the agency of the extinct Columbæ is invoked.
As illustrating the methods of observation and experiment employed by the author, the Leguminous shore-plants Afzelia bijuga, Cæsalpinia bonducella, and Entada scandens are discussed at length; and in the chapters on the enigmas of the Leguminosæ in the Pacific it is pointed out that the behaviour of the plants of this order is a source of much perplexity, and that they conform to no single rule of dispersal.
Coming to the inland plants of this region, the Fijian, Tahitian, and Hawaiian groups are taken as the chief centres of distribution in the Pacific. After discussing the relative sizes, the altitudes, and the climates of these three archipelagoes, it is shown that Hawaii, on account of the far greater altitude of the islands, is characterised by a special mountain flora, and that it is comparable with Fiji, and to a great extent also with Tahiti, only as regarding the plants of the levels below 4,000 or 5,000 feet.
The first era of the plant-stocking is designated the Age of Ferns, and it is observed that, whilst in Hawaii nearly half of the ferns and lycopods are peculiar to that group, very few new species have been developed in the Fijian and Tahitian regions.
The next era in the floral history of these islands is represented in the first era of the flowering plants. This is indicated by the endemic genera, which are particularly numerous in Hawaii, relatively scanty in Fiji, and very few in Tahiti. On account of their preponderance, the era is designated the Age of Compositæ and Lobeliaceæ. The genera of these two orders, though mainly characteristic of Hawaii, are also to be found in the Tahitian region, but they are absent from the Fijian area. Chiefly American in their affinities, their dispersion over the Pacific took place during the Tertiary submergence of the archipelagoes of the Western Pacific, in which are included the groups of the Fijian area (Fiji, Samoa, Tonga). These early forms of Compositæ and Lobeliaceæ are often arborescent in habit; and it is observed that Tree-Lobelias also occur high up the slopes of lofty mountains in tropical regions, as in Equatorial Africa, under conditions similar to those prevailing on the slopes of the Hawaiian mountains, where the Tree-Lobelias, termed by Dr. Hillebrand “the pride of our flora,” abound.
The other Hawaiian endemic genera, marking the first chapter in the history of the flowering plants, arrange themselves in two groups, one chiefly American in general affinities, and containing highly differentiated Caryophyllaceæ, Labiatæ, &c.; the other largely Malayan, and indicating the close of the first era of the flowering plants, when the main source of the plants was shifted from America to the Old World. The Fijian endemic genera, which are few in number, miscellaneous in appearance, and disconnected in character, are regarded as having probably acquired their endemic reputation through their failure at their sources in the regions to the west.
The second era of the flowering plants is indicated by the non-endemic genera. Here we are concerned on the one hand with a mountainous flora mainly Hawaiian, in which genera from the New Zealand and Antarctic floras take a conspicuous part, and on the other with a low-level flora chiefly derived from Indo-Malaya, and including the plants of the lower slopes of Hawaii below 4,000 and 5,000 feet, and the floras in mass of Fiji and Tahiti.
On account of their lower altitude, the extensive mountain flora of Hawaii is but scantily developed in Tahiti, and is represented by a mere remnant in Fiji and Samoa. Two-thirds of the Hawaiian non-endemic mountain genera contain only species restricted to the group, and, although amongst these disconnected genera, Acæna, Gunnera, Coprosma, Lagenophora, &c., of the New Zealand and Antarctic floras take a prominent part, a large proportion of the genera like Ranunculus, Rubus, Artemisia, Vaccinium, and Plantago represent generally the flora of the north temperate zone on the summits of tropical mountains. The Tahitian mountain flora, scanty as it is when judged by the non-endemic genera, displays much kinship with the Hawaiian mountain flora; but this kinship is mainly confined to genera from high southern latitudes, such as Coprosma, Cyathodes, Astelia, &c. In the possession on its mountain slopes of the three genera of the Coniferæ, Dammara, Podocarpus, and Dacrydium, the Fijian region is distinguished from that of Tahiti and Hawaii; and it is assumed that they mark the site of a continental area in the Mesozoic period, when the Tahitian and Hawaiian groups did not exist.
The era of the non-endemic genera, in so far as it is concerned with the low-level flora of Hawaii and the floras in mass of the areas of Fiji, Samoa and East Polynesia, is termed Malayan, because many of the genera are thence derived. Here we are dealing with all the oceanic groups of the tropical Pacific, and not with a portion of them, as in the case of the Age of Coniferæ, in the Secondary period, that was limited to the Western Pacific, or in the case of the Age of Compositæ and Lobeliaceæ that was restricted during the Tertiary epoch to the Hawaiian and Tahitian regions. The first part of this era, as is indicated by the endemic species, is an age of complete isolation in Hawaii, and of partial isolation in the groups of the southern region. Amongst the genera typical of this period are Pittosporum, Gardenia, Psychotria, Cyrtandra, and Freycinetia. A later period in this era of the general dispersal of Malayan plants over the Pacific is one where the extremely variable or polymorphous species plays a conspicuous part, as represented in such genera as Alphitonia, Dodonæa, Metrosideros, Pisonia, and Wikstrœmia, the general principle being that each genus is at first represented by a widely ranging very variable species, which ultimately ceases to wander and settles down, and becomes the parent of different sets of species in the several groups.
The facts of distribution in this age of general dispersion are just such as we might look for in the case of a general dispersal over the oceanic groups of the Pacific, with the altitudes of the islands playing a determining part. But it should be remarked that the greater number of the genera that have entered the Pacific from the Old World have not advanced eastward of the Fijian region, half of the Fijian genera not occurring in the Hawaiian and Tahitian regions. The explanation of this is to be found, not in any lack of capacities for dispersal, but in a want of opportunities. The story of plant-distribution in the Pacific is bound up with the successive stages of decreasing activity in the dispersing agencies. The area of active dispersion, as illustrated by the non-endemic genera, at first comprised the whole of the tropical Pacific. It was afterwards restricted to the South Pacific, and finally to the Western Pacific only. The birds that carried seeds all over this ocean became more and more restricted in their ranges, probably on account of increasing diversity of climatic conditions. The plants of necessity responded to the ever narrowing conditions of bird-life in this ocean, and the differentiation of the plant and the bird have taken place together.
During the stages of decreasing activity in the dispersing agencies, the widely-ranging highly variable species continued to be an important factor in the development of new species in the different groups. The rôle of the polymorphous species has always been a conspicuous one in the Pacific.
Yet, as in the case of the Cyrtandras, it is shown that the display of great formative power within a genus is not a peculiarity of an insular flora; that the isolation of an oceanic archipelago does not exclusively induce “endemism,” but only intensifies it; that the development of new species may be nearly as active on a mountain in a continent as on an island in mid-ocean; and that this is equally true of a land genus, like Embelia, exposed to an infinite variety of conditions, and of an aquatic genus, like Naias, where the conditions of existence are relatively uniform all the world over.
In framing a scheme by which the eras of the floral history of the Pacific are brought into correlation with those of geological time, the age of the Coniferæ is placed in the Secondary period, that of the Compositæ and Lobeliaceæ in the Tertiary period, whilst the era of Malayan immigration is regarded as mainly post-glacial. The age of the Coniferæ is concerned only with the Western Pacific, since the Hawaiian and Tahitian islands had not then been formed. The age of the Compositæ and Lobeliaceæ is concerned only with Hawaii and Tahiti, since the islands of the Western Pacific were then more or less submerged. That of the Malayan plants affects the whole Pacific as at present displayed to us.
In the chapter on the viviparous mangroves of Fiji it is shown that both the Asiatic and the American species of Rhizophora (R. mucronata and R. mangle) exist in that group, and that there is in addition a seedless form, the Selala, which, although intermediate in character between the two other species, comes nearest to the Asiatic plant. Reasons are given for the belief that the Selala is derived from the Asiatic species (R. mucronata), not as the result of a cross but as connected with its dimorphism; and in support of this it is pointed out that on the Ecuador coast of South America, where only the American species exists, a dimorphism is also displayed, one of the forms approaching in several of its characters the Fijian Selala, though fruiting abundantly and bearing the impress of a closer connection with the typical American species than with the Asiatic plant. The view that Rhizophora mangle reached the Western Pacific from America is rejected, and it is considered that this species was originally as widely diffused in the Old World as in America, and that it now survives only in a few places in the tropics of the Old World. The results of detailed observations on the modes of dispersal and on the germinating process both with Rhizophora and Bruguiera are given; and the absence, as a general rule, of any period of rest between the fecundation of the ovule and the germination of the seed is established.
A special chapter is devoted to the significance of vivipary, and it is considered that a record of the history of vivipary on the globe is afforded in the scale of germinative capacity that begins with the seedling hanging from a mangrove and ends with the seed that is detached in an immature condition from an inland plant. It is suggested that with the drying up of the planet in the course of ages the viviparous habit, which was once nearly universal, has been for the most part lost except in the mangrove swamp, which to some extent represents an age when the earth was enveloped in cloud and mist and the atmosphere was saturated with aqueous vapour. The lost habit is at times revived in the abnormal vivipary of some inland plants, and traces of it are seen in the abnormal structure of the seeds of some genera of the Myrtaceæ, like Barringtonia, and in the seeds of genera of other orders. With the desiccation of the planet and the emergence of the continents there has been continual differentiation of climate resulting in seasonal variation and in the development of the rest-period of the seed.
With the secular drying of the globe and the consequent differentiation of climate is to be connected the suspension to a great extent of the agency of birds as plant-dispersers in later ages, not only in the Pacific Islands but over all the tropics. The changes of climate, bird, and plant have gone on together, the range of the bird being controlled by the climate, and the distribution of the plant being largely dependent on the bird.
The history of climate, the history of the continents and of the oceans, the history of life itself, but only in the sense below defined, all belong to that of a desiccating world, or rather of a planet once sunless and enveloped in mist and cloud, that through the ages has been drying up. Life’s types were few and the sea prevailed, and one climate reigned over the globe. With the diminution of the aqueous envelopes the continents began to emerge, climates began to individualise, and organisms commenced to differentiate, and thus the process has run on through the past, ever from the general to the special both in the organic and in the inorganic world.
The same story of a world drying up is told by the marine remains left stranded far up some mountain slope, or by the bird akin to no other of its kind that Time has stranded on some island in mid-Pacific. The bird generalised in type that once ranged the globe is now represented over its original range by a hundred different groups of descendants, confined each to its own locality. Climate, once so uniform, now so diversified, has by restricting the range of the bird favoured the process of differentiation, and the plant dependent on the bird for its distribution has in its turn responded to these changes.
The rôle of the polymorphous species belongs alike to the plant and to the bird. A species that covers the range of a genus varies at first in every region and ultimately gives birth to new species in some parts of its range. Then the wide-ranging species disappears and the original area is divided up into a number of smaller areas each with its own group of species. Each smaller area breaks up again, and forms, yet more specialised, are produced; and thus the process of subdivision of range and of differentiation of form goes along until each island in an archipelago owns its bird and each hill and valley has its separate plants. This is not the path that Evolution takes, since beyond lies extinction whether of plant or of bird. Such is the upshot of the process of differentiation exhibited in the development of species and genera in the Pacific Islands, or, indeed, in any oceanic groups. It can never do more than produce a Dodo or a Kiwi, or amongst the plants a Tree-Lobelia.
Evolution here and elsewhere is a thing apart from species and genera, which are but eddies on the surface of its stream. It is a scheme of life introduced into a much conditioned world, and adaptation in endless forms is the price it has had to pay. The whole story of life on this earth is a story of a sacrifice, of an end to be won, but of a price to be paid. Immortality is in the scheme, but death is the price of adaptation. The same theme runs through our conceptions of the spiritual life. There is the same duality, evolution adapting its scheme to the exigencies of the physical world, the good principle ever in conflict with the evil, and at times compelled to adapt itself to attain its ends. There is the tale of adaptation in the one case and of sacrifice in the other, and success is reached in both.