VIII. THE LITTORAL PLANTS AND THE CURRENTS OF THE PACIFIC
The working value of the currents as plant-dispersers.—The relation between the currents and the distribution of shore-plants.—The clue afforded by the American plants.—Two regions of tropical shore-plants, the American and the Asiatic.—America, the home of the cosmopolitan tropical shore-plants that are dispersed by the currents.—Hawaii and the currents.—Summary.
ACTIVE as the currents are in dispersing seeds and fruits over the Pacific, it should be remembered that those plants that owe their distribution to this agency are only shore-plants, and not, indeed, all the shore-plants, but only those with buoyant seeds or fruits. Even the coral atoll owes a great deal to the agency of the fruit-pigeon and of other birds; for instance, their species of Ficus, Eugenia, and Pisonia. In order, therefore, not to form an exaggerated notion of the efficacy of the currents, it will be necessary to obtain some numerical idea of what they have really accomplished in transporting seeds and seedvessels over the oceans in a state fit for successful germination on the shores upon which they are stranded. It is requisite to make this proviso, because in some cases the currents work to no purpose. Thus, the empty nuts of Aleurites moluccana are carried far and wide over the Indian and Pacific Oceans, and are stranded on the beaches of the various islands, as I have found myself in the cases of Keeling Atoll, Java, and Fiji. The Coco-de-Mer, or the Double Coco-nut Palm, is another apt instance. Though its fruits have been carried far and wide over the Indian Ocean, the species is restricted to the Seychelles. So also the acorns of various species of Quercus are widely but ineffectually distributed by the currents both in temperate and tropical regions. (This subject of useless dispersal is dealt with in Chapter XIII.)
It is essential to bear in mind at the outset that for their inland plants the Pacific islands can draw on the floras of a relatively large portion of the globe. Such plants, having as a rule fruits or seeds that sink in sea-water, or are incapable of floating for long periods, could only have arrived at these islands, where man’s interference is excluded, through the agencies of winds and birds, assisted by other lesser agencies, as those of bats, insects, &c. On the other hand, for their littoral plants, which are for the most part dispersed by the currents, the source of supply is very restricted. The shore-plants with buoyant seeds or fruits of the islands of the tropical Pacific, that are here dealt with, number only about seventy, and it is not likely that this number will be greatly increased, since, whatever may be the deficiencies in our acquaintance with the inland floras of these islands, we have a fairly complete knowledge of the strictly littoral plants.
I do not suppose, indeed, that the number of such plants with seeds or fruits capable of being transported unharmed over wide tracts of sea would much exceed 100 for the whole Indo-Pacific region from India to Tahiti. Professor Schimper gives a list containing 117 tropical plants distributed far and wide over the shores of this region, and made up of species dispersed by currents, birds, and man. Taking a liberal estimate, not over two-thirds of the plants mentioned in this list are dispersed by currents. Then, again, if the flora of a coral atoll, like that of Diego Garcia or of the Keeling Islands, is taken as affording an index of the work of the currents, the number of plants dispersed by the currents would appear to be indeed restricted, since in either case their indigenous flowering plants, including those of both the buoyant and non-buoyant groups, do not exceed fifty.
About twenty years ago, Mr. Hemsley, who, in his work on the botany of the Challenger Expedition, prepared the way for the investigation of this subject, made a list of not less than 120 plants, almost all tropical, that are “certainly or probably dispersed” by the currents (Introd. Chall. Bot., p. 42). This is admittedly only a preliminary list, and as the result of recent investigations some plants have to be omitted and others to be added; but I doubt whether, numerically, it is far below the mark. The relative efficacy of the currents seems to have been first systematically discussed by De Candolle in his Géographie Botanique, which was published in 1855. Data were then very scanty, and out of a list of nearly 100 inter-tropical species (Old World plants found in the New World and New World plants found in the Old World) he designates nine only as exclusively dispersed by the currents. Even this list, in one respect, needs correction (see Note 33); but it is of interest to note that this eminent botanist from the first never looked upon the agency of the currents as a very important factor in plant-dispersal; and, finding in the specially directed and carefully performed experiments of Thuret confirmation of his views, he reiterated his opinion in a note to that author’s paper in 1873 (cited in Chapter III.).
However, De Candolle was quite right in minimising the effect of currents on the distribution of plants. His extensive survey of the plant-world from the standpoint of dispersal gave him that sense of proportion in assigning values to dispersing agents which enabled him to feel his way almost intuitively, even where exact data were often lacking. It is, however, a little disappointing to find such a slight treatment of the subject in Kerner’s great work on the Natural History of Plants, though one can scarcely controvert his opinion that the dispersion of plants, as a whole, is not appreciably affected by this process. Numerically speaking, this is in the main correct; yet it is here that the genius of Schimper led him to recognise and to mark out a line of investigation, fruitful in important results, in connection with the weighty question of “Adaptation.” If the author of this work has been able to add a little to our acquaintance with this subject, he owes much to the inspiration he received from Schimper’s memoir on the Indo-Malayan Strand-Flora.
Still, it must be admitted that the effectual operations of the currents as plant-dispersers are limited to the shore-plants with buoyant seeds or fruits. If we were to include in our list the shore-plants of temperate regions that possess seeds or fruits capable of floating in sea-water for long periods, and of afterwards germinating, the total for the whole world would not, I imagine, reach 200. We cannot here concern ourselves with those purely river-side plants that contribute their buoyant seeds and seed-vessels to river-drift, since there is no evidence indicating that river-side plants are effectively dispersed by the currents unless they also frequent the estuary and the coast-swamp; and in that case they come under the head of littoral plants. The total for the whole British flora would probably not far exceed a dozen, and nearly all of them are very widely dispersed.
The working value of the currents as plant-dispersers in the Pacific can be rudely estimated by the number of littoral plants with buoyant seeds or fruits that occur in the various groups. Most of these plants hail from the Indo-Malayan region. Speaking generally of the extension eastward of the Indo-Malayan strand-plants over the Pacific, Prof. Schimper (page 195) remarks that they become fewer and fewer in number as they extend farther from their original home, their number shrinking to a very few in the most remote groups of the Marquesas and the Hawaiian Islands. This is well illustrated in the following numerical results that I have prepared. Of the whole number, some seventy in all, of the littoral plants of the tropical Pacific with buoyant seeds or fruits, Fiji possesses about sixty-five, Tahiti about forty, and Hawaii only about sixteen. As shown, however, in Chapter VII., some of the Hawaiian littoral trees that are useful to the aborigines were probably introduced by them. The number actually introduced through the currents into Hawaii in all likelihood therefore does not exceed ten. There is a method in this diminution in numbers, as the plants migrate eastward and northward over the Pacific, which has been described in detail in the preceding chapter. The efficacy of the currents as plant-dispersers in the tropical Pacific therefore diminishes as we proceed eastward.
In the South Pacific the littoral plants preserve their Old World origin as far as the Polynesian archipelagoes extend eastward across to Pitcairn, Elizabeth, and Ducie Islands, where we find in one or other of them such characteristic Indo-Malayan beach trees as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa, Hernandia peltata, and Tournefortia argentea (see Note 34). In the more distant Easter Island there is a suspicion, for the first time, of immigration from South America in the presence of Sophora tetraptera. In the islands relatively close to the American continent, as in Juan Fernandez and in the Galapagos group, the Indo-Malayan strand-plants are no longer represented.
We come now to consider the relation between the distribution of the shore-plants and the currents. It is quite legitimate to discuss the currents of the Pacific from the botanist’s point of view, that is to say, from the standpoint of the distribution of littoral plants with buoyant seeds or fruits. For ages the buoyant seeds and fruits of the strand-plants of the tropical Pacific have been drifting over that ocean, and we have the results now before us in the dispersal of the species to which they belong. There is no necessity to endeavour to make the distribution of such littoral plants square with the arrangement of the currents as shown in a chart. The usual result of such a comparison has been to lead the investigator, whether an anthropologist, a zoologist, or a botanist, to find his facts at variance with the course of the prevailing currents. Man, animals, and plants have entered the Pacific from the west, whilst the most available currents are from the east; and one may be perhaps permitted the solecism that the Pacific islands have apparently been stocked with their shore-plants, with their aborigines, and with much of their fauna by currents running in the wrong direction. These Pacific islands could only have had a direct communication with the Old World, from which they have mainly derived their shore-plants, by the currents; but since both the aborigines and the plants have forced their way across the ocean to the Tahitian region in the teeth of the regular currents, indicated as such in the chart, we are compelled to assume that they have availed themselves either of the Equatorial Counter-Current or of the occasional easterly drift currents that mark the prevalence of westerly winds during the short season of the year when the easterly trade-winds do not prevail.
The Equatorial Counter-Current hypothesis would involve a preliminary crossing of the whole breadth of the Pacific Ocean, that is to say, a voyage of some 8,000 miles, before the drifting seed doubled back to the Polynesian Islands. The other view is a much more probable one, as is sufficiently indicated by the following extract from the “Admiralty Sailing Directions for the Pacific Islands” (II., p. 25, 1900).... “In the western part of the Pacific these trades ... are frequently interrupted by winds which blow from west or north-west, especially during the months of January, February, and March, when the north-west monsoon of the Indian Ocean extends out in the Pacific as far as the Samoa Islands.” In various works on this region one may find reference to canoes blown off the shore during this season and carried some hundreds of miles to the eastward. A ship can then sometimes sail with a fair wind from the southern end of the Solomon Group to the Fijis; and as we learn from Mariner, the crocodile may be at such times carried away from the Solomon Islands and stranded in Fiji. Mr. Hedley, in his exceedingly interesting paper on a zoogeographic scheme for the mid-Pacific (Proc. Linn. Soc. N.S.W., 1899), gives many details of this nature; but there is no space to deal further with the matter here.
After all, the botanist must take his cue from the drifting seed and the distribution of the plant. He finds the seed floating in the open sea as well as stranded on the beach. He then discovers the plant growing on the beaches, and by experiment he tests the floating capacity of the fruit or seed. Finally he ascertains the home of the plant. He does this for all the littoral plants with buoyant seeds or fruits, and he forms his own conclusions of the efficacy of the currents independently of the current-chart, remembering that he has in Time an important factor that the geographer does not possess in dealing with the currents. The effect of time has often been to obscure the differential results of the operations of the currents in the case of those species that, like Barringtonia speciosa, are almost universally distributed in the islands of the Pacific. It is obvious that such plants cannot aid us much in the matter of ascertaining the track followed by the drifting seed in entering this ocean. But if we find a littoral plant with buoyant seed or fruit that has only partially performed the traverse we shall possess in the interrupted operation an important piece of evidence.
Several years ago, in my paper on Polynesian plant-names, read before the Victoria Institute, I developed this argument when endeavouring to find in the floating seed a clue to the route pursued by the Polynesians in entering the Pacific. Since that time my acquaintance with these islands and their plants has been considerably extended; but no important modification of the principal argument is now needed. It was then pointed out that in Nipa fruticans, the swamp-palm of the Malayan Islands and of tropical south-eastern Asia, we have a plant well fitted for the purpose and one well known to be dispersed by the currents over small tracts of ocean. The Nipa Palm has attempted to enter Polynesia from the Malayan region by two routes, namely, by Melanesia and by Micronesia. Along the first route it has in the course of ages reached the Solomon Islands, where I found it in 1884. Along the second route it has extended its range to Ualan at the eastern end of the Caroline Group, where it was observed by Kittlitz many years ago, as indicated in the narrative of his voyage (Reise nach russische America, nach Mikronesien, etc., 1858, ii. 35), and in Dr. Seemann’s English edition of the same author’s Vierundzwanzig Vegetationsansichten ... des stillen Oceans.
The question now arises as to which of these two routes was taken by the drifting seed. In my paper I adopted the view that the shore plants reached Fiji and Samoa by Micronesia, that is to say, by the Caroline, Marshall, and Gilbert Groups. This is the route which, as mentioned by Mr. Hedley in the paper above quoted, Mr. Woodford prefers for some of the Lepidoptera; and it is the one that is favoured by Mr. Wiglesworth for the birds, since in his memoir entitled Aves Polynesiæ he remarks that certain indications tend to show that the Pelew Islands have served as a sort of bridge for the spread of species from Indo-Austro-Malaya right across the Pacific. Though I still think that the beach trees, most of which would find a home on the numerous coral atolls of the Marshall, Gilbert, and Ellice Groups, often followed that track, yet I am now inclined to consider that the mangroves and their associates, plants which find their most suitable home in the estuaries of large elevated islands, like those of the Solomon Group, in all probability reached Fiji in the mass by the Melanesian route.
Although the Old World has supplied to the Pacific islands most of their littoral plants that are dispersed by the currents, that is to say, the plants with buoyant seeds or seed vessels, yet there is an appreciable American element, and it is with the plants occurring in the New World that we are now concerned. The total number of the littoral plants of these islands that possess buoyant seeds or fruits is, according to the lists given under Note 35, about seventy. Of these about forty-five are exclusively Old World species, sixteen occur in both the Old and New Worlds, three are exclusively American, and six are Polynesian.
The question we have now to ask ourselves is whether the shore plants common to both the Old World and America have their homes in America, or whether they have been derived from the other hemisphere. With one or two exceptions, as in the cases of the Australian genera Dodonæa, Scævola, and Cassytha, which, as shown in a later page in this chapter, present no great difficulty, there does not seem to be any serious objection, as far as the numerical distribution of the species is concerned, in regarding America as a possible home of the genus. It is not often we shall come upon such a striking instance of the principle that where the species are most numerous there is the home of the genus, as in the instance of Cocos. The Coco-nut palm has been carried around the world through the agencies of man and the currents, whilst the home of the genus is in America.
Now assuming that in having to choose between the Old World and the New World as the home of most of the genera in the list we selected the latter, we have to ask ourselves in what degree this would be consistent with the place America holds with regard to the distribution of tropical shore-plants dispersed by the currents and with reference to the arrangement of the currents. If we except the African continent, there is no part of the world that bears such a definite relation to the currents as America, and with an ordinary chart of these regions their arrangement is to be understood at a glance. Yet strange to say, as far as the distribution of tropical littoral plants is concerned, America holds a position that the present system of the currents on its coasts will not altogether explain. Within the lifetime of the species of mangroves and other plants of the coast swamps that are found on both the Pacific and Atlantic coasts of tropical America the two continents of this name have been united by the emergence of the Isthmus of Panama.
Few things are more significant in plant-distribution than the arrangement of the tropical littoral plants with buoyant seeds or fruits, a subject that is discussed with some detail by Professor Schimper in his work on the Indo-Malayan strand-flora (page 190). These plants group themselves into four sections:—
(a) Those of the Pacific and Atlantic coasts of tropical America (including the West Indies) and of the West Coast of Africa. They include mostly plants of the mangrove-swamps and their vicinity, such as Anona paludosa, Avicennia tomentosa, A. nitida, Conocarpus erectus, Laguncularia racemosa, Rhizophora mangle, etc.
(b) Those of the Old World excluding the African West Coast and extending from the East Coast of Africa eastward to the Pacific islands. This is much the largest group and comprises many of the plants named in the list given in Note 35 under Old World species. One may cite as examples of plants ranging almost all over this area, Barringtonia speciosa, B. racemosa, Bruguiera gymnorhiza (in its most comprehensive sense), Carapa moluccensis, Derris uliginosa, Guettarda speciosa, Hernandia peltata, Heritiera littoralis, Pemphis acidula, Rhizophora mucronata, etc. Plants of the mangrove-swamp and of the beach are, therefore, here included.
(c) Those occurring all around the tropics and including many of the plants mentioned under Note 35 as Pacific island shore-plants found also in America. Most of them belong to the Leguminosæ, and there may here be mentioned Canavalia obtusifolia, Cæsalpinia Bonducella, Entada scandens, Gyrocarpus jacquini, Ipomœa pes capræ, Sophora tomentosa, and Vigna lutea.
(d) Those confined to a portion of the two great regions, such as Nipa fruticans in the Old World, and the Manchineel (Hippomane mancinella) to tropical America.
It is to be noted that the ubiquitous species do not include any of the mangroves. Each of the two regions has its own species, none being common to both the American and Asiatic regions, although, as is shown in Chapter XXX., the American species of Rhizophora is now seemingly breaking its bounds and intruding into the Pacific islands. On the other hand, some of the mangrove genera, Avicennia, Carapa, and Rhizophora, are found all round the globe, whilst others are restricted to one or other of the two regions, Bruguiera, Lumnitzera, and Sonneratia, for instance, to the Old World region, and Laguncularia to the American and West African region.
For convenience we may designate the two great regions of tropical strand-plants, with buoyant seeds or fruits, the American and the Asiatic regions, remembering that the first includes both coasts of America as well as the African West Coast, whilst the second extends from the East Coast of Africa to Polynesia. Excluding the ubiquitous species, these two regions are well distinguished from each other. If we look at the chart of the currents we perceive the reason of the American region including the West African Coast, and we see why none of the indigenous plants of this region occur on the African East Coast. So also with the Asiatic region, a glance at the chart will show that all the portions of its area are in connection with each other directly or indirectly through the currents, and that only time is required for the transport of buoyant seeds over most of the region.
Hitherto I have mainly followed Professor Schimper in this matter; but since my visit to Ecuador and the Panama Isthmus some further considerations have presented themselves to me. If the reader will look again at the map of the currents, he will observe that there is little reason for supposing that the Asiatic region can lend its littoral plants to the American region. On the other hand there are greater facilities, as far as currents are concerned, for America supplying the Asiatic region, namely by means of the great equatorial currents that course westward across the Pacific to the tropics of the Old World.
It would therefore seem that the American region can receive nothing by the currents from the Asiatic region. If accordingly it gives but gets nothing back, we are compelled to assign an origin in the American region to all littoral plants dispersed by the currents that are found in the tropics around the globe. This is what we have already regarded on other grounds as possible for nearly all the littoral plants of the tropical Pacific with buoyant seeds or seedvessels that are found in America. These plants are practically the same as those distributed around the tropical zone which are enumerated in the list given under Note 35, b. With their home in America, by crossing the Pacific they would ultimately arrive at the East African coast, where their course westward would terminate; whilst commencing their journey from the east side of the American continent they would reach the West African coast; and their distribution around the tropics of the world would be explained. There follow from these considerations the corollaries that a tropical strand-plant dispersed by the currents which has its birthplace in Asia could never reach the American region, and that American strand-plants are for the most part native-born, excepting those, if there are any, that hail originally from the African West Coast.
It is necessary in passing to explain the similarity of shore plants on the Pacific and Atlantic coasts of Tropical America. For the mangroves and their accompanying plants inter-communication between the two coasts is now impossible; and a communication between the two oceans must be postulated within the lives of the existing species. For the plants like Entada scandens and Ipomœa pes capræ, which occur inland as well as at the coast, it is easy to show that in the case of the Panama Isthmus, their seeds could be readily carried into the Atlantic and Pacific Oceans by rivers draining the opposite slopes of the same “divide,” so that the dispersal of the same species from a common centre into two oceans may be seen in operation in our own day. My observations on this subject are given in Chapter XXXII., to which the reader is referred.
I have now gone far enough to indicate the place that America holds with regard to the distribution of tropical shore-plants dispersed by the currents and with regard to the currents. There is every probability, as I venture to think I have shown, that the Pacific islands have derived most of their ubiquitous shore-plants with buoyant seeds or fruits from America. But one of the results of our discussion of America in this double aspect was that excepting in the case of the African West Coast it gives but does not receive plants from the Old World. We apply this test, with perhaps a little hesitation, to the shore-plants of the Pacific islands that are dispersed by the currents; and we find, as will be seen below, that it is responded to in a remarkable manner.
It has been observed in the previous chapter that scarcely any of the large-fruited beach-plants of the South Pacific islands, that could only have been dispersed by the currents, have reached Hawaii. We do not find amongst the truly indigenous coast flora of this group any of the following trees: Barringtonia speciosa, Calophyllum Inophyllum, Cerbera Odollam, Guettarda speciosa, Hernandia peltata, Ochrosia parviflora, Pongamia glabra, Terminalia Katappa, Terminalia littoralis, &c. It was also noted that the currents had not only failed to establish these plants in Hawaii, but that they had also failed to establish them in America, the suggestion being that the Hawaiian Islands had been, in part at least, stocked by the currents from America. That the Indo-Malayan strand-plants in their extension eastward over the Pacific should have failed to reach America, is a result we might have expected from the arrangement of the currents. Yet mingled with them we have plants like Ipomœa pes capræ, Canavalia obtusifolia, and Sophora tomentosa, that also occur in America. Since, however, their seeds are not better adapted for accomplishing the passage across the Pacific from the Old World to America than the equally buoyant fruits of the above-named littoral trees that have failed, the presumption arises that their home is in America, and that they have performed the easier passage across the Pacific westward from America to the Old World.
The exclusion of so many characteristic shore-trees from America that range often over the whole tropical region from the African East Coast to the islands of the Central Pacific, is not a matter of seed or fruit-buoyancy, but a matter concerned with the home of the species, and with the arrangement of the currents. Those shore-plants of this region that occur also in America have their home in that continent, and have subsequently been carried across the Pacific by the currents westward to the Asiatic shores.
The only exceptions, that I can recall, to the rule that America does not receive shore-plants dispersed by the currents from the Old World, are presented by the three Australian genera, Dodonæa, Scævola, and Cassytha, of which widely spread littoral species occur in America, namely, Scævola Lobelia, Dodonæa viscosa, and Cassytha filiformis. They offer, however, but little difficulty, since, as pointed out in other parts of this work, Dodonæa viscosa has probably been in part dispersed by man, whilst the other two species are as well fitted for dispersal by birds as by currents. The occurrence therefore of these species in America does not necessarily raise the question of the currents.
The same exclusive principle is illustrated in the scanty littoral flora of Hawaii. Deprived, like America, of the characteristic large-fruited beach-trees of the South Pacific, species that could only have reached it through the agency of the currents, it is scarcely to be expected that it would have received its few littoral plants with buoyant seeds from the source which has failed it in the cases of the numerous absentees. It is to America therefore that we look for the source of its littoral plants as far as the agency of the currents is concerned.
The Hawaiian Islands contain about twelve plants, named in the list given in Note 36, that possess seeds or fruits known to be dispersed by the currents, and capable, as experiments indicate, of floating in sea-water for prolonged periods. Not all of them are at present littoral in their station in this group; but their claim to be considered such in other regions is established in the Note above mentioned. Of these plants, seven at least are found in America, five in the Old World also, and two exclusively in America. This proportion of American plants is far greater than that characterising the whole littoral flora of the Pacific islands dispersed by currents, where out of some seventy species only nineteen are found in America (see Note 35). As far as the distribution of the plants is concerned, it is therefore quite possible that Hawaii has received most of its plants that are dispersed by the currents from tropical America.
We will now consider how such a possibility is in accordance with the arrangement of the currents in the North Pacific. If we look at the Quarterly Current Charts for this ocean published by the British Admiralty we notice that all through the year the Hawaiian Group lies more or less within the area of currents flowing from the West Coast of America, the Northern Equatorial Currents as they are collectively named. Except in the winter months these currents come from the N.E. and E.N.E., and bring drift from the coasts of British Columbia, Oregon, and Northern California. It is then that they pile up huge pine logs on the shores of the Hawaiian Islands, as I have described in Chapter VII. and in Note 30; and, according to Dr. Hillebrand, they transport this drift timber much farther south to the shores of the Marshall and Caroline Groups. One might cite other facts illustrative of the working of these currents, such as one finds in the pages of Fornander and other authors; but this would scarcely come within the province of this work. I may here remark that when in Honolulu I was informed that a bell-buoy which had got adrift on the Californian coast was subsequently washed up on the coasts of Kauai. It is stated in Findlay’s “North Pacific Directory” (1886, p. 1068), that a junk carrying nine hands that had been blown off the south coast of Japan in a typhoon, anchored, after ten or eleven months at sea, in December, 1832, near Waialea in Oahu, the view taken of its course being that after drifting along in the Japan Current it came within the range of the south-west current that carries pine timber to Hawaii from the West Coast of America.
The portion of the Northern Equatorial Current that strikes the Hawaiian Group during the greater part of the year is no doubt a south-westerly deflection of the Japan Current from the American West Coast; and it would be impossible to find any tropical drift mingled with the pine logs stranded on the islands during that period. However, in the winter months, centering in January, the Japan Current flows down the West Coast of America to about the latitude of Cape Corrientes on the coast of Mexico, before being deflected westward. Here it meets with a portion of the Peruvian Current, and both flow westward, the united stream striking probably only the southernmost islands of the Hawaiian Group. It is at this season alone that there would be any likelihood of drift from tropical America being stranded on the Hawaiian beaches, and it is quite possible that at such a time the Northern Equatorial Current may carry intermingled in its stream pine logs from Oregon and seed-drift from Panama.
I am not inclined to attach any value except in the Western Pacific to the agency of the Equatorial Counter-Current in transporting seeds and fruits over the Pacific. It presents seemingly the only opportunity of the transportal of the seeds and fruits of Asiatic littoral plants to America; but if at all effective in this way, it would have endowed the littoral flora of the western shores of tropical America with many of the trees so characteristic of the coral islands of the Pacific. In this sense, it has failed completely as an effective agency in plant-dispersal; and judging by results we may, I think, dismiss it from our consideration. However, Dr. Hillebrand (p. xv.) assumes that during the prevalence of south-westerly gales in winter in the Hawaiian Islands, the Equatorial Counter-Current would be pushed northward so as to mingle to the east of the group with the North Equatorial Current. In this manner it is supposed that seed-drift brought direct from the Asiatic side of the Pacific would be stranded on these islands. This appears to me to be most improbable, since some ten or twelve degrees of latitude usually intervene between the Hawaiian Group and the Equatorial Counter-Current (see Admiralty Sailing Directions, Pacific Islands, 1900, II., 31, and the Quarterly Current Charts; also Encyclopædia Britannica, vol. 18, p. 118).
The most serious objection from the botanist’s standpoint against such a view as that of Dr. Hillebrand is the absence from Hawaii of most of the shore-plants that we should expect the currents to have brought from the Old World. It is also evident that as far as the currents are concerned the Hawaiian Islands are far more likely to receive littoral plants from America than from the Old World. Though no tropical drift has yet been found stranded on the coasts of these islands, yet it is not unlikely that future investigators may find some seed-drift from Central America on the most southerly coasts of the group, as on the south-east shores of the large island of Hawaii. It would only be stranded in the winter months and then probably in small quantities.
Summary of the Chapter.
(a) Since the effective operations of the currents are limited to the shore-plants with buoyant seeds or fruits, such plants forming but a small proportion of any flora, it must be acknowledged that, numerically speaking, the results of the dispersing-agency of the currents on plant-distribution in general are but slight.
(b) Yet the importance of the subject is by no means to be measured by a numerical scale of results, a line of inquiry being here opened up leading to fields of investigation full of promise for the student of plant-distribution.
(c) Whilst dealing with the relation between the distribution of shore-plants and the arrangement of the currents, it is quite legitimate to discuss the currents of the Pacific from the point of view of the botanist, who, after all, must take his cue from the drifting seed and the resulting distribution of the plant.
(d) The shore-plants of the Pacific islands that are dispersed by the currents being mainly Indo-Malayan in origin, it follows that they have extended eastward over the Pacific to the Tahitian islands against the stream of the South Equatorial Current and against the trade-wind. It is, however, shown that they could have availed themselves of the interval between January and March when the North-west Monsoon reaches the Pacific.
(e) It is claimed that whilst the mangroves and their associated plants have for the most part entered the Pacific by the Melanesian route through the Solomon Islands, the beach-plants have also followed the route through Micronesia by the Caroline, Marshall, and Ellice Groups.
(f) A small number of the strand-plants of the Pacific islands that are dispersed by currents occur in America as well as in the Old World; and questions of prime importance arise when we have to decide whether their home is in the Old World or in the New World.
(g) Good reasons are given for regarding them as chiefly of American origin; and it is shown that America with regard to the arrangement of the currents stands in the singular relation of being a disperser but not a recipient of shore-plants.
(h) It is pointed out that the tropical shore-plants that are distributed by currents belong to two great regions which are the effect of the present arrangement of the currents, viz., the American including the West Coast of Africa, and the Asiatic comprising the remainder of the tropical zone. Each region has its own plants, and those that occur in both, being in fact distributed all round the tropics, are regarded, according to the principle above stated, as having their home in the American region.
(i) The occurrence of the same strand species on the Pacific and Atlantic coasts of tropical America is regarded as indicating that the arrangement of the existing species of its shore-plants, more particularly of the mangroves, antedates the emergence of the Panama Isthmus. This hypothesis is not needed for the coast plants like Entada scandens that occur inland, since we can now observe their seeds being carried down into the Atlantic and Pacific Oceans by rivers draining the opposite slopes of the same “divide” in the Panama Isthmus.
(j) It is shown that the currents of the Pacific have failed to establish the numerous beach-trees (possessing buoyant fruits) of the Pacific islands, not only in the Hawaiian Group, but also on the coast of America; and it is therefore argued that we should expect the Hawaiian Group to have received through the currents its shore-plants with buoyant seeds or fruits from the tropical west coasts of America.
(k) In support of this contention it is pointed out that most of the Hawaiian strand-plants that are dispersed by the currents are found in America, and some indeed in America to the exclusion of the Old World.
(l) The arrangement of the currents in the North Pacific also favours the view that the Hawaiian Islands are more likely to receive plants by the agency of the currents from America than from the Asiatic side of the Pacific.
IX. THE GERMINATION OF FLOATING SEEDS
Germination in the floating seed-drift of tropical estuaries.—A strain of vivipary.—Abortive germination of seeds in warm seas.—A barrier to plant dispersal.—The borderland of vivipary.— Summary.
THE tendency of the floating seed or fruit to germinate in the estuaries of tropical rivers is especially characteristic of the plants of the mangrove-swamps and of their borders. In the Fijian rivers, and particularly in the estuary of the Rewa, where the river-water is usually mixed with that of the sea, there are frequently to be found in a state of germination floating fruits of Barringtonia racemosa, Carapa obovata, Clerodendron inerme, Derris uliginosa, Smythea pacifica, &c.; whilst the floating fruits of more characteristic beach-trees like Barringtonia speciosa and Cerbera Odollam, that grow also on the sides of the estuaries, were never noticed in this condition. That this tendency should be restricted to the plants of the mangrove-formation and is not to be observed in the beach-trees is a singular fact. There is, however, an intermediate group of littoral plants mostly belonging to genera of the Leguminosæ and Convolvulaceæ, such as Mucuna and Ipomœa, where germination of the floating seed is apt to begin but ends abortively, and results in the sinking and death of the seed. The subject of the germination of seeds in the floating drift of tropical estuaries presents itself, therefore, in three aspects:—
(1) As concerning the plants of the mangrove-formation, where, excluding the viviparous species (when germination takes place on the plant), germination is frequent in the water:
(2) As concerning the beach-trees where it is rare or absent altogether:
(3) As concerning certain Leguminous and Convolvulaceous littoral plants where germination is not infrequent but always abortive.
Dealing first with the plants of the mangrove-formation, it may be remarked that the same tendency of the floating fruits or seeds to germinate, which is above noticed in the case of the estuaries of Fiji, came under my observation in the floating drift of the estuary of the Guayaquil River in Ecuador, the germinating fruits and seeds being carried far out to sea. The seeds of Anona paludosa, which float in quantities in the river-drift, were often found germinating; and the same may be said of the fruits of Laguncularia racemosa and of the “joints” of Salicornia peruviana which abound in the creeks of the mangrove-delta and are carried out to sea in the germinating condition.
It might be expected that this readiness to germinate in the brackish water of estuaries would prove to be a formidable obstacle to the dispersal of these plants over wide tracts of ocean. The exposed portions of the seedling might be deemed ill-suited to withstand, without injury, the “wear-and-tear” of transport by currents over long distances, even when not affected by the sea-water; and it might be thought that they would be often nibbled off by fish or destroyed by other aquatic animals. Only the specially organised seedlings produced by a viviparous process on the tree, such as those of Rhizophora and Bruguiera, might be regarded as able to survive the effects of prolonged immersion in the oceanic currents.
Observation, indeed, shows that such seedlings are exposed to and suffer from these perils; yet it is evident from the distribution of the species that, whether in the germinating condition or not, the seeds and fruits of Anona paludosa and Laguncularia racemosa have been carried by the currents from America to the West Coast of Africa. The seedlings of Avicennia and of Rhizophora mangle have also performed the same trans-Atlantic voyage. Those of both these mangroves are to be observed floating off the coasts and in the estuaries of both coasts of America. The seedlings of Avicennia are particularly abundant in the mangrove-creeks of the delta of the Guayaquil River; and I observed them in a healthy condition, ten to twenty miles out at sea, floating together with those of the Rhizophora. Since, as in the case of Rhizophora, germination occurs normally on the plant, Avicennia can only be dispersed by its floating seedlings. Yet it is noteworthy that although Avicennia seedlings appear, to a marked degree, less fitted for ocean transport than those of Rhizophora and Bruguiera, the species have a much wider distribution. Avicennia officinalis has a cosmopolitan distribution in the tropics and beyond, occurring as it does on the Atlantic and Pacific coasts of America, on both coasts of Africa, over Asia and Australia, as well as in New Caledonia and New Zealand, but not in Polynesia (Bot. Chall. Exped., III., 178).... I have now gone far enough to show that the tendency displayed by the seeds and fruits of several of the plants of the mangrove-formation to germinate either on the tree or in the floating drift of estuaries has not affected the general distribution of the species in its main outlines. Few fruits are found more often in a germinating condition in the floating drift of the Rewa River in Fiji than those of Barringtonia racemosa, yet the species ranges from the African East Coast eastward to Polynesia. Seedlings as well as seeds or fruits, whether or not in a germinating condition, are, therefore, able in such cases to disperse the species.
This readiness of the floating fruits of plants of the mangrove formation (excluding the viviparous species) to germinate in the estuaries is, I am inclined to think, due in the main to the strain of vivipary that runs through nearly all the plants of the mangrove-swamp and of its borders. It would, indeed, appear that the viviparous habit (the capacity of germinating on the plant) which finds its extreme development in Rhizophora and Bruguiera of the Fijian swamps is represented in its earliest stage in the readiness of the floating fruits of Barringtonia racemosa, Carapa obovata, &c., to germinate in the Fijian estuaries, and as remarked in Note 37 there is a suspicion of vivipary in the instances of both the species just named. Intermediate cases, as that of Laguncularia in the Ecuador swamps, occur in other regions with species where germination only takes place at times on the plant. This subject is, however, generally discussed in Chapter XXX. and need not be further dealt with here.
A predisposing cause of the germination of floating seeds and fruits in tropical estuaries would seem to be afforded by the super-heating of the water of the estuary. This came under my notice both in the Rewa River in Fiji and in the Guayaquil River in Ecuador, where the water of the estuary is often noticed to be some degrees warmer than that of the sea outside, and of the water from the river above the estuary. (See Note 38.)
We come now to the subject of the non-germination in tropical estuaries of the floating fruits of the beach-trees, such as Barringtonia speciosa and Cerbera Odollam, that in the Pacific islands may contribute to river-drift. Such trees may grow on the banks of the estuary, and their fruits would thus readily fall into the water; but in the Rewa estuary in Fiji it was evident that the fruits and seeds of beach-plants, such as Scævola Koenigii, are also brought in by the tide. The seeds of Morinda citrifolia were often noticed in the Rewa drift together with the fruits of Heritiera littoralis, which is both a beach and a swamp plant, but never in a germinating condition. The same remark applies also to the fruits of beach trees found afloat in the sea between the islands, such as Cordia subcordata, Guettarda speciosa, and Terminalia. It is possible that a few of these plants, as in the case of Barringtonia speciosa, display traces in the structure of their fruits of a lost viviparous habit. (See Note 50.) It is pointed out in discussing Guettarda that germination is much more easily induced than one would expect in the case of fruits with such a hard ligneous putamen.
An interesting subject is presented in the abortive germination of the floating seeds of many plants of the Leguminosæ and Convolvulaceæ both at sea and in a tropical estuary. My conclusions on this matter are based partly on observations made in Fiji, but mainly on the results of numbers of experiments, this being unavoidable, since the abortive germination causes the sinking of the seed. The principal determining cause of the germination in water of one of these floating seeds is evidently to be sought in the temperature of the water, it being immaterial for the earliest stage of germination, as many of my experiments indicate, whether the seed or fruit is afloat in the sea or in the river. In these flotation experiments, when conducted under warm conditions with sea-water, the earliest signs of germination were frequently displayed in the softening, swelling, and sinking of the seed. If the swelling seed is taken out in time and planted after a preliminary soaking in fresh water, the germinating process is at once resumed and is often successfully and rapidly completed; but if the seed is allowed to remain in the vessel after it has absorbed sea-water the vitality of the embryo is destroyed and the seed decays.
That many seeds would fail from this cause to cross an ocean my experiments repeatedly demonstrated. Nor does the appearance of a seed afford any indication of its probable failure to cross an ocean. Some seeds of Mucuna, as far as their hard coverings could guide one, would seem to be quite secure from such a risk. The stony seeds, for instance, of M. urens D.C. look as if they might safely be transported by the currents round and round the globe; and De Candolle very rightly placed this species in his scanty list of plants dispersed by currents. Yet few seeds are more treacherous when their buoyancy in sea-water is tested in a warm place, as in a hot-house. They may take up water, swell, and sink in a week, or they may float unharmed for a year.
The seeds most exposed to this risk are those of the Leguminous giant climbers, the lianes of the coast and inland forests of the islands of the tropical Pacific. They belong to the genera Mucuna, Strongylodon, &c.; and thus several of the plants that constitute for the student of plant-dispersal the enigmas of the Pacific are here included. The seeds of Mucuna are especially liable when afloat in sea-water under warm conditions to display the early signs of germination, swelling up and sinking to the bottom of the vessel, a process, however, soon arrested and followed by the death of the embryo unless the seed is removed in time. Yet the seeds of this genus are notably long “floaters.” Those of an American species, variously designated as Mucuna pruriens D.C. and M. urens D.C., have long been known to be washed ashore together with the seeds of Entada scandens on the western shores of Europe, and particularly on the Scandinavian coast, where they form regular constituents of what the Scandinavian botanists correctly term the Gulf-stream Drift.
Mucuna urens D.C. occurs with other American shore-plants that are dispersed by the currents on the African West Coast; and there is no reason to doubt that its seeds perform the trans-Atlantic voyage. It is found in Polynesia, in Hawaii, in the Marquesas, and according to Reinecke also in Samoa; and probably it occurs in other groups. The specific determinations of the genus, however, need thorough overhauling, so that it is not possible to deal more than in general terms with the distribution of a species. The distribution of Mucuna urens in the Pacific is, however, irregular, and no doubt this is to be connected with the uncertain behaviour of its seeds when transported by tropical currents. The seeds would, I venture to think, often sink through abortive germination in the warm areas of equatorial seas.
When in Hawaii I kept ten of the seeds of this species (M. urens D.C.) in sea-water for four and a half months, none of them sinking in that period, the temperature of the water rarely reaching over 80°F., the average daily temperature being 76-77°. However, when four years afterwards in England I placed five of the seeds obtained at the same time in sea-water under conditions where the water-temperature ranged for the first few weeks between 75° and 90°, three of them began to swell within ten days, and on removal at once germinated healthily. The remaining two were afloat at the end of twelve months, and when planted one of them germinated a month afterwards.
Having experimented on the seeds of about half a dozen different species of Mucuna in sea-water, all with buoyant qualities, it is possible for me to lay down the general rule for the buoyant seeds of the genus that sinking is the result of an attempt at germination, which, as before observed, proves abortive unless the seed is removed in time. It is obvious that the gardener wishing to raise plants of this genus without delay might profitably adopt the method of keeping them afloat in water at a temperature of 80-90° F. until they begin to swell, which may happen in some cases in a few days. Sea-water seems to produce the most rapid results.
When on Keeling Atoll in the Indian Ocean I collected, amongst the stranded seed-drift brought by the currents to those islands, the seeds of five or six species of Mucuna, two of which were identified at Kew as M. macrocarpa, Wall., and M. gigantea D.C. (see my paper on the dispersal of plants at Keeling Atoll). No plant of this genus appears up to that time to have been recorded from the Keeling Islands, so that at all events most if not all of the seeds had been brought by the currents from the Indian Archipelago, some 700 miles away. It may be added that amongst the drift gathered by me on the south coast of Java the seeds of three species of Mucuna were identified at Kew, including the two above-named species from Keeling Atoll.
These current-borne seeds of the Keeling beaches had probably performed an ocean journey of a thousand miles, since the route could scarcely have been direct. Yet their behaviour when placed eighteen months after in sea-water in a hothouse in England was most erratic. Of three seeds of Mucuna gigantea all swelled and sank within eight days. Two seeds of M. macrocarpa sank after floating from sixty to a hundred days; whilst of two seeds of another species both remained afloat after a year. In a sea-water experiment in England on five Hawaiian seeds of M. gigantea, under the conditions referred to in the Mucuna urens experiment, one sank within ten days, whilst three of them were afloat after twelve months, one of them subsequently germinating. This species, it may be remarked, is widely distributed as a coast plant over tropical Asia, Australia, and in Polynesia. It seems to take the place in the Old World which Mucuna urens takes in America, and it is curious that they meet in Polynesia, being sometimes associated as in Hawaii. In the chapter on my observations in Ecuador and in Panama it is remarked that Mucuna seeds are frequent constituents of river, sea, and stranded drift. I, therefore, have enjoyed the opportunity of observing the behaviour of the seeds of this genus in a variety of localities, namely, in the Keeling Islands, in West Java, in Fiji, Hawaii, and tropical America; and this may be pleaded as an excuse for entering into so much detail respecting them.
The large seeds of Strongylodon lucidum (S. ruber), a Leguminous liane that ranks with the species of Mucuna amongst the huge climbers of the forest of the Pacific islands, behaved in a similar way in my flotation experiments in sea-water. Though, as shown in Note 3, these seeds can float for a year and retain their germinating power, some of them brought their buoyant capacity prematurely to an end by an abortive attempt at germination. These black rounded seeds form a common object amongst the river seed-drift stranded on some of the Fijian beaches in the vicinity of estuaries. They are so hard and durable that they are mounted in brooches in Honolulu. Yet these pebble-like seeds will sometimes begin to swell in a few days in sea-water. Out of five seeds placed in sea-water in England under warm conditions (the water temperature for the first few weeks ranging between 75° and 90° F.), one swelled and sank within ten days, another did so after two months, whilst the other three were afloat after twelve months, and one of them subsequently germinated. There is some disagreement amongst botanists as to the limits of the specific characters of the plants of this genus (see Note 39); but the plan seemingly most in accord with the fundamental principles regulating plant-distribution in this region of the Pacific is to regard the forms found in Hawaii, Tahiti, and Fiji, as referable to one species. In addition to the Polynesian forms there are only two or three species, found in the Philippines, Madagascar, and Ceylon, and it is with the species from the last-named locality that the Polynesian species is by some identified.
The seeds of several other Leguminous climbers would probably act in a similar way, for instance, those of Entada scandens; but the seeds of this plant experimented on by me were too few to enable an opinion to be formed. Of four seeds of Dioclea violacea from Fiji that were subjected to the same experiment as those of Strongylodon lucidum, all floated in sea-water after a year, with the exception of one that did not swell and sink until after ten months. On the other hand, in my experiment in Fiji on the fresh seeds of Canavalia obtusifolia, a plant found on tropical beaches all round the globe, seventy per cent. sank in the first six or seven weeks, swelling and displaying the first signs of germination, but quite ten per cent. were afloat after three months.
My experiments on the foregoing and other littoral species of the Leguminosæ merely indicate that under the ordinary temperature of tropical currents a portion of the seeds will probably sink owing to abortive attempts at germination. It is likely that if in the experiments in England a constant temperature of 85° to 90° F. had been sustained throughout, most if not all of the seeds would have swelled and sunk within a month or two. The temperature of the experiments in Fiji and Hawaii did not exceed that of many tropical currents; but there are areas of superheating in equatorial seas, which I think would prove insurmountable barriers in the path of most drifting Leguminous seeds, a subject to which further reference will be made.
Coming to the Convolvulaceæ, my experiments show that the buoyant seeds often lose their floating powers from the same cause. Those of Ipomœa pes capræ may be taken as an example. I was surprised to find when experimenting on the buoyancy in sea-water of these seeds in Fiji and Hawaii that a considerable proportion, about a third, sank in the first two months, swelling and sinking to the bottom. That this swelling represented the early stage of germination was well brought out in parallel experiments in fresh water and sea-water made in England on the buoyant seeds of the British littoral species, Convolvulus soldanella. A good proportion of the seeds in the first part of the experiment absorbed water, swelled, and sank, those in fresh water proceeding at once to germinate healthily at the bottom, whilst those that sank in sea-water merely decayed. Of the survivors about fifty per cent. in either case floated after six months. It may be added that the seeds of other tropical littoral species, such as those of Ipomœa glaberrima and I. grandiflora, behaved in the same way.
It would appear from my experiments, and it is a result that we should expect, that buoyant seeds of the Leguminosæ and Convolvulaceæ would often float for much longer periods under cool than under warm conditions. There must be areas of high temperature in mid-ocean that would prove much more fatal to the chances of a drifting tropical seed than the icy waters of a Polar current. In my paper on Keeling Atoll I have described how I procured the germination of a seed of Ipomœa grandiflora, Lam., after a year’s flotation in sea-water in London, which included a period of three weeks when the water temperature was at or about 32° F. These seeds from this point of view would be exposed to much more risk of sinking through abortive attempts at germination when drifting across some parts of the Pacific Ocean. It would appear from the Admiralty Chart of Surface-Temperatures, published in 1884, that such an area with a surface-temperature of 83° to 86° throughout the year extends north and east of New Guinea well into the Pacific, reaching in the first half of the year as far east as the Tahitian region. It would seem highly probable that the immersion of Leguminous or Convolvulaceous seeds for many months in these tepid waters would in most if not in all cases induce incipient germination which would lead to the sinking of the seed. There are, however, exceptional cases, as that of Cæsalpinia bonducella, which, as my experiments recorded in Chapter XVII. indicate, appear to be quite proof against any conditions of temperature such as are likely to be found in tropical seas in the present day.
There are a few general considerations arising out of the foregoing observations to which reference may now be made. The study of the behaviour of the floating seed or fruit often carries us, as I have before implied, to the borderland of vivipary. When from a canoe on a Fijian river we lift up the germinating fruit of Barringtonia racemosa from amongst the drift floating past in the stream and pull down from the branches overhead the seedling a foot in length of Rhizophora, we hold in our hands the two extremes of the series of vivipary. With many of the plants of the mangrove-formation there is a fine adjustment with respect to the germinating capacity of the seed, or in other words a delicate balancing of organisation on one side and of physical conditions on the other. A slight disturbance of the equilibrium would produce great results in plant distribution. Thus, an elevation of the temperature of the sea-water in the tropics to 90° F. would, I apprehend, produce the abortive germination of nearly every floating seed and fruit in equatorial seas, even of those of the beach-trees like Barringtonia speciosa and Terminalia littoralis that are regarded as proof against such risks under existing conditions where the surface-temperatures would average 78° to 80°.
There would thus be a barrier to the dispersal of plants by currents as effective as that of a frozen ocean. In the warm, humid climates of the early geological ages, seed-transport by currents may have been often impossible, since the seeds that did not begin to germinate on the plants of the swamps would probably do so in the tepid water of the sea. Viviparous plants would, however, be placed at no greater disadvantage than they are at present, since the genera Rhizophora, Avicennia, and others are now only dispersed by the floating seedlings. But such an increase of temperature at the present time would mean the death in the current of the floating seeds and fruits of nearly all non-viviparous shore-plants. As a rule every Leguminous and Convolvulaceous seed would swell up and go to the bottom; whilst fruits like those of Barringtonia racemosa and Carapa obovata, that often germinate afloat in tropical estuaries, would invariably do so under the changed conditions, and the seedlings not being adapted for ocean transport would perish.
Yet we know that with the seeds of many inland plants temperature has seemingly very little to do with starting the process of germination. We are familiar with the fact that the seeds of many plants that fail to germinate in the summer of their production habitually germinate under apparently less favourable conditions of temperature in the following spring. This is attributed by botanists to the immaturity of the seed on first falling from the plant, a further period of maturation being necessary before, under any conditions, germination is possible.
We see this also well illustrated in the floating seeds and fruits of the Thames drift. Most of them fail to germinate in the drift at the end of the summer and the beginning of autumn, and defer the process until the following spring, when they germinate freely in the water under much cooler conditions than those which they experienced in the early part of their flotation in the drift. There are, however, exceptions to this rule. Plants like Caltha palustris, for instance, are rarely represented in the spring seed-drift of ponds and rivers, because most of the fruits or seeds germinated soon after falling into the water in the previous summer.
In most of my sea-water experiments in England the immersion had a very marked influence, not in causing premature germination and destroying the germinating capacity, as often happens with the floating seeds of Convolvulaceæ and Leguminosæ, especially in the tropics, but in postponing without injury to the seed the process of reproducing the plant. Such seeds or fruits when placed in fresh water after many months of flotation in sea-water germinated very freely in a few days, whilst those left in the sea-water under precisely the same conditions remained unchanged. This is true of many of the seeds and fruits found in the Thames drift, such as those of Ranunculus repens, Lycopus europæus, Rumex, &c. A striking instance was also afforded by the seeds of Arenaria (Honckeneya) peploides, where seeds transferred directly to fresh water, after many months flotation in sea-water, germinated in a few days; whilst those left in the sea-water remained unchanged. This subject is discussed at length in Note 19, and needs no further mention here.
If the seeds of many plants in Great Britain postpone through immaturity their germination to the following or even to the second spring, it goes without saying that this does not exclude temperature as the ultimate determining factor in germination. The immaturity of seeds adds another link to the series of the germination-range in plants. This range begins with the plants where germination takes place on the tree and the seedlings hang suspended from the branches, as in the typical mangroves Rhizophora and Bruguiera. Here, as is shown in Chapter XXX., there is evidently no period of repose between the completion of the maturation of the seed and the commencement of germination. The range ends with the detachment of immature seeds which ripen apart from the parent plant, and may postpone the germinating process for months and often for years. All intermediate stages exist between these two extremes. Thus the seedling may at once detach itself from the parent as in Avicennia, or the germinating process on the plant may be limited to the protrusion of the radicle as in Laguncularia, or the seeds may be quite mature and ready to germinate as soon as they fall to the ground, as we find with many small seeded plants. All the stages, of which only a few are here indicated, are full of suggestiveness for the student of plant-life.
This subject is dealt with from other standpoints in Chapter XXX., but the reader will now see more clearly what was meant when I said that the study of the behaviour of the floating seed leads us to the borderland of vivipary. In this range of the germinating process we may possess an epitome of the history of the climatic conditions of plant-life from an early era in the world’s story, beginning with those ages when perhaps under the uniform conditions that then prevailed, all plants were more or less coast-plants and more or less viviparous, and coming down to the present era when with an extensive and varied land-surface there is great variety both in climate and in the range of germination. The mangrove-swamp and its viviparous trees would thus represent from this point of view a condition of things once more or less universal on the globe.
Summary of the Chapter.
(a) The tendency of the floating seed or fruit to germinate in the brackish water of tropical estuaries is especially characteristic of the plants of the mangrove-swamp and their vicinity; but with those of the beach trees that occur in the river-drift it is rarely if at all to be observed.
(b) From the wide distribution of plants of the mangrove-formation it is evident that this readiness of the floating seed or fruit to germinate is not prejudicial to the dispersal of the species.
(c) It may perhaps be in the main attributed to a strain of vivipary running through all the plants of the mangrove-formation, which finds its extreme development in the viviparous species, where germination takes place on the tree. But it is probably favoured by the superheating of the waters of tropical estuaries.
(d) In the case of the buoyant seeds of several climbers and creepers of the Leguminosæ and Convolvulaceæ, more or less littoral in their station, it is shown that in warm water, whether fresh or salt, a good proportion are apt to sink through incipient germination, which results when the experiment is made in sea-water in the death of the embryo.
(e) Though in tropical currents of ordinary temperature a good number of such floating seeds would escape this risk, it is argued that there are certain warm areas in the tropical seas that would prove much more fatal to the chances of these drifting Leguminous and Convolvulaceous seeds than the icy waters of a polar current. It is thus held that these seeds often sink in mid-ocean in tropical latitudes through abortive germination.
(f) The study of the behaviour of the floating seed or fruit leads us to the borderland of vivipary. In the scale of the germinative capacity of plants it is possible to arrange a continuous series that commencing with the mangroves, where germination takes place on the tree, ends with those numerous inland plants where seeds are liberated in an immature condition.
(g) It is suggested that the viviparous habit may have been the rule under the uniform climatic conditions of early geological periods and that with the differentiation of climates that marked the emergence and extension of the continental areas the viviparous habit has been lost, except in those regions of the mangrove-swamps which to some extent retain the climatic conditions once general over the globe. With differentiation of climate the true seed-stage with its varying rest-periods has been developed.
Chapter X
THE RELATION OF THE BUOYANCY OF SEEDS AND SEEDVESSELS TO THE DENSITY OF SEA-WATER
The general principles concerned.—The subject assumes a statistical character.—Seeds and seedvessels are as a rule either much heavier than sea-water or much lighter than fresh water.—The present littoral plants with buoyant seeds or seedvessels could be equally well dispersed by currents in oceans of fresh water.—Seed-buoyancy has no relation either in the present or in the past to the density of the sea.—Though an accidental attribute, the specific weight of seeds has had a profound influence on plant-distribution.—Summary.
TO find amongst the results of my numerous experiments examples illustrating the influence of density on flotation has not been so easy as I at first imagined. Excluding all adventitious causes of buoyancy, a matter discussed in Note 40, it may be inferred that the great majority of seeds and fruits sink both in fresh water and sea-water. Of those that are buoyant many float indefinitely in both waters, whilst in a very few cases, where the floating power is derived from an outer fleshy covering, as with the fruits of Potamogeton natans, the fruits float a much shorter time in sea-water than in fresh water, on account of the injurious effect of the salt upon their coats.
Experiments have to be specially directed towards this subject. It would be useless to experiment in fresh water at one time and in sea-water a month later. Nor would it answer to employ seeds and fruits from different localities, since variations in this way sometimes occur. It is necessary that the experiments should be made on seeds or fruits collected at the same time and place, and that they should be simultaneous and carried on under the same conditions. As the discussion proceeds, the reader will perceive that many interesting points are opened up, and that such an investigation, instead of being, as the title of this chapter might suggest, an abstruse and disconnected inquiry, is of considerable importance in relation to the dispersal of plants through the agency of currents.
Guided by the results of my experiments in this direction I will proceed to lay down certain general principles:—
(A) In the first place it may be accepted as a general rule that seeds or seedvessels that sink in fresh water sink also in sea-water, the difference in density between the two being rarely a factor of any importance in determining buoyancy. The great majority of seeds and fruits come under this category, since, as is pointed out in Chapter VIII., only a small proportion of the whole, say a tenth, possess floating power. We might cite, as illustrative of this principle in temperate regions, almost all the 240 species included in the non-buoyant group of the British plants experimented on (see Chapter III. and Note 10). As a general rule this is true alike of the small seeds of the Cruciferæ and Scrophulariaceæ, of the nutlets of the Labiatæ and Boragineæ, of the genus Scirpus, and of the dust-like seeds of Juncus. The results of my experiments on the plants of the tropical Pacific are no doubt typical of other tropical regions; and if I wished to quote instances, I should have to enumerate not only most of the plants without buoyant seeds or fruits that are mentioned in the Fijian and Hawaiian lists given under Notes 2, 4, and 6, but also to appeal to tropical regions generally.
(B) One can carry the principle above-named yet further and say that not only as a rule do seeds or fruits that sink in fresh water sink also in sea-water, but that so far as tested many of them sink in water of much greater density than that of ordinary sea-water (1·026). Thus, for instance, the seeds of Nuphar luteum, Scrophularia aquatica, and Stellaria aquatica, the nutlets of Polygonum persicaria, and the achenes of Aster tripolium sank in sea-water the density of which had been raised to 1·050, the limit of the experiment. The minute seeds of Juncus communis and J. glaucus and the larger seeds of Luzula campestris, even after drying for six months, sank in salt water having a density of 1·075. It would, however, seem probable that for most of these small seeds and seedvessels a density of 1·100 would prove to be the critical point. If this is so, then most of those that sink in sea-water would float in the dense water (1·160) of the Dead Sea.
However, my investigations have only gone a small way in this direction; and perhaps some of my readers will pursue the inquiry. I will take the case of the nutlets of Scirpus palustris. They sink in fresh water and in sea-water, or may float in rare cases for a day or two. Out of 100 of these seed-like fruits, 25 floated in salt water of a density of 1·075, 13 in water of 1·050, 7 in sea-water (1·025), and 3 in fresh water, (1·000). It would thus appear that the proportion of buoyant nutlets is doubled with every increase of ·025 of the density scale. At this rate of increase they would all float in salt water of a density of 1·125, which may be regarded as the suitable medium for the flotation of the fruits of this Scirpus.... The seeds of Glaucium luteum, the Sea-Poppy, have no buoyancy either in fresh water or in sea-water even after prolonged drying. They all sank in water of a density of 1·050, but 18 per cent. floated when the density was raised to 1·075. At the rate of increase noticed in the case of Scirpus palustris, all the seeds would float in water of a density of 1·130-1·140.... The acorns of the Common Oak (Quercus robur) have usually but little buoyancy unless they have been long drying. After soaking in fresh water for half an hour 100 mature fruits, without the cupule, that had been kept a fortnight, I found that only 2 floated in fresh water, 6 in sea-water (1·025), and 18 in water of 1·050. At this rate of increase all would float in water having a density of 1·080-1·090.
(C) There is also another general rule, and it is this:—Seeds or fruits that float for a long time in sea-water usually float almost as long in fresh water. Here belong the greater number of buoyant seeds and fruits, those only able to float for a few weeks being comparatively few. Now with the long-floating seeds and fruits, those for instance that float in the drift of English rivers from the autumn to the spring, or those that are transported by currents over the tropical zone, there is, as a rule, but a slight difference between their flotation periods in fresh water and sea-water. If one of them sinks after floating for several months in fresh water, it will sink in sea-water a few days after. Fruits of Scævola Kœnigii, pyrenes of Morinda citrifolia, and seeds of Thespesia populnea, Ipomœa grandiflora, Cæsalpinia bonducella, and of different species of Mucuna, that had been kept afloat for a year in sea-water, floated just as buoyantly in fresh water at the close; and in those cases where any sank during the course of the experiment, it was ascertained that they were able to float in fresh water almost to the end.
That many of the seeds and fruits of tropical littoral plants that are known to be dispersed by the ocean-currents will float well in fresh water is shown in the constant occurrence in the floating drift of Fijian estuaries, where the water may be quite fresh or brackish, of the seeds and fruits of plants like Cerbera odollam, Clerodendron inerme, Entada scandens, Heritiera littoralis, Ipomœa pes capræ, Morinda citrifolia, Mucuna, Vigna lutea, &c. In the same way I noticed afloat in the Guayaquil River in Ecuador, when the water was quite fresh, seeds and fruits characteristic of the sea-drift, such as those of Anona paludosa (seeds), Entada scandens, Ipomœa, Mucuna, Vigna, &c.; and when we supplement observation with experiment, as for instance in the case of Anona paludosa, we find that they will float equally long in fresh and sea-water.
The same rule prevails with most of the buoyant seeds and seedvessels of plants of the British flora—seeds and fruits, as I may remind the reader, that are mostly to be found in river and pond drift. I am not able to distinguish any difference of importance in the results of the separate fresh-water and sea-water experiments. Thus with the seeds or seedvessels of Bidens cernua, several species of Carex, Galium palustre, Iris pseudacorus, Lycopus europæus, Ranunculus repens, and numerous others, the difference after a flotation of many months was but slight. If the results of the separate experiments were to be compared, there would be at least ninety afloat in fresh water for every hundred afloat in sea-water; and if at the end of a sea-water experiment, whether occupying three, six, or twelve months, the seed or fruits were to be placed in fresh water, quite nine-tenths and sometimes more would remain afloat. A striking illustration of the principle that the excess in density of sea-water, as compared with fresh water, adds but little to the floating capacity of seeds is to be found in the results given in Note 41 of simultaneous experiments made some years since by Mr. Millett and myself at Marazion and in London on the seeds of Convolvulus soldanella.
(D) In their relation, therefore, to the density of fresh water and sea-water, most seeds and seedvessels may be placed in two principal classes, the first including quite four-fifths of the total, where they are much heavier than sea-water, and the second comprising most of the remainder, where they are much lighter than fresh water.
(E) It would be surprising, however, if there were not some seeds or seedvessels that come between these two extreme groups; some, indeed, that have a specific weight approximating to that of fresh water, or to that of sea-water, or fluctuating between them, and presenting such evidence of a fine adjustment that the observer, forgetting that they are members of a series, might be apt to regard them as specially adaptive in their origin. It will thus be seen that this subject is gradually assuming a statistical character; and in truth we shall ultimately recognise here the play of the laws of numbers.
As an example of the plants where the specific weight of the seeds or fruits is near that of fresh water, Alisma plantago may be taken. In the course of an experiment, by lowering the density of the water from 1·025 to 1·020, I sent a shower of floating carpels to the bottom. The results vary considerably, as one might expect; but, generally, during the first few days of an experiment about twice as many (sometimes in all as much as 80 per cent.) sank in fresh water as in sea-water, a few only floating in either water for long periods.... The seeds of Arenaria peploides present an example where the specific weight is between that of fresh water and of sea-water. For the purposes of dispersal they may be considered as heavier than fresh water and lighter than sea-water. The details are given in Note 18; but it may be remarked here that plants possessing seeds or fruits that sink in fresh water and float in sea-water are very rare. As indicated below, this is what we might look for on statistical grounds.
Plants whose seeds or fruits are not much lighter than sea-water are exceptional. In such cases the effect of increased density of the water is to extend the period of flotation. Thus, in my experiments on the nutlets of Scirpus maritimus, the majority of the fruits floated in fresh water only eight to ten days; whilst in ordinary sea-water they floated in most cases two to three weeks; but when the density was raised to 1·050, the greater number of them were afloat after two months. In a few plants, as with Spiræa ulmaria, the effect of the difference in density between fresh and sea-water was not to extend the period of flotation, but to increase the number that floated for a given period, the extreme limit of the buoyancy of the carpels in either water with this species being about three weeks.
Amongst tropical plants, as illustrated by those of the Pacific islands, cases also came under my notice where the mean specific weight of the seed is somewhere between those of fresh water and sea-water. The seeds of Afzelia bijuga, an inland as well as a littoral tree in Fiji, offer an interesting example. If we place 100 seeds of a littoral tree in sea-water, we find that on the average about 70 float. If then we lower the density gradually, some of the seeds begin to sink at once; and on the removal of the survivors to fresh water, about 47 will remain afloat. The results may thus be stated:—Out of 100 littoral seeds, 30 are specifically heavier than sea-water (1·025); 23 are between sea-water and fresh water in specific weight; whilst 47 are lighter than fresh water (1·000). When, however, we take 100 seeds of inland trees, we find that on the average 87 are heavier than sea-water, 5 are in weight between sea-water and fresh water, and 8 are lighter than fresh water. The significance of these figures becomes evident when we arrange them in curves. The combined result for littoral and inland seeds is given in the diagram below; and we see there, what is also indicated with the separate curves that we are dealing with a double series, one concerned with seeds lighter than fresh water, and the other with seeds heavier than sea-water. The reader can himself supply the separate curves for the littoral and inland seeds. The point, however, to notice is that if a botanist with a statistical bent were to make a miscellaneous collection of the seeds of the Vesi (Afzelia bijuga) in one of the Fijian islands, in order to test their buoyancy, he would obtain such a result as is given in this diagram. Two varieties of the tree would be at once indicated, and further research would indicate that these varieties were connected with littoral and inland stations. This subject is further dealt with in Chapter XVII.
+------------------------------------------------------------------------------+ |Combined results for 200 seeds of Afzelia bijuga (100 littoral; 100 inland). | +--------------------------------+------------------------+--------------------+ | | | | | |Percentage.| Heavier than sea- | Between sea-water and | Lighter than fresh | | | water, or +1·025. | fresh water in weight. | water, or -1·000. | +-----------+--------------------+------------------------+--------------------+ | | | | | | 100 | | | | | | | | | | +--------------------+------------------------+--------------------+ | | | | | | 80 | | | | | | | | | | +--------------------+------------------------+--------------------+ | | | | | | 60 | | | | | | | | | | +--------------------+------------------------+--------------------+ | | . | | | | 40 | . | | | | | . | | | | +--------------------+------------------------+--------------------+ | | . | | | | 20 | .| | . | | | |. | . | | +--------------------+------------------------+--------------------+ | | | . . | | | 0 | | . | | | +--------------------+------------------------+--------------------+ | | | | | +-----------+--------------------+------------------------+--------------------+
It might seem strange that the seeds of Entada scandens should come into the category of seeds with a specific weight near that of fresh water; yet my observations in Fiji indicate that such is the case. In the discussion of this plant in Chapter XVII. it is pointed out that, as a rule, not more than a fourth will float in a river when they are first freed from the pod, and not more than fifty per cent. will float in the sea. Those that float, however, in either water will usually float indefinitely. The seeds also of Mucuna gigantea D.C. are not very much lighter than fresh water. Out of six seeds that floated in sea-water buoyantly, five floated in fresh water, but heavily.
It is of interest to notice in this connection that the mangrove-seedlings produced by germination on the tree, as in the case of Rhizophora and Bruguiera, have a mean specific weight somewhere between fresh water and sea-water. This is often illustrated in a curious way, when the seedling has not been prematurely detached from the tree. Thus in the sea off the coast of tropical America, as well as amongst the Fijian Islands, the seedlings of Rhizophora mangle are as a rule to be observed floating horizontally; whilst in the fresh or brackish water of the estuaries of these regions they assume a more or less vertical position, only the plumular portion protruding above the water. This is also true of the seedlings of Rhizophora mucronata, the Asiatic mangrove, and of Bruguiera rheedii. This subject is discussed in detail in Chapter XXX.; but it may be here remarked that a good proportion of Rhizophora seedlings, when detached in the mature condition from the tree, have no buoyancy, between 20 and 50 per cent. going to the bottom when they fall into a river, and between 5 and 10 per cent. when they drop into the sea. The navigator might often obtain an indication of the density of the sea-surface when approaching the mouth of a large river by observing the floating Rhizophora seedlings (a foot long) which are carried out to sea in numbers. If he sees them from the deck of his ship floating horizontally he will infer that the surface-water is mainly sea-water. In ordinary fresh water when they float vertically he would not be able to distinguish them from floating seeds or fruits.
It has only been possible to treat this subject in an illustrative manner. More details might have been given; but I have gone far enough to bring the following points into relief and to justify one in drawing the conclusions to be now stated.
Most seeds and seedvessels in respect of their floating powers tend to gather around two centres or means and to form two groups, the sinking group and the buoyant group.
In the sinking or non-buoyant group, which includes 80 per cent. of the whole, the mean specific weight is considerably greater than that of sea-water (1·026), which would require its density to be raised to 1·100 in order to serve as a floating medium for many of them.
In the buoyant group the mean specific weight is much lighter than that of fresh water (1·000); and from this it is to be inferred that in oceans of fresh water the same fruits and seeds in the mass would be distributed by the currents that are transported by them at the present day. Even though it arose from an ocean of fresh water, the coral island would receive the same littoral plants through the agency of the currents that it receives under its existing conditions.
The number of plants with seeds or fruits between fresh water and sea-water in specific weight is very small, probably not over 2 per cent. of the total. Most seeds or fruits that sink in fresh water sink also in sea-water, and most that float in sea-water float also in fresh water. Nature has thus created a wide gap between the sinking and the floating seed; and nearly all of the work of the present currents in plant-dispersal might have been effected, so far as the density is concerned, in fresh water. She has not arranged seeds and seedvessels in what the statistician would term “a good series.” As indicated in the diagram below, there are two series that meet in the neutral region where the density is between fresh water and sea-water, but with culminating points placed on the one side far above the density of sea-water and on the other far below that of fresh water.
+-------------------------------------------------------------------------------+ | Relation of the specific weight of seeds and fruits to the density of | | fresh and sea-water. | +--------------+--------------------+-----------------------+-------------------+ | Percentage. | Heavier than sea- |Between fresh and sea- | Lighter than fresh | | | water, or +1·026. | water, 1·000-1·026. | water, or -1·000. | +-------------+--------------------+-----------------------+--------------------+ | | | | | | 100 | | | | | | | | | | +--------------------+-----------------------+--------------------+ | | | | | | 80 | | | | | | | | | | +--------------------+-----------------------+--------------------+ | | . | | | | 60 | . | | | | | . | | | | +--------------------+-----------------------+--------------------+ | | . | | | | 40 | . | | | | | .| | | | +--------------------+-----------------------+--------------------+ | | | . | | | 20 | | . | | | | | . | | | +--------------------+-----------------------+--------------------+ | | | . | . | | 0 | | . .| | | | | . | | | +--------------------+-----------------------+--------------------+ | | | | | +-------------+--------------------+-----------------------+--------------------+
I do not, therefore, think that the buoyancy of seeds and fruits has had any relation either in the present or in the past to the density of the sea. Nor is it to be supposed that any slight variations in density in the course of ages would have materially affected the dispersal of plants by currents. It is to be inferred that the physicist and the geologist would be prepared to grant only small variations, such as a change from 1·020 to 1·025. It will be gathered from what has been said before that changes of this nature would have a very slight influence on the buoyancy of seeds and fruits, since the plants they would affect would be very few. The change that the student of plant-dispersal would require to produce any marked alteration in distribution would be in amount alarming to the physicist.
Whether or not the oceans have been getting fresher or salter in the course of ages (see Note 42), we will be moderate in our demands, and will listen to the physicist when he argues that a diminishing density, for instance, from 1·035 to 1·025, in the course of ages might explain some of the peculiar features in the present isolation of insular floras. Many seeds, he would contend, that could float across an ocean having a density of 1·035 would be unable to accomplish it when the density fell to 1·025. It has, however, been remarked that the critical point of density for the flotation of seeds or fruits that sink under present conditions is probably about 1·100. Cases of such a fine adjustment to the density of sea-water are too few to endow this argument with any weight. Or it might be suggested that with a gradual increase in density in the lapse of ages seeds might float now that sank before, or they might float for a longer period. Such a change, however, would not have much effect, since nearly all the seeds and seedvessels that sink in our rivers sink also in our seas, and a much greater increase of density is required to make any difference.
Yet, although we might term the sinking of a seed or fruit an accidental attribute of certain plants, just as we might regard the floating of a log as an accidental attribute of a pine, since in either case the specific weight might have been acquired without any direct relation to the density of water, still the sinking of the seed or fruit signifies a profound distinction not only, as is stated below, in plant distribution, but, as we shall see later on, in plant-development. Especially striking, says Prof. Schimper (p. 153), is the dependence between an over-sea area of distribution and a station at the coast in the case of species of the same genus of which some belong to the littoral and some to the inland flora. In the first place, as has been often remarked in these pages, we have a wide distribution generally associated with considerable buoyancy of the seeds or fruits. In the second case the areas are usually very restricted and there is little or no buoyancy. The better fitted a seed or fruit is for dispersal by currents the greater, therefore, is the area of the plant. Whether such an important relationship depends on an accidental attribute of the seed or fruit is the question that immediately presents itself. But it is obvious that in raising such a question we touch on a very vital point in adaptation, since if attributes developed in one connection have a profound influence in another we may have to rearrange some of our fundamental notions of the inner workings of Nature.
Let us, therefore, look a little closer into this matter, and turn again to the Pacific islands. The present state of things may be thus tersely described. Whilst the shore-plants dispersed by the currents have remained relatively the same, changes of all kinds, from the production of a variety and of a species to the development of a genus, have taken place in the inland floras. Now, let us imagine that all this is altered and that every seed or fruit is buoyant. There would then be but little distinction between the strand and inland floras, since they would be in a constant state of interchange, and most species would be widely distributed. A relatively monotonous aspect would belong to all insular floras, and indeed to much of the plant-world, since isolation, one of the principal conditions for the origin of new species and new genera, would often not exist.
On the other hand, let us suppose that all seeds and fruits were non-buoyant. The agency of birds would then be alone available for stocking new islands with most of their plants. The conditions of isolation would be intensified. There would be no widely-ranging strand-flora, since every island and every stretch of continental sea-board would possess its own littoral plants that could only reflect the peculiarities of the inland flora. The only determining factor between coast and inland plants would be the presence or absence of the capacity or organisation for occupying a station on the sea-shore.
We have now proceeded far enough to disclose the far-reaching influence on plant-distribution and on plant-development that the relation between the specific weight of seeds and fruits and the density of sea-water must possess. Yet it has been shown that when such a relation is viewed statistically it has an accidental aspect. We will accordingly devote the next few chapters to the discussion of the buoyancy of seeds and fruits from the structural standpoint.
Summary of the Chapter.
(a) The great majority of seeds and seedvessels (quite 80 per cent.) are much heavier than sea-water, but a noticeable proportion are considerably lighter than fresh water, whilst those with a specific weight near that of fresh water or of sea-water are very few.
(b) The buoyancy of seed and fruit has no direct relation to the density of sea-water, and even if the ocean was deprived of all its dissolved salts, the agency of the dispersal of plants by currents would not be materially affected.
(c) Small changes in sea-density, such as the physicist would allow, would, therefore, have no appreciable influence on the operations of the currents as plant dispersers; and only great changes in density, such as are presented by the waters of the Dead Sea, would add materially to the number of floating seeds and fruits.
(d) Although the specific weight of seeds and fruits in its relation to sea-density may be regarded as an accidental attribute, their non-buoyancy in the great majority of plants has had a far-reaching influence not only on plant-distribution, but on plant-development. The plant-world would be transformed if all seeds and fruits floated in sea-water.
(e) If the floating seed or fruit displays a quality that, so far as the density of the sea is concerned, has been developed in quite another connection, we have next to inquire whether the structure of such buoyant seeds and fruits also affords evidence of non-adaptation.
XI. ADAPTATION AND MEANS OF DISPERSAL
Nature has never concerned herself directly with providing means of dispersal.—Fleshy fruits not made to be eaten.—Nor “sticky” seeds to adhere to plumage.—Nor prickly fruits to entangle themselves in fur and feathers.—The dispersal of seeds a blind result of the struggle between the intruding Evolutionary power and the controlling influence of Adaptation.
BEFORE entering into a discussion of the causes of the floating powers of seeds, it is necessary that I should state my general position on the relation between capacities of dispersal in the organic world and the question of adaptation. Adaptation runs through all the organic and inorganic worlds, and we cannot conceive an universe without it. The naturalist who looks only for the end in the purpose served makes but a partially legitimate use of the phrase. On the other hand, it has been improperly appropriated by those who hold to the theory of Natural Selection, as indicating the result of small fortuitous variations that have chanced to be of service to the species in the struggle for existence. There is no question here of any end in view. Nature is represented as working blindly, and the result of such “fortuitous variation” is termed an adaptation. We cannot, however, pick and choose only adaptations that are very evident in their character. We must include everything in the organic world as an adaptation, whether apparent or not, that is in direct relation with the organism’s conditions of existence. It is not conceivable that an organism can be adapted to conditions outside its environment, and yet many so-called adaptations are of this character.
Nature—and I here confess my belief in a determining agency working above and through all living and dead matter, but largely controlled and checked by the laws of the physical world—Nature, as I apprehend, has never concerned herself directly with providing means of dispersal either for plants or animals. With regard to plants, she makes no direct provision for the distribution of their fruits or seeds. If she had done so, she would have employed some uniformity in her methods, as in the instance of the means of reproduction; whereas the modes of dispersal are almost infinite in their variety. When I say that Nature makes no direct provision for the dispersal of plants and animals, I mean not in the sense that a bird is adapted for an aerial life, or an aquatic plant for a more or less submerged existence. That a bird is often able to distribute its kind over a great area is the “accident” of its conditions of existence. In a similar way the wide distribution of the “ticks” that they carry round the world is due to the parasitical habits of these insects, habits that have been acquired without any view to their mode of dispersal by birds.
Similarly it cannot be said of seeds or fruits that are transported by birds, whether adhering to their plumage by means of hooks or hairs, or through some viscid excretion, or inclosed in soil adhering to the feet or legs, or carried in the stomach and intestines, that Nature has made any special provision for their dispersal. The dispersing agencies take advantage of certain capacities or characters of a seed or fruit that have been developed in the plant for quite other reasons and in conformity with quite other principles. There may be mentioned as examples the mucosity of seeds, the fleshiness of fruits, the occurrence of hairs and prickles, &c. Yet as far as their connection with dispersal is concerned, such capacities and characters are blind results in the history of the plant’s development, the dispersing agencies making use of what was not intended for them.
“Adaptation to definite life-purposes,” as Sachs terms it (Physiology of Plants, 1887, p. 122), is seen everywhere; but it is adaptation restricted to the organism’s conditions of existence. It is not conceivable, as I have said, that an organism can be adapted to conditions outside its environment. If there is such a seeming adaptation, it is but a blind result, the accidental outcome of collision or contact between two sets of conditions. If we represent a number of these sets of conditions by several circles gradually increasing in size until they encroach on each other, we find that the circles lose their form and acquire a polygonal shape. All characters seemingly connected with modes of dispersal have only this indirect relation to such agencies; and their utility in these respects is an accident in the plant’s life. They have not been acquired in connection with the dispersing medium, but are the products of the laws of growth and heredity, guided by a determining agency, and acting within the organism’s conditions of existence. It is within these narrow limits that all evident adaptations lie. In matters outside the conditions of the development of seeds and fruits, the evolutionary or determining principle “lets them go.” Detached from the plant, they come in contact with conditions for which they were never created. The predominant power in Nature, that brings to a successful issue the development of an organism, has its limitations, and this is one of them, the evolutionary or determining influence being ever checked and hampered by the laws of the inorganic world.
I can only refer briefly to some of the reasons that have led me to apply this view of the duality of forces in Nature to the subject of plant-dispersal. The principles of evolution and adaptation rule the world except in matters of dispersal. Take, for instance, the fleshy fruits which the gardener often makes more attractive to birds than they are in the wild condition. The result is certainly to increase their facilities for dispersal by birds; but such a result was as little intended by man as it was by Nature when species of Cornus, Ficus, Prunus, Viburnum, and other genera matured their drupes, berries, and fleshy fruits in the Cretaceous epoch.
Children are now taught in several excellent little books on “Nature-Study” that fleshy fruits are specially adapted to be eaten by animals to secure the distribution of the seeds. We read in one book that plants produce these fruits “on purpose to be eaten,” in another that they are “intended to be eaten,” and in a third that the seed-coverings are adaptations, all with the ulterior object of distribution by frugivorous animals. I must be pardoned if I venture to express my dissent from these statements, more especially since they are made by authors from whom it might be thought almost impertinent for me to differ. Yet authority can be claimed for holding the opposite view.
When the botanist speaks of “useless secretions” in a plant, he is alluding amongst other things to the sugar and organic acids of fruits. “How and why all these substances originate is,” as Professor Sachs observed in the work before quoted, “not known.” It is, however, suggested by Dr. Kerner, in his Natural History of Plants (Engl. edit. i, 460-462), that such secretions, though useless to plants, may exist for the purpose of alluring animals to assist in seed-dispersal. There are some botanists, it may be remarked, that would reject such a view of the nature of fruits. Dr. Stapf in his memoir on the flora of Kinabalu observes in this connection that the fact that a fruit is fleshy and attractive to birds is “no proof that it is really devoured by them, and still less that it is dispersed by them.” Neither in fleshy fruits, nor in minute seeds, nor in seeds capable of being transported by the wind does he regard the general object of the particular character as primarily to act as a means of dispersion.
The same plea is made for the mucosity of seeds like those of Capsella and Plantago (see Note 43), or for the “stickiness” of other seeds and fruits like those of Pisonia, qualities that favour adherence to passing objects. This is the reason, we are told, why seeds are “sticky.” Such secretions I infer are often materials lost to the plant; and being in that sense excretory we are not called on to supply a use for them. They can, therefore, not be regarded as having any teleological significance, since adaptation arises only from the requirements of the plant’s conditions of existence. If they are serviceable in assisting the distribution of seeds, such an event can only be described as an accident in the plant’s life arising from chance contact with another environment.
The appendages of seeds and fruits, such as hooks and hairs, that render them liable to adhere to fur or feathers, are also regarded as special adaptations to this end. Without entering into the physiological significance of hairs and prickles generally, concerning which, as many of my readers will know, much might be said not in favour of such a view, I would refer to cases like that of Cæsalpinia Bonducella, where the large prickly pods could not possibly be intended to aid the plant’s dispersal, whilst the leaf-branches are also prickly, and the seeds are well known to be distributed by the currents. There are other cases like that of Bidens cernua where the achenes, by reason of their barbed bristles, and on account of a layer of “buoyant tissue” in the fruit-coats, are dispersed both by birds and by water. We may fitly ask to which capacity the theory of adaptation should be applied. Spiny fruits may be sometimes so large, as in the instance of Trapa natans, that the question of adaptation to dispersal cannot be raised.
The great variety of the modes of dispersal of seeds is in itself an indication that the dispersing agencies avail themselves in a hap-hazard fashion of characters and capacities that have been developed in other connections. Seeds and fruits, having developed certain characters under a particular set of life-conditions, on being detached from the parent plant are brought into contact with conditions quite outside their original environment. Qualities and capacities are then brought into play which have no connection with the life-history of the plant. The care with which the mother plant guards the maturing seeds, and the protection of the environment, are at a certain period withdrawn, and the seeds are left to take their chance under strange conditions. It would be idle to see anything purposeful in the waste that results. Rather we would see in it the effect of one of the numerous limitations of the determining or evolutionary power in Nature. Such a power has to adapt its workings to the laws of the physical world, checked here, frustrated there, at times, as in this particular case, losing all control, but in the end prevailing.
My general position may be thus summarised. As concerning the distribution of fruits and seeds, the dispersing agencies take advantage of characters and capacities that were never intended for them, characters and qualities indeed that are often only brought out in relation to another environment. Thus no question of adaptation as regards means of dispersal can arise, since such capacities for dispersal have no connection with the plant’s life-history. That seeds are dispersed at all is a blind result of the ever-continued struggle between the opposing forces of evolution and adaptation; that is to say, between the determining power that lies behind organic life and the physical conditions to which it has to adapt its ends.
Chapter XII
THE CAUSES OF THE BUOYANCY OF SEEDS AND FRUITS OF LITTORAL PLANTS WITH ESPECIAL REFERENCE TO THOSE OF THE PACIFIC ISLANDS
The classification of buoyant seeds and fruits.—The first group, where the cavity of the seed or seedvessel is incompletely filled.—The second group, where the kernel is buoyant.—The third group, where there is air-bearing tissue in the seed-tests or fruit-coats.—The buoyant seeds and seedvessels of the littoral plants of the British flora.—Summary.
IN the following pages I have adopted in its main features the classification of buoyant seeds and fruits employed by Professor Schimper in his work on the strand-flora of the Indo-Malayan region. The causes of buoyancy, as he points out, are very various, but they can be arranged in a few categories; each category, however, usually admitting great variety within its limits. It is this want of uniformity that first attracts our attention when we come to study the structure of seeds and fruits from the standpoint of their buoyancy. Whilst in the Pacific I went over most of the field traversed by Professor Schimper in Malaya (the majority of littoral plants of these regions being common to both), and as a result I have added not a few plants to his original groups.
It will be seen from the following synopsis that there are three principal groups. The first group includes those seeds and fruits where the buoyancy is derived from unfilled space in the seed or fruit cavity. The second group comprises those seeds or fruits where the floating power is due to the buoyant kernel or nucleus. The third group includes those where the buoyancy arises from the existence of air-bearing tissue in the coverings of the seed or fruit.
The first two groups I will term the mechanical or non-adaptive groups, not only on account of the structure inducing the buoyancy, but because, as Professor Schimper remarks, the same structure often occurs with inland fruits and seeds possessing little or no floating power. In many of these cases, as he points out, the question of adaptation to dispersal by ocean currents cannot, therefore, be raised. The third group may be named the adaptation group, because it is on these examples of buoyant seeds and fruits that this investigator chiefly based his contention that in the main the structures concerned with buoyancy represent adaptations to dispersal by currents effected through the agency of Natural Selection. It is accordingly to this group that Professor Schimper especially directed his attention, and the result of his observations made in the home of the plants and of his investigations in the laboratory has been the elucidation of many difficult points in the structure of their fruits and seeds. To the two “mechanical” groups he did not pay the same attention; and as their examination came more within the limits of my own capacity as an inquirer I have worked them out with some detail, the subdivisions of the first group being my own as well as much of the material.
Synopsis of the buoyant fruits and seeds of littoral plants of the tropical Pacific classified according to the cause of buoyancy. (The authorities are indicated by the initial letter, S = Schimper, G = Guppy. Details are given under some of the species in latter part of volume.)
FIRST GROUP.—The floating power is derived from unoccupied space in the cavity of the seed or fruit, no part of the seed or fruit as a rule possessing independent floating power.
SUB-GROUP I., where the seed is concerned.
SECTION I. The seeds have little or no albumen, and neither the tests nor the seed-contents have any buoyancy. The cotyledons are generally large, foliaceous, and crumpled or folded, or otherwise arranged, so that the seed-cavity is incompletely filled.
S. G. Hibiscus tiliaceus. G. Hibiscus diversifolius. S. G. Thespesia populnea. S. Suriana maritima. G. Kleinhovia hospita, variable. S. G. Colubrina asiatica. S. Dodonæa viscosa. G. Argyreia tiliæfolia, variable. G. Ipomœa bona nox, variable. G. Ipomœa glaberrima, Boj. S. G. Ipomœa grandiflora. S. G. Ipomœa pes capræ. G. Ipomœa turpethum, variable. G. Cassytha filiformis. S. Euphorbia atoto.
Notes.—The species marked “variable” have seeds that sometimes sink and sometimes float. With the exception of Kleinhovia they are only at times littoral in station.
The plants of the British flora are represented by Convolvulus soldanella and C. sepium, the last being “variable” and not a littoral species.
SECTION II. All the seeds belong to the Leguminosæ. Neither the tests nor the seed-contents have any buoyancy, the floating power arising from a large central cavity produced by the bending outward of the cotyledons during the final shrinking stage of the maturation of the seed.
S. Mucuna (generically). G. Mucuna urens D.C. (Hawaii). G. Mucuna, species of. S. G. Vigna lutea. S. G. Cæsalpinia bonducella. G. Cæsalpinia bonduc. G. Entada scandens.
SUB-GROUP II., where the fruit is concerned.
SECTION III. The seed only partially fills the fruit-cavity, and as a rule is not buoyant. The fruit shell, usually woody, may be also buoyant.
S. G. Heritiera littoralis. G. Smythea pacifica. G. Dalbergia monosperma. S. G. Derris uliginosa. S. G. Pongamia glabra. G. Desmodium umbellatum. G. Gyrocarpus jacquini.
SECTION IV. The floating power is derived from empty seed-cavities, where owing to abortion of the ovule or some similar cause the seed is not developed.
S. G. Morinda citrifolia. G. Premna tahitensis.
Note.—Professor Schimper, in the case of Morinda citrifolia, holds the view that we have here a special adaptation to dispersal by currents.
SECOND GROUP.—Here the floating power is due mainly or entirely to buoyant kernels. In the case of seeds the tests are non-buoyant; but with “stones” the floating capacity may be aided by a layer of air-bearing tissue inside the shell.
SECTION I. Non-Leguminous.
S. G. Ximenia americana (drupe). S. G. Calophyllum inophyllum (drupe).
Note.—Professor Schimper would place these two plants in the second section of the third group on account of the layer of air-bearing tissue inside the shell of the “stone”; but they are assigned to this section, since the floating power is mainly due to the buoyant kernel.
Arenaria (Honckeneya) peploides, a British beach plant, belongs here.
SECTION II. Leguminous seeds.
G. Dioclea. G. Strongylodon lucidum. S. Canavalia (generic). G. Canavalia sericea. S. G. Canavalia obtusifolia. S. Erythrina (generic). S. G. Erythrina indica. P. Erythrina ovalifolia (Penzig). S. G. Sophora tomentosa. G. Afzelia bijuga. G. Lathyrus?
THIRD GROUP.—The floating power is due to the presence of air-bearing tissue in the seed-tests or fruit-coats.
SECTION I. The buoyant tissue occurs at the outside or forms the periphery of the seed or fruit. Unless otherwise indicated the fruit is implied in the list below.
S. G. Carapa moluccensis (seed). S. G. Carapa obovata (seed). G. Inocarpus edulis. G. Serianthes myriadenia. G. Parinarium laurinum. S. G. Barringtonia speciosa. G. Barringtonia racemosa. S. G. Pemphis acidula (seed). S. Terminalia (generic). S. G. Terminalia katappa. G. Terminalia litorea. S. Lumnitzera (generic). S. G. Lumnitzera coccinea. S. G. Guettarda speciosa. G. Wedelia strigulosa. S. G. Scævola Kœnigii. S. G. Cerbera Odollam. G. Ochrosia parviflora. S. G. Cordia subcordata. S. G. Tournefortia argentea. S. G. Clerodendron inerme. G. Vitex trifolia. G. Vitex trifolia, var. unifoliolata. G. Tacca pinnatifida (seed). S. Nipa fruticans. S. Cocos nucifera. G. Scirpodendron costatum.
Additions of shore-plants from Malaya and tropical America mostly given in Schimper’s work on the Indo-Malayan strand-flora.
S. Cynometra cauliflora. S. Conocarpus erectus. S. G. Laguncularia racemosa. S. Lumnitzera racemosa. S. Sonneratia (seed). S. Barringtonia excelsa. S. Scyphiphora hydrophyllacea. S. Wollastonia glabrata. G. Hippomane mancinella.
Note.—Here belong a species of Vitex, probably V. agnus castus, the fruits of which occur in the stranded drift of the Sicilian beaches, and also the British littoral shore-plants, Cakile maritima, Crithmum maritimum, Matricaria inodora, and Scirpus maritimus.
SECTION II. The buoyant tissue forms a layer inside the hard test of a seed or inside the shell of the “stone” of a drupaceous fruit, and to this cause the floating power is mainly or entirely due.
G. Mucuna gigantea (seed). S. Hernandia peltata. S. Excæcaria agallocha. S. Cycas circinalis. S. Pandanus odoratissimus. G. Anona paludosa (seed) of tropical America.
Note.—I have followed Schimper in respect to Pandanus, but it might be by some placed in the first section of this group.
Here belongs Euphorbia paralias, a British littoral plant, the buoyant seeds of which occur in the stranded seed-drift of English and Mediterranean beaches.
In the following general discussion of the groups, reference will be made only to the plants best illustrating the different varieties of structure connected with buoyancy; whilst mention of the other plants will in some cases be found in other parts of this volume, as shown in the Index; and the matter is discussed at some length in not a few of the species.
THE FIRST GROUP.
Of the first group, where the floating power is due to the unoccupied space in the cavity of the seed or fruit, the Convolvulaceæ offer the most typical examples. Here as a rule the crumpled embryo fills the seed-cavity more or less incompletely; and it is on the relative size of the unoccupied space that the sinking or floating of the seed depends. In those plants where the seed sinks the seed-cavity may be almost filled, as in Ipomœa tuberculata, or densely packed, as in Ipomœa pentaphylla, and in species of Cuscuta. When the seed floats, as with Ipomœa pes capræ, I. glaberrima, &c., the unoccupied space is relatively large; and when, as with I. bona nox and I. turpethum, the behaviour of the seeds is irregular, some floating, and others sinking, a corresponding variation exists in the extent to which the seed-cavity is filled. This applies also to the irregular behaviour of the seeds of Ipomœa peltata and of Argyreia tiliæfolia. A singular instance is afforded by the seeds of Ipomœa insularis, collected by me in Fiji and Hawaii. Those from Fiji were incompletely filled, and consequently buoyant. Those from Hawaii were more densely packed and sank.... The three British species of Convolvulus illustrate the same principle, namely, C. arvensis, with non-buoyant seeds; C. soldanella, with buoyant seeds; and C. sepium, with seeds irregular in behaviour.
In the case of plants of the Convolvulaceæ, possessing buoyant seeds, there is always evidence of marked shrinking of the seed-contents before the final setting and hardening of the seed-coats. The embryo often appears shrivelled and dried up, and is almost brittle, so that large spaces are produced in the seed-cavity. If we partly divide such a seed and place it in water, the embryo absorbs water rapidly, and within an hour is soft, healthy-looking, and much swollen, the interspaces being filled with a jelly-like mucilage. It is therefore evident that absolute impermeability of the seed-coats is essential for the successful transport by sea-currents of the floating seed; and we can only suppose that the shrinking of the seed-contents takes place before the final setting of the tests. That with the buoyant seeds the coats are quite waterproof was illustrated in many of my experiments where, after a period of flotation covering several months, and sometimes a year or more, the seed-contents were still quite dry and shrunken. The limit of buoyancy, as I have shown in Chapter IX., depends on an attempt at germination on the part of the floating seed, which then absorbs water, softens, swells, and sinks.
It is, therefore, not a matter of surprise that non-buoyant seeds of the Convolvulaceæ do not gain floating power after prolonged drying of many months. It is also to be expected that, as we find in Fiji, when a characteristic shore-species with buoyant seeds like Ipomœa pes capræ extends far inland, the seeds retain their floating powers. Seed-buoyancy of this description is, on the face of it, purely mechanical.
EXPLANATION OF THE DIAGRAMS ILLUSTRATING THE CAUSES OF SEED-BUOYANCY
1. Entada scandens (natural size): (a), the shell; (b), the kernel; (c), the intercotyledonary cavity. The shell consists of three coats—an outer and an inner hard chitinous coat, and an intermediate layer of brown cellular tissue containing little or no air. The buoyancy is due entirely to the central cavity, neither the seed-tests nor the seed contents possessing any floating power (see page 181).
2. Mucuna urens, from Hawaii (natural size). The kernel (b) sinks, and the shell has no floating power except where it possesses (under the raphe) a layer of dark brown, air-bearing, spongy tissue (a). This, however, is not sufficiently developed to endow the seed with buoyancy, which is due to the intercotyledonary cavity (c). (see page 111).
3., 4. Mucuna gigantea, from Fiji (natural size). The kernel (b) sinks, and the seed owes its floating power entirely to the existence in the shell (a) of a layer of brown, spongy, air-bearing tissue which is mostly developed at the circumference and is almost wanting at the flat sides of the seed (see page 115).
5., 6. Dioclea (violacea?), from Fiji (natural size). Here the kernel (b) is buoyant and endows the seed with floating power. Though the shell (a) possesses a thick layer of reddish-brown cellular tissue, this tissue contains but little air and aids the floating power but slightly (see page 113).
7. Strongylodon lucidum, from Fiji (natural size). The floating power is due entirely to the buoyant kernel (b). There is a very scanty amount of loose brown tissue (a) under the raphe; but it has no appreciable effect on the buoyancy (see page 113).
8., 9., 10. Cæsalpinia bonducella and C. bonduc, from Fiji (natural size). Neither the seed-tests (a) nor the kernel (b) have any floating power in themselves, the buoyancy being connected with a large internal cavity (c), which normally is intercotyledonary, as in Fig. 8 (C. bonducella). With both plants, but more especially with C. bonduc (Figs. 9 and 10), there may be a lateral cavity (d), or the kernel may be loose in the shell (Fig. 10), but this does not necessarily imply buoyancy (see page 194).
11., 12. Arenaria peploides (enlarged: seeds 4 mm. in size). Here the curved embryo (a) sinks, and the spongy air-bearing albumen (b) gives buoyancy to the seed (see page 116).
13. Euphorbia paralias (enlarged: seeds 3 mm. in size). The kernel (b) sinks, and the seed owes its buoyancy to a layer of air-bearing tissue (a) in the shell (see page 116).
14. Morinda citrifolia (enlarged pyrene 7 mm. long). The floating power is due to the bladder-like air cavity (a). The seed (b) proper is enclosed in the woody tissue behind the bladder (see page 112).
15. Cucurbita (seed enlarged), from the Valparaiso beach-drift (see page 125). The kernel (b) has no buoyancy. The shell (a) is formed of two layers of air-bearing tissue, the outer composed of prismatic cells and the inner of a spongy vacuola-material.
[To face page 111.
Another type of the buoyant seeds of the first group is presented by several species of Leguminosæ, as with Entada scandens, some species of Mucuna, and Cæsalpinia bonducella. As with the Convolvulaceous seed, the embryo sinks and the seed-shell has no buoyancy; but here the floating power is due to the existence of a more or less symmetrical long central cavity produced by the arching or bending outwards of the large cotyledons which lie usually in close contact with the seed-shell. This arching outward of the cotyledons depends on a shrinking process in the setting or final stage of the maturation of the seed. The stages of the process may be traced in the immature seeds, which are much larger and in some cases twice the size of the mature seed. In this immature condition the seed-coats are soft, and the flabby fleshy and thick cotyledons fill up the seed-cavity. As the hardening and setting process continues, the cotyledons diminish in size, become firmer, and gradually bend outward, leaving a central cavity. This arching outwards is no doubt in part the result of the contraction of the seed-tests during the shrinking process. Considerable variation prevails in the results, and where the cavity is very small the seed sinks. Further details relating to this subject will be given in my treatment of some of the plants, and especially under Cæsalpinia. But it may be here remarked with reference to Hawaiian seeds of Mucuna urens D.C., that although they are strictly referable to this group, they display beneath the hard test, on the side beneath the raphe, a scanty layer of dark spongy air-bearing tissue which is sufficiently buoyant to float up detached portions of the test, but does not of itself give buoyancy to the seed. The significance of this structure will be subsequently pointed out. The seed owes its floating power to the large central cavity, but this layer of spongy tissue adds to its buoyancy.
The section where the buoyancy of the fruit is connected with unoccupied space in the fruit-cavity is extremely heterogeneous in its composition. Every fruit has a method of its own, and the great variety of causes of buoyancy of a mechanical character is here exemplified. For instance, with Gyrocarpus jacquini and Cassytha filiformis the cause of buoyancy is in the main the same as that described in the case of the Convolvulaceæ. The origin of the floating power of the pods of Derris uliginosa is two-fold. In the first place the seed or seeds but partly fill the pod, and in the second place the seed is able to float of itself by reason of its possessing, as in the seeds of Entada scandens, a large central cavity produced by the arching out of the cotyledons during the final stage of maturation. A double cause is also to be assigned to the buoyancy of the fruits of Heritiera littoralis and of Smythea pacifica, where, in addition to the unoccupied space produced by the shrinking of the seed, the fruit-case itself floats, though nothing but a mechanical explanation is to be given of the floating of empty ligneous fruits.
One of the most suggestive types of buoyancy belonging to the first group is presented by those cases, which are, however, not very frequent, where the floating power is to be attributed to empty seed-cavities produced by the abortion of the ovule or failure of the development of the seed. A significant instance of this is afforded by the fruits of Premna taitensis, a coast plant. The buoyant “stone” of the drupe, which is often found afloat in the Rewa estuary in Fiji, is 4-locular, each cell containing normally one seed, but as a rule only one cavity contains a mature seed, the three other cavities becoming more or less empty through the failure of their seeds. It can be proved that neither the seeds nor the substance of the “stone” are buoyant, and that the “stone” owes its capacity of floating for months to the empty cavities arising from the failure in development of three out of the four seeds. In Fiji we see the rivers distributing these small fruits, and we find the “stones” stranded on the beaches and floating in the currents amongst the islands; and there can be no doubt that this is one of the effective modes of dispersal of the species; yet, if there was ever a case of accidental buoyancy concerned with dispersal by currents, we have it here. Further details are given in Note 32.
It is probably also to the abortion of the ovule, or to the failure of the seed, that the remarkable air-cavity (see Note 8) to which the pyrenes of Morinda citrifolia owe their floating power, is to be attributed. To this structure Professor Schimper (pp. 165, 183, 200) attaches considerable importance as an example of special adaptation to dispersal by currents through the influence of Natural Selection. He suggests, however, that possibly its morphological significance may be found in its being a peculiarly modified seed-chamber. The case of Premna taitensis above cited indicates that the latter view is the most probable. The subject awaits a careful microscopical study of the seed-development of the genus Morinda since, as elsewhere remarked, the non-buoyant pyrenes of inland species have not such an air-chamber. An outline sketch of a pyrene of Morinda citrifolia is given in the preceding plate. A good figure of it occurs in Schimper’s Plant Geography, p. 28. A very suggestive instance of this nature is described under Brackenridgea in Note 46 and in Chapter XIII.
THE SECOND GROUP.
Here are included those seeds and stone-fruits that possess buoyant kernels. Professor Schimper points out that since this is a feature both with inland as well as coast plants such a character cannot be viewed as an adaptation to dispersal by currents. The plants concerned belong mostly to the Leguminosæ, and we find here some of the most widely spread of strand species, such as Canavalia obtusifolia and Sophora tomentosa, as well as some of the giant climbers of the coast forests belonging to the genera Dioclea and Strongylodon. The kernels when divested of their coverings float buoyantly, but they soon absorb water and sink usually in a day or two, a circumstance indicating that it is to the impervious coverings that they indirectly owe their capacity to keep the seed or fruit afloat. It is noteworthy that seeds of Strongylodon lucidum from Fiji display beneath the raphe a trace of an internal layer of loose cellular tissue which, however, has no appreciable effect on the buoyancy; whilst with seeds of Dioclea (violacea?) from the same locality there is a thick layer of loose tissue which aids the floating power of the kernel but is not of itself sufficiently aeriferous to buoy up the seed.
This leads one to refer to two other plants belonging to this group, Calophyllum inophyllum (Guttiferæ) and Ximenia americana (Olacineæ), where, though the floating power is mainly due to the buoyant kernel, it is also aided by a layer of air-bearing tissue inside the hard shell of the “stone” of the drupe. Professor Schimper places these fruits in the third or adaptive group on account of the layer of buoyant tissue, but it would be more correct to class them according to the predominant cause of their buoyancy. It can be shown that with a non-buoyant kernel the “stone” no longer floats. This double cause of the floating power renders an explanation very difficult, since it would seem indefensible to give conflicting interpretations of their nature. With Ximenia americana there is another great difficulty. Its drupes are known to be dispersed by fruit-pigeons (Introd. Chall. Bot. p. 46); and judging from the rare occurrence of the “stones” in the drift there is good reason to believe that bird agency in the Western Pacific is predominant in the dispersal of the plant. It is by such test cases as this that we must put to the proof the reality or non-reality of the influence of adaptation on seed-buoyancy.
THE THIRD GROUP.
We have here those plants where the floating-power is entirely or mainly due to an air-bearing tissue in the seed-tests or fruit-coats. Several of the fruits are figured in Schimper’s Indo-malayische Strand-flora, and one or two are figured in the English edition of his work on Plant-Geography, p. 29.
In the first section, where the buoyant tissue occurs at the outside or forms the periphery of the seed or fruit, are included several of the most familiar of the littoral trees and shrubs of the Pacific islands, such as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa, Pemphis acidula, Scævola Kœnigii, Terminalia katappa, and several others named in the synopsis. I cannot enter into detail here, but the reader will find fuller particulars of each plant in most cases in Professor Schimper’s work, and in some instances in my separate discussion of the plants concerned. In nearly all cases we are concerned here with the fruits, and only in a few cases with the seeds, as with Carapa and Pemphis acidula.
This investigator observes that to this sub-group belong the fruits and seeds usually described in systematic works as provided with corky or suberous coverings; but he points out (p. 167) that the resemblance is nearly always quite superficial, and is limited to colour and consistence, suberous tissue occurring in only a few cases, as in the fruit-coats of Clerodendron inerme. The buoyant tissues, he remarks, are often more or less ligneous, and in those cases where there is no lignin reaction they resist the action of sulphuric acid much more effectively than pure cellulose; whilst in their physical characters, as well as in their behaviour with reagents, they differ just as much from ordinary cork. Thus, they are but little elastic and often easily crumble away; whilst in large fruits, like those of Cerbera and Terminalia, they would soon be stripped off entirely when subjected to the “wear-and-tear” of transport by currents, if they were not traversed by numbers of stout, tough fibres which hold the materials together. Where the buoyant tissues are firmer, as with Clerodendron inerme and Cordia subcordata, the fibrous framework is scanty or absent, whilst very small seeds or fruits, like those of Tournefortia argentea and Pemphis acidula, where the “wear-and-tear” would be comparatively slight, often possess no protecting fibres in the buoyant tissues.
In one or two fruits, like those of Cerbera Odollam, these tissues display large intercellular spaces; but in the majority of cases such spaces are insignificant in size or absent altogether. Speaking generally, however, there is, as Professor Schimper observes, great similarity in the structure of the buoyant tissue in the coverings of these fruits and seeds. The cell-walls are thin or only slightly thickened, and detached air-bearing portions of the tissue will float for many weeks. The great floating capacity of these fruits and seeds is stated by this investigator to be entirely due to the tenacity with which the air is retained in the covering tissues. It is, however, noteworthy that in the case of Scævola Kœnigii the fruits are just as well suited for dispersal by frugivorous birds as by the currents, a significant circumstance discussed in the next chapter.
The second section contains those plants where the buoyant tissue occurs inside the hard shell of the fruit or seed, such as is found, for example, in Anona paludosa, Mucuna gigantea, Hernandia peltata, Cycas circinalis, &c. Professor Schimper here includes Calophyllum inophyllum and Ximenia americana; but I have before remarked that the buoyancy of their fruits is mainly due to their buoyant kernels. This aeriferous tissue forms a layer between the seed or nucleus and the hard outer shell. It is described by the above-named authority as soft or friable and dark brown. The cells contain air and may be closely arranged or separated by small interspaces, their walls being neither woody nor suberous.
_The structure of the buoyant seeds and seedvessels of the littoral plants of the British flora._
The littoral plants with floating seeds or fruits form but a section of the strand-plants of the British flora, scarcely a third, as is pointed out in Chapter IV., of the total number. Though small in number they exhibit great variety in structure; and notwithstanding that as far as they have been examined they may all be referred to one or other of the groups and sections of the classification adopted in the synopsis for the plants of the Pacific islands, nearly every plant presents in the structure of its seeds or seedvessels a type of buoyant structure different from the others.
The first group is represented by the seeds of Convolvulus soldanella, which owe their floating power to the incomplete filling of the seed-cavity. The second group, where the buoyancy arises from the buoyancy of the kernel or nucleus, is illustrated by the seeds of Arenaria (Honckeneya) peploides, but in a fashion quite unique. The test is thin but impervious, and has no buoyancy; the curved embryo also sinks; and the floating power arises from the air contained in the loose spongy albumen, around which the embryo is coiled (see figure). A more normal component of the second group is represented in some Leguminous seeds, perhaps of Lathyrus maritimus, that occur regularly amongst the stranded seed-drift of the north coast of Devon. Here the kernel of the seed is buoyant. The seeds of Euphorbia paralias are indebted for their floating capacity to a layer of spongy tissue containing large air-spaces placed between the kernel and the chitinous outer test, neither of which possess any floating power (see figure). They thus belong to the second section of the third group.
The fruits of Cakile maritima, Crithmum maritimum, Matricaria inodora, and Scirpus maritimus, all belong to the first subdivision of the third group where the air-bearing tissue exists in the peripheral coverings, the seed or nucleus in all cases sinking. With Cakile maritima there is a light spongy outer case of aeriferous tissue, which, however, soon loses the epidermis, a circumstance that probably explains the limited period of flotation of about a week. The walls of the mericarp of Crithmum maritimum are composed of spongy cellular air-bearing tissue with a persistent epidermis, and the floating powers of the fruits are consequently great. The achenes of Matricaria inodora have beneath the epidermis a layer of buoyant tissue, and their structure is similar to that found with the buoyant achenes of littoral species of Wedelia, plants of the same order of Compositæ that are found on the Pacific islands. The cause of the floating power of the fruits of Scirpus maritimus lies entirely, according to Kolpin Ravn, in the air-bearing cells of the epidermis. The reader will find the results of my experiments on the buoyancy of the seeds in Notes 16, 17, and 18.
Summary of the Chapter.
(1) Following the main lines of Schimper’s classification of those of the Indo-Malayan region which possesses for the most part the same species, the buoyant seeds and fruits of the littoral plants of the Pacific islands are classed in three groups: the first where the cavity of the seed or fruit is incompletely filled, the floating power arising from the empty space; the second where the buoyancy is derived from the buoyant nucleus or kernel; and the third where it arises from air-bearing tissues in the coats of the seed or fruit.
(2) The first and second groups, in which the question of adaptation to distribution by currents through the agency of Natural Selection is not raised, since the same structural characters are found in seeds and fruits of inland plants not dispersed by the currents, are termed the mechanical or non-adaptive groups. The third is distinguished as the adaptive group, because it is here that Schimper finds evidence in favour of the Selection Theory.
(3) The first group is best represented by the Convolvulaceous and the Leguminous types. In the former, which is well illustrated by Ipomœa pes capræ, the seed-cavity is imperfectly filled by the crumpled embryo, the result of the shrinking process during the final setting of the seed. In the latter, which is exemplified by Entada scandens and Cæsalpinia bonducella, the seed displays a large central cavity produced by the arching outward of the cotyledons during the shrinking process accompanying the last stage of the maturation of the seed. As an instance of fruits belonging to the group, those of Heritiera littoralis may be cited. An uncommon type is presented in the “stones” of the drupes of Premna taitensis, and in the pyrenes of Morinda citrifolia, where the buoyancy arises from empty seed-cavities resulting from the failure of some of the seeds.
(4) The second group with buoyant kernels includes mostly widespread Leguminous species, such as Canavalia obtusifolia and Sophora tomentosa.
(5) The third or “adaptive” group comprises many of the characteristic littoral trees and shrubs of the Pacific islands, such as Barringtonia speciosa, Guettarda speciosa, Terminalia katappa, Tournefortia argentea, &c., that contain in their fruit-coverings a buoyant cork-like material often bound together by fibres, but which proves on examination to resemble cork only in appearance. In another type, illustrated by the fruits of Cycas circinalis and the seeds of Anona paludosa, the buoyant tissue forms a layer inside the shell of the seed or “stone.”
(6) Some fruits like those of Ximenia americana and Calophyllum inophyllum illustrate both the so-called mechanical and adaptive principles in their structure; whilst with the first-named species they are as well adapted for dispersal by frugivorous birds and are known to be a favourite food of fruit-pigeons. The same difficulty arises with the fruits of some other characteristic littoral plants, as with Scævola Kœnigii, the drupes of which are equally well fitted for dispersal by birds and currents.
(7) The same general principles have been at work in determining the structures concerned with the buoyancy of the fruits and seeds of British littoral plants. Although the species are few in number they exhibit in this respect great variety, eight species illustrating six or seven types of buoyant structure.
XIII. ADAPTATION AND SEED-BUOYANCY
The question of the operation of Natural Selection.—Are there two principles at work?—The presence of buoyant tissue in the seed-tests and fruit-coats of inland plants, both wild and cultivated.—Useless buoyancy.—The buoyancy of seeds and fruits is not concerned with adaptation.—Summary.
WHEN we speak of a certain structure as an adaptation to dispersal by currents through the agency of Natural Selection, it is necessary at the outset to be quite clear as to what is implied. Professor Schimper, who brought his great and varied knowledge of many other phases of plant-life to bear on this subject, is careful to clear the ground of preliminary erroneous conceptions in such a perspicuous and impartial manner that we cannot do better than follow his guidance. There are, he observes (p. 178), many mechanisms or contrivances in plants, which, though they seem to have arisen with a fixed purpose, can in no wise be regarded as having been developed for that end, since they were produced in quite a different connection and have merely acquired a new or supplementary function, of which they are the cause and not the effect.
This is very much the position that I have taken up for the whole subject of the relation between plants and their dispersing agencies, and it will be found discussed in Chapter XI. It involves, as I venture to think, a dominant principle in the organic world, which it is one of the objects of this work to emphasise, namely, that Nature in dispersing plants habitually makes use of structures and capacities that were originally developed in quite another connection. Behind this change of function, this new purpose, lies the secret of the organic world. There is for me no more pregnant fact in plant-life than the thistle-seed blown before the wind, or the seed of our sea-convolvulus floating in the sea. It proves to my mind that the evolutionary power in nature is checked and hampered by conditions not of its own creation, and that two opposing forces are ever at work, the one creating and the other limiting the creative power, the actual mode of dispersal being but a blind and accidental result of the struggle.
The question of the operation of Natural Selection is not raised, as Professor Schimper indicates, until we consider whether the new function has had any bettering influence on the structure or mechanism with which it has come to be concerned. If such a modification is thus brought about it might be legitimately claimed as a result of this agency, and the term “adaptation” could be used. But if there is no evident change produced, we should be compelled to assign very subordinate limits to the capacity of Natural Selection; and in the instance of buoyant fruits and seeds it would be restricted to determining a plant’s station by the water-side and in increasing its area. It is only in the first case that we could speak of them as adaptations in the meaning attached to this term in the language of the Selection Theory. It would at first sight seem easy to ascertain whether the characters of fruits and seeds, to which the buoyancy is due, are adaptations in this sense of the word; but in reality it is far from being so. We can, however, proceed with unanimity up to a certain stage in the argument; but there agreement ends.
It has been before established that in the Pacific islands, and indeed in the tropics generally, the plants with buoyant seeds or seedvessels are mainly stationed at the coast. It has also already been shown that this littoral station is often associated with a special buoyant-tissue in the coverings of the seed or fruit; and it will now be pointed out that this tissue is, as a rule, absent or but scantily developed in the case of inland species of the same genus. Of great importance, remarks Professor Schimper (p. 179), in relation to the Selection Theory and the development of adaptations, is the comparison of the fruits and seeds of strand-plants with those of allied inland species; and he finds here evidence in support of the Darwinian view. He takes the cases of the genera Terminalia and Calophyllum, which contain both inland and littoral species; and he shows that although the same buoyant-tissue occurs in the fruit-coats of inland species, it is there much diminished, and in consequence the floating powers are considerably lessened or lost altogether (see Chapter II.). It is not pretended that this tissue has had any connection in its origin with dispersal by currents, but merely that its greater development in the shore species is an adaptation to this mode of transport.
Further testimony is adduced by this investigator (p. 182) in supporting his view in the fruits of the genera Barringtonia, Clerodendron, Cordia, and Guettarda, where the buoyant tissues extensively developed in the coast species are either non-existent or only represented by a trace in the inland species of the same genus, a difference in structure associated with the loss or great diminution of the floating capacity of the fruits concerned. I have been able to establish other examples in the cases of the genera Scævola and Tacca, which will be found referred to in Chapter II.
Professor Schimper (p. 200) points to the circumstance that the “adaptations” in these fruits all belong to the diagnostic marks of the genera and the species, and contends that these plants abundantly prove the erroneous nature of the contention that Natural Selection could have played no part in the elimination of the strand-flora. My own contention is that Natural Selection has played such a part, but that in doing so it has merely availed itself of characters previously existing, without originating, modifying, or improving them in any way. The foregoing evidence might with equal fitness be employed to show, as pointed out in Chapter II., that in the course of ages there has been a great sorting process by which, excluding the mangroves, plants of the xerophilous habit possessing buoyant seeds and fruits have been sorted out and placed at the coast. Direct evidence does not lead us farther than to the establishment of a littoral station for plants thus endowed. The problem whether the characters of their fruits and seeds that are concerned with buoyancy may be regarded as adaptive in the Darwinian sense lies beyond the reach of direct testimony. We can, however, approach it from the outside by several directions, and from some of these we will now proceed to deal with it.
There is first the singular circumstance that in Fiji, when the littoral plants with buoyant seeds or fruits leave the beach and extend far inland, they, as a rule, retain their floating powers and, of course, their buoyant structures. I found this to be true of Cassytha filiformis, Cerbera Odollam, Ipomœa pes capræ, Morinda citrifolia, Scævola Kœnigii, and one or two other plants mentioned in Note 44, where this subject is discussed. My experiments on these plants indicated that their fruits or seeds floated equally long, whether obtained from coast or from inland plants. This, at first sight, appears to present a serious objection to the adaptation theory; but it was not so regarded by Professor Schimper, who in a letter to me, dated March 8th, 1900, observed that he did not see “why littoral plants growing inland should lose their adaptations to littoral life, especially if those adaptations are not conflicting with the conditions of life beyond the littoral zone, and if the competition does not require special adaptations.”
My view, however, is that any process of adaptation is unnecessary. All these plants, it is contended, were originally inland plants that acquired the buoyant qualities of their seeds and fruits in the inland stations, and ultimately found a station at the coast through the sorting process above referred to. In the case of plants like Ipomœa pes capræ and Cassytha filiformis this would be conceded, since they belong to the acknowledged non-adaptive groups discussed in the preceding chapter. It is only to some of these plants, such as Scævola Kœnigii and Cerbera Odollam, that the adaptation view of Professor Schimper is applied; and the question arises whether we are justified in making such a distinction, or, in other words, whether it is antecedently probable that two independent principles have been at work in determining the fitness of seeds and fruits for dispersal by the currents.
The plants for which the influence of adaptation through Natural Selection is claimed belong, as stated in Chapter XII., almost entirely to the third group. It is admitted that with the other two groups the utmost that any sorting or selecting process would effect would be to determine a station at the coast and to extend the area of distribution. The numerical aspect of the question therefore acquires some importance; and the reader’s attention is accordingly directed to the results tabulated in Note 45, where it is shown (assuming for the time that there is no difference of opinion about the adaptive significance of the seeds and fruits concerned) that the plants of the third or adaptive group make up only about half the total. It would therefore appear that if the agencies of Natural Selection have been at work here either in bettering or in developing buoyant structures, half of the shore-plants with buoyant seeds or fruits have not come within their influence.
But the subject takes another aspect when we reflect that in some buoyant fruits, as with Ximenia americana and Calophyllum inophyllum, the two principles would seem to have been at work. Whilst from this standpoint Natural Selection is regarded as having either developed or increased in amount the layer of buoyant tissue in the fruit-coats, the buoyant kernels are not viewed as adaptive in their origin. In the case of Ximenia americana the dispersing agency of frugivorous birds adds another factor, since, as before stated, its drupes are known to be dispersed by fruit-pigeons. In the cases of Scævola Kœnigii and of Vitex trifolia, two plants belonging to the adaptive group, Professor Schimper (pp. 156, 188) admits also the dispersing agency of frugivorous birds, and he claims it for Morinda citrifolia, in the pyrenes of which he also detects a special adaptation to dispersal by currents. It may be added that, as he also points out, fruits of the non-adaptive group of littoral plants, such as Premna integrifolia (P. taitensis) and Cassytha filiformis, would sometimes also attract birds. In fact, those of the last-named have been found in the crops of pigeons (Introd. Chall. Bot., p. 46).
Looking at all these littoral plants with fruits that are equally fitted for dispersal by birds and by currents, we may now ask, Where does the general principle of adaptation to dispersal lie? Whatever view we adopt, we must apply the same view to all, whether it be a question of dispersal by birds or by currents. We cannot choose between two sets of principles determining the buoyancy of seeds and fruits any more than we can regard a fleshy drupe and a buoyant seed as illustrating different principles regulating the dispersal of plants. Nature works with uniformity in these matters, and if the Natural Selection theory is held to explain one case it ought to account for all. Yet nobody would go so far as this; and this view of dispersal is on many grounds antecedently improbable. These difficulties disappear if we assume that in all cases the dispersing agencies have without modification made use of characters and capacities that were developed, as we now see them, in quite other connections and under quite other conditions.
It will now be necessary to look a little closer into the subject of the buoyant tissue, to the existence of which in their coats about half of the littoral plants concerned owe the floating power of their fruits or seeds. In the first place, it is to be remarked that in the case of some of the seeds of the plants of the non-adaptive groups it is also represented to a small degree in the seed-coats, although, as with Strongylodon lucidum and Mucuna urens, it is not present in sufficient amount to float the seed. In the next place, it should be noted that with some genera possessing, like Terminalia, both inland and coast species it is to be found alike in the fruit-coverings of inland and of littoral plants, though in a less degree in the case of the fruits of inland species, the floating power of which is proportionately diminished. There are, however, a few cases where this buoyant tissue is developed in inland species which belong to genera or subgenera that have no littoral species. This is what we would expect, if Natural Selection has merely concerned itself with placing plants of xerophilous habit possessing buoyant seeds or fruits at the coast. Under such conditions we would now and then expect to find an inland plant possessing buoyant fruits or seeds of this description that has never been able to establish itself at the coast.
A good instance is afforded by Pritchardia Gaudichaudii, a fan palm peculiar to Hawaii, the drupes of which float for several weeks and have a covering of spongy buoyant tissue (see Chapter XXV.). The seeds of Hibiscus Abelmoschus, a species distinguished subgenerically from the littoral Hibiscus tiliaceus, offer another example. They float for months, and owe their buoyancy to a layer of air-bearing tissue between the kernel and the test, in this respect differing from the seeds of the littoral species, where the floating power is due to unoccupied space in the seed-cavity. The buoyancy of the seeds of Hibiscus Abelmoschus thus offers another example of ineffectual floating power, since it is not a littoral plant, is often cultivated, and has accompanied aboriginal man over much of the tropical zone.
A singular instance of the dispersal by currents of an inland plant that occurs both wild and cultivated in tropical America, the West Indies, and on the West Coast of Africa, is afforded by Spondias lutea, Linn., which is referred to at the end of Chapter XXXII. Its “stones,” which are provided with a cork-like covering much as we find with those of Cordia subcordata and Guettarda speciosa, possess great buoyancy, and are found in the river and beach drift of those regions with the seeds in a sound condition.
A very remarkable case of ineffectual buoyancy is presented by the seedvessels of Brackenridgea, which have been found floating in the drift off the coast of New Guinea. They owe their floating power to closed cavities which would seem to arise from the failure of one of the seeds or from the abortion of an ovule. But, according to Beccari, their fleshy coverings would aid their dispersal by frugivorous birds; and since the species are all much localised and are rarely littoral in their habit, it is very probable that birds have mainly effected the dispersal of the genus (see Note 46). It has, however, been shown in the previous chapter that Premna taitensis and Morinda citrifolia owe their dispersal by currents to similar cavities in the seeds or “stones.”
Amongst the inland plants possessing seeds or fruits that are dispersed by the currents without aiding the distribution of the species may be recognised types of both the adaptive and non-adaptive groups. A singular instance is afforded by the large seeds almost an inch long of a huge pumpkin (Cucurbita) which, in sound condition, form one of the commonest constituents of the beach drift on the coast of Chile from Valparaiso northward to Iquique. The fruit is commonly eaten by the lower classes. The seeds, which are very buoyant, contain a kernel that does not float, the buoyancy being due to the water-tight coats which, as shown in the plate in Chapter XII., possess well developed air-bearing tissues. It may here be observed that Martins refers to the germination of seeds of Cucurbita pepo after 45 and 93 days’ flotation in sea-water.
One sometimes finds buoyant tissue developed in the seeds of bottle-gourds, where it can serve no useful purpose of dispersal. Thus small bottle-gourds, seemingly of the genus Cucurbita rather than of Lagenaria, are to be commonly found afloat in the Guayaquil River and stranded on the Ecuador beaches. They will float for many months, and contain the seeds dried up into a small loose compacted mass in their interior. These seeds, which contain a layer of spongy air-bearing tissue in their coverings, will in several cases float for months. Some that I had been keeping two months afloat in sea-water germinated freely. It is shown in Note 47 that bottle-gourds containing sound seeds are dispersed far and wide by the currents. In some species the seeds are buoyant, and in others they sink in sea-water; but the gourds themselves will float for probably a year or more, and the floating capacity of the seeds when it exists is too insignificant to affect the fruit’s buoyancy.
Other instances of the useless buoyancy of fruits of inland plants are afforded by different species of Citrus. In the floating drift of the Fijian rivers the fruits of the wild and indigenous Shaddock (C. decumana) and of an inedible Orange, also wild and indigenous (C. vulgaris?), are at certain times to be found, the latter often in numbers. The first-named floats four to five weeks in sea-water, and the last-named nearly two months, and both are to be observed floating out at sea between the islands. The fruits of the Tahitian Orange, a variety of C. aurantium, floated in sea-water between three and four weeks. The seeds of these and other species of Citrus sank in from a few hours to a day or two. The buoyancy of the fruit depends on the rind—the thicker the rind the greater the floating power. This was not only shown in the length of the period of flotation, but also in the buoyant behaviour of the fruit. With the Tahitian Orange, where the rind is relatively thin, the fruits floated heavily in sea-water and only protruded slightly above the surface. With the Shaddock and with the other indigenous species of Citrus, the fruits floated lightly and protruded half-way out of the water.
There is nothing trivial in these examples of buoyant fruits. That they have at times aided in the dispersal of the genus, with man’s assistance in planting the seeds of the stranded fruits, I cannot doubt; but unaided by man such buoyant capacities would be useless for purposes of effective dispersal by currents. Between the two genera Terminalia and Citrus there is this great distinction, that the former is more or less halophilous, some of its species being at home on the sea-beaches, whilst the latter, as Schimper would term it, is salt-shy, and includes no halophytes or plants of the sea-shore amongst its species. The only effect of buoyancy of the fruits on the distribution of the species of Citrus would be to place them by the side of the river and the pond. This has evidently been its result in the case of the Shaddock in Fiji, where, as Seemann remarks, it often thickly lines the banks of the rivers.
As also indicating that the buoyancy of the seed or fruit would never, apart from the halophilous habit, endow an inland plant with a littoral station, the examples of the Oak (Quercus robur) and of the Hazel (Corylus avellana) may be taken. As shown in Note 48, these fruits acquire floating power by drying, on account of the space formed by the shrinking of the kernel. They occur commonly in beach drift, but rarely in a sound condition; yet experiment has proved that they will sometimes germinate after prolonged sea-water flotation. The fruits of other species of Quercus are also transported in tropical regions by the currents, but never, as far as I could learn, effectively. The Amentaceæ as an order are “salt-shy,” and with only a few exceptions shun the sea-beach.
In the great sorting-process, by which xerophytic plants with buoyant seeds or fruits have been placed at the coast, and hygrophytic plants with similar fruits or seeds have been stationed at the riverside or by ponds and lakes, one might expect to find that other influences may have at times been in conflict with the selecting operation here indicated. To this cause may probably be attributed the cases of “useless buoyancy” above referred to. Here we find in some inland plants fruits and seeds with buoyant tissues in their coverings that in the case of littoral plants would have been regarded as the result of adaptation to dispersal by currents. Such cases go to emphasize the conclusion already indicated that these tissues could not have been developed through the agency of Natural Selection. But the great objection against the application of the Darwinian view to the general subject of the buoyancy of the seeds and fruits of littoral plants lies in the circumstance that quite half of the plants concerned are admitted to be outside the scope of the theory, and that for these another explanation has to be found. I think we may fairly claim that in a matter which finally resolves itself into a question of buoyancy one explanation should cover all. We have thus to decide whether to regard as adaptations to dispersal by currents the structures of the buoyant seeds and fruits of littoral plants; or whether to hold the view that as far as dispersal by currents is concerned such structures are purely accidental, and that Nature has never directly concerned herself in the matter at all. The first explanation lies under the disadvantage above alluded to, and it remains to be learned whether the second view could be made to cover all cases of dispersal by currents. Further investigation on many points is yet required; but, apart from the evidence against Natural Selection as the principal agency that has been produced in this chapter, a powerful argument in favour of the view that the buoyancy of seeds and fruits is not concerned with adaptation is, that as a rule the floating capacity of the seed or fruit has no direct relation with the density of sea-water. Generally speaking, as shown in Chapter X., these seeds and fruits are much more buoyant than they need to be, that is to say, if they owe their floating power to adaptation to dispersal by currents. This is quite in accordance with the argument developed in Chapter XI. with regard to the general question of plant-distribution, that dispersing agencies make use of characters and capacities of seeds and fruits that were never intended for them.
Summary of the Chapter.
(1) There are many mechanisms or contrivances in plants that now serve a purpose for which they were not originally developed.
(2) Of this nature, it is contended, is the relation between fruits and seeds and the agencies of dispersal.
(3) If, however, the structure or mechanism is made more effective by the new function, such a modification may be regarded as an “adaptation” in the language of the theory of Natural Selection.
(4) It is held by Professor Schimper that the structures connected with the buoyancy of the fruits or seeds of several tropical littoral plants are, in the above sense, adaptations; and he points to several genera where the buoyant tissues in the coverings of the fruits or seeds of the coast species are scantily represented or absent in the inland species of the same genus, a difference corresponding with the loss or diminution of the floating powers.
(5) This contrast in structure and in floating capacity between the fruits or seeds of inland and coast species of the same genus is beyond dispute, and the author adduces fresh data in support of it.
(6) But he contends that it is not proved that the relatively great development of buoyant tissues in the case of littoral plants is the effect of adaptation; and that if the selecting process had been confined to sorting out the xerophilous plants with buoyant seeds or fruits and to placing them at the coast, the same contrast would have been produced.
(7) In support of this contention he points out that when such littoral plants extend inland the floating capacity and the buoyant tissues are as a rule retained; and that in those exceptional cases where inland plants possess buoyant fruits or seeds these tissues are sometimes well developed under conditions in which they could never aid the plant’s dispersal.
(8) But the most serious objection against the adaptation view is that admittedly only about half of the shore-plants with buoyant fruits or seeds come within its scope. Therefore a second explanation has to be framed for the other plants concerned.
(9) As showing the difficulties raised by regarding some of the structures connected with buoyancy as “adaptive” and others as “accidental,” it is pointed out that some fruits possess the two kinds of structure. It is also shown that in several cases fruits endowed with buoyant tissues are just as well adapted for dispersal by frugivorous birds; and the instance of Ximenia americana is cited where a drupaceous fruit, known to be dispersed by fruit-pigeons, possesses also in its “stone” both the “adaptive” and “non-adaptive” types of “buoyant structures.”
(10) It is urged that whatever is the relation between the buoyancy of the seeds and fruits of shore-plants and dispersal by currents, there has been a uniform principle affecting all.
(11) The weight of evidence is regarded as adverse to the Natural Selection theory, an inference which is consistent with the conclusion arrived at in Chapter X. that there is no direct relation between the density of sea-water and the buoyancy of seeds and fruits, the floating capacities being as a rule far greater than the adaptation view would explain. Nature, it is held, has never made any provision for dispersal by currents, the buoyancy of seeds and fruits being, as concerns the currents, a purely accidental quality.
XIV. THE RELATION BETWEEN LITTORAL AND INLAND PLANTS
Professor Schimper’s views.—Great antiquity of the mangrove-formation.—Problem mainly concerned with the derivation of inland from littoral plants.—Grouping of the genera possessing both coast and inland species.—Scævola.—Morinda.—Calophyllum.—Colubrina.—Tacca.—Vigna.—Premna.
IN discussing the relation between the littoral and inland floras in the Pacific it will be at first necessary to pick up some of the threads of the various lines of investigation dealt with in the previous portion of this work. Apart from considerations connected with the genetic history of the plants concerned, when we come to inquire into the sources of any individual strand-flora, whether in the temperate or in the tropical regions, we arrive at the rough and ready inference that it is composed of “what the sea sends and the land lends.” But it has been already shown that the relative proportion of the current-borne and in consequence widely dispersed plants in a strand-flora varies greatly in different regions. Thus in the Pacific islands, as typified by those of Fiji, about 90 per cent. have buoyant seeds or seedvessels originally brought from distant localities; and in the tropics, as a rule, the average would probably be never under 75 per cent. On the other hand, in a temperate region the plants derived from inland would be most predominant, making up probably some three-fourths of the whole, whilst the proportion of current-dispersed plants hailing from distant places would be relatively few.
It is on this account that there is such uniformity in the general composition of the strand-flora over a large part of the tropics, since current-dispersed plants are widely spread. But in the temperate regions we find a great contrast in this respect. There are, it is true, a few current-borne plants that one meets everywhere. For instance, Convolvulus soldanella is to be gathered on English beaches and on those of New Zealand and of the coast of Chile. But these littoral plants with buoyant fruits hardly give a feature to the strand-flora. A multitude of intruders, either characteristic of the inland flora of the region or confined only to the seaboard of that part of the world, also make their home on the beach and frequently endow a beach-flora with its leading features. The possible associations of plants on a beach in a temperate region are thus very great; and I have already discussed this in part in Chapter IV. as concerning the British shore-flora. One has only to look at a work like that of Dr. Willkomm on the vegetation of the strand and steppe-regions of the Iberian peninsula to realise how the few littoral plants familiar to the English eye cut but a sorry figure amongst the numbers of strange intruders from the arid regions inland. So again, as I found on the Chilian beaches, Convolvulus soldanella finds odd associates amongst the species of Nolana and Franseria that are peculiar to the coasts of that part of the globe (see Chapter XXXII.); and different grotesque American forms of the Cactaceæ with a Mesembryanthemum and a host of strange-looking plants descend from the arid slopes of the hills behind to keep company with the far-travelled English beach-plant (see Note 49). Or again, a glance at the pages of Professor Schimper’s great work on Plant-Geography will bring the same fact home in a still more varied fashion.
Yet on tropical coasts the intruding inland element is also distinguishable, though it may influence only to a small degree the general character of the strand-flora. Dividing it, as we have described in Chapter V., into the plants of the sandy beach and of the mangrove-swamp, we find in the mangroves the most stable element and in the beach-plants those most liable to change. Professor Schimper observes that whilst the physiognomy of the beach-flora varies to some extent with the alterations in the inland flora, the mangrove-formation makes but a slow response to such changes. As he points out in his work on the Indo-Malayan Strand-Flora (p. 199), seeds and seedvessels are being continually brought down to the sea-coast through the agencies of rivers, winds, and birds; and in this manner, in the course of ages, the beach-flora is recruited from the inland plants. But for the mangroves such additions to their numbers are rarely possible. Whilst the same genera are often shared by both the beach and inland floras, we have in the mangrove-formation families, sub-families, and genera almost peculiar to itself, and including plants, like those of the Rhizophoreæ, that in their characters betray but little kinship with others and give but little indication of their descent. The mangroves have remained through the ages as something apart from other coast-plants, isolated both in their history and in their characters, and especially distinguished by their “adaptations” to their surroundings.
Such is the line of argument followed by this eminent German botanist in his account of the development of a tropical strand-flora. In various parts of this work I have ventured to suggest that the mangroves may be the remnant of an ancient flora widely distributed over the lower levels and coastal regions of the globe in an age when vivipary (meaning, thereby, germination on the plant) was the rule rather than the exception. At such a period, as I imagine, the climatic conditions of the earth were much more uniform than they are at present, at least in the lower levels; and a warm atmosphere, charged with aqueous vapour and heavy with mist and cloud, enveloped a large portion of the globe. The mangroves, it may be remarked, are by no means universally distributed on tropical coasts in our own time. (Professor Schimper describes their distribution in his Indo-malayische Strand-Flora, pp. 85, 86, and in the English edition of his Plant-Geography, p. 409.) They are not found on rainless coasts even when under the Line, except where there happen to be large estuaries; but where a rank and luxuriant inland flora betokens a high degree of humidity, there they thrive. This is well illustrated on the rainless shores of tropical Peru, a locality described in Chapter XXXII. of this work.
Yet if, as it is here contended, the mangroves form a remnant of a once widely spread viviparous flora, it might be expected that the beach-plants of that age would have been also viviparous, and that with their present descendants, as well as with some of the inland plants allied to them, we ought to find in the anomalous structure of the seed some indication of the lost viviparous habit. This appears to be the case, as described in Note 50, with the Barringtoniæ, a tribe that has supplied some of the most characteristic beach-trees, and also with some genera of the Guttiferæ. Perhaps, indeed, when the seeds of several other littoral beach-trees come to be examined, for instance, Guettarda, analogous structures may be found.
Although the beach-flora of the tropics is less stable in its composition than the mangrove-formation, it is not to be assumed that in the Pacific region or in the tropics generally it is at all modern in its character. Though in the main, no doubt, more recent than the mangroves, since it is likely that in early geological periods the swamp rather than the sandy beach formed the predominant feature of the sea-border throughout the tropics, yet it bears in several respects the impress of a high antiquity. There are few beach plants in the tropical Pacific that are not found over the tropics of a large portion of the globe, a circumstance that would in itself warrant our assigning a great age to the beach-flora; and it is highly probable that some at least of the beach plants of the Pacific that occur on the east and west coasts of tropical America are, for reasons given in Chapter XXXII., older than the barrier now interposed by Central America between the Atlantic and Pacific oceans. There are, it is true, a few species, like Acacia laurifolia and Drymispermum Burnettianum, which, on account of their restriction to the beaches of the Western Pacific and their lack of capacity for dispersal by currents, may be regarded as local productions; but for the great majority, ranging as they do over much of the tropics, it is not possible to determine when and where they assumed their littoral habits. That except in a few instances their home in some bygone age lay outside the Pacific can scarcely be doubted.
It is therefore to be expected that in a discussion of the relation between the strand and inland floras in the Pacific islands the problem will be mainly concerned with the possible derivation of inland from littoral plants. In such a discussion the relation between the beach and inland species of the same genus becomes a subject of great interest. It is a subject that had a peculiar fascination for Professor Schimper, who refers to it more than once in his pages; and though never able to take it up, he viewed it as a very promising field of inquiry. The question has been frequently alluded to in this work; and it is especially dealt with in one connection in Chapter II. It is there shown that whilst, as a general rule, the seeds or seedvessels of the coast species possess great floating power, those of the inland species of the same genus have little or none, and that both may have independent modes of dispersal, the first by currents, and the last through frugivorous birds.
A close connection between the beach and inland floras is apparently displayed in the circumstance that quite a third of the genera of the Pacific insular floras containing littoral species (some 70 in all, excluding the mangroves) possess in this region also inland species. But the further examination of this interesting group of genera, which are enumerated in the list below, goes to show that the connection between the inland and coast species of a genus is by no means always so close, or of such a character, as one might have expected. It will not be possible, however, to do much more than indicate in this chapter the results of this inquiry; but the details will usually be found either in the separate discussion of the genus or in other parts of this work. For convenience of treatment these genera may be grouped in the following sections.
_Grouping of the Plant-Genera of the Islands of the Tropical Pacific that possess both Littoral and Inland Species._
Section I. Where the littoral and inland species are most probably of independent origin, both possessing their own means of dispersal; Calophyllum, Hibiscus, Colubrina, Morinda, Scævola, Cordia, Ipomœa, Vitex, Tacca, Casuarina.
Section II. Where the littoral species have probably given rise to inland species, and both still exist in the group of islands: Vigna, Premna.
Section III. Where inland species have been probably developed from littoral species no longer existing in the group: Canavalia, Erythrina, Sophora, Ochrosia.
Section IV. Where the littoral and inland species are evidently of independent origin, and there is no means of accounting for the existence of the inland species by agencies of dispersal at present in operation: Barringtonia, Pandanus.
Section V. Where in the same genus some inland species are derivatives of the coast species and others are of independent origin: Guettarda.
Section VI. Where the coast species, having little or no capacity for dispersal by currents, are regarded as derived from the inland species in one group of islands and as afterwards distributed to those in the vicinity: Eugenia, Drymispermum, Acacia.
SECTION I
This group, which includes those genera where the coast and inland species are regarded as of independent origin, both possessing their own means of dispersal, contains about half of the total number of genera here concerned. We will first deal with the genera Calophyllum, Morinda, and Scævola, where the littoral species have buoyant fruits or seeds that are dispersed by currents, whilst the inland species have more or less non-buoyant fleshy fruits that could only be dispersed by frugivorous birds. Here the inland and coast species could have arrived independently at the island, and we are not called upon either on this ground or by reason of affinity of characters to connect the one with the other.
The genus Scævola is very typical of its kind and has been already in part discussed in Chapter II. The wide-ranging shore-species, S. Kœnigii, that is distributed over the Pacific may sometimes, as in Hawaii, be accompanied by numerous inland species, all endemic, seven of them being enumerated by Hillebrand; or, as in Fiji and Tonga, there may be associated with it a solitary inland species, S. floribunda (see Note 51); or, as in Tahiti, it may exist by itself. On the other hand, as in the Kermadec Islands, a single inland peculiar species may alone represent the genus. The inland species have fleshy drupes which, as far as examined, have no floating power and possess no buoyant tissues in their coverings; and their independent dispersal by birds cannot be doubted. The endemic character of most of the inland species of the Pacific islands is most probably due to the suspension of the transporting agency of frugivorous birds, just as the wide range of the solitary littoral species may be attributed to the uninterrupted agency of the currents. There is nothing in the description of the endemic species given in Hillebrand’s Hawaiian Flora to indicate any especial genetic connection between the inland species and the beach plant, S. Kœnigii; and the occurrence of a solitary inland peculiar species in the Kermadec Islands clearly proves an origin independent of any littoral plant.
Morinda is another critical genus in this discussion. Besides the widespread littoral species (M. citrifolia) that is distributed by the currents and is also dispersed by man, there are in the Pacific islands a number of inland species, mostly climbers and denizens of the forests. In the Index Kewensis six are accredited to Fiji and five to New Caledonia. Hillebrand gives a peculiar Hawaiian species, and there is a widespread species (M. Forsteri) that ranges over the South Pacific from New Caledonia to the Marquesas and the Paumotu Islands. Since, as indicated in Chapter II. and in Note 8, the pyrenes of the fruits of the inland species are not dispersed by the currents and could readily be transported by frugivorous birds, we are not called upon to connect them in their origin with M. citrifolia, the wide-ranging species of tropical beaches.
The fact of the dispersal of certain inland species of the genus over large areas of the tropics, such as in the case of Morinda umbellata through tropical Asia and Malaya, and M. Forsteri in the Pacific, is indeed sufficient proof that these inland plants are independent of any littoral species in the Pacific and possess their own means of distribution. Though the genus, comprising at least forty species, is mainly confined to the Old World, there are a few species in America; but M. citrifolia, the familiar beach species of the Old World and the Pacific, is not indigenous there, and, as far as I can gather, all the American species belong inland. Facts of distribution of this nature negative the possibility that the Pacific islands have received their inland species of Morinda through the intervention of the far-ranging littoral plant.
As respecting Calophyllum, which is represented all over the tropical South Pacific by the wide-ranging C. inophyllum and by a tree of the inland forests found also in Malaya and in Ceylon (C. spectabile), there are, apart from questions of affinity, grave objections against the derivation of the same inland species from the coast species all over this area. The fruits of the two inland species of Fiji, C. spectabile and C. burmanni, have sappy outer coverings and are quite suited for dispersal by fruit-pigeons. As observed in Chapter II. and Note 9, they have limited floating capacities and their dispersal by birds is necessary to explain their distribution. Since the timber is greatly valued by the Polynesians, it is not unlikely, however, that those islanders have assisted in the distribution of the inland species. It is not possible to do more than touch on this subject here; but it may be inferred that the history of Calophyllum in the Pacific has not been one that would warrant our regarding the inland trees as derivatives of a coast species.
There are other genera of this section where, for reasons of a different character, there is no cause for assuming that the inland species are derived from the coast species, or vice versâ. Thus, in Fiji, Casuarina equisetifolia, a widely distributed species of the Old World, occurs at the coast and in the scantily wooded plains behind; while C. nodiflora, a New Caledonian species, finds its home in the lower forests. There are many endemic species in Australia and New Caledonia; and we are not called on to connect together these two species in Fiji. In the same way we are not under any obligation in the case of the numerous inland species of Ipomœa of the Pacific islands to connect them with the coast species. They are all widely ranging species, and their seeds have been carried to the islands, each in its own fashion. So again with the inland species of Hibiscus found in the Polynesian islands and often cultivated, we cannot either from the point of view of dispersal or of affinity connect them with the far-ranging littoral species, H. tiliaceus, which belongs to a section of the genus distinct from those sections to which the inland species belong.
In a similar way there is no ground for supposing that Cordia aspera, an inland species confined to Fiji, Tonga, and Samoa, is derived from C. subcordata, the widely distributed littoral species of the Pacific and of the Old World, since they belong to different sections of the genus. But, apart from any question of affinity, the drupes of inland species of Cordia are known to be well suited for dispersal by frugivorous birds, though, unlike the littoral species above named, not adapted for transportal by the currents. The genus Vitex, which is represented by a wide-ranging littoral species in the Pacific (V. trifolia), appears to be associated with inland species only in Fiji, where one or two, seemingly endemic, occur. But there is nothing in Dr. Seemann’s description of V. vitiensis, one of these species, that at all suggests its derivation from the strand species, a very variable plant that often extends far inland into the plains, adopting a different habit of growth in those localities. It is known that Vitex fruits can be dispersed both by birds and by currents. This genus is more fully discussed in a later chapter.
Of the genus Colubrina there seem to be only two Pacific species known—one the widely distributed shore-plant, C. asiatica, a straggling shrub with alternate leaves found in all the Pacific groups and on the beaches of much of the tropics of the Old World; the other a tree, C. oppositifolia, with opposite leaves, that is peculiar to the Hawaiian islands, where it frequents the open-wooded and scrubby inland districts. The seeds of the shore-plant float unharmed for many months, whilst the fruits of the inland plant, which differ in some important respects (see Note 52), would float only for a week or two. The strand species is also quite at home inland in many parts of the world; and there is nothing from the standpoint of affinity to indicate that in Hawaii it has given birth to an inland species so divergent in habit and in character. There is of course the difficulty of explaining how a plant like C. oppositifolia, with such a dry, unattractive fruit, could be indebted to birds for its original introduction into the group; but the same difficulty arises with a host of Hawaiian plants. It is, however, evident from its distribution over the islands of this archipelago that it possesses or has possessed some means of inter-island dispersal, and since it is not of much service to the aborigines we must look therefore to the bird.
In the instance of the genus Tacca there is in Fiji an inland species, T. maculata, associated with a wide-ranging beach species, T. pinnatifida, which also grows inland. The first-named is recorded from the north coast of Australia and from Samoa, and though, unlike the beach plant, its seeds are unfitted for dispersal by currents (see Chapter II.), they might be distributed by birds. Dr. Reinecke describes another inland species from Samoa, T. samoensis. The beach plant, T. pinnatifida, grows so typically (sometimes side by side with T. maculata) in the inland plains of Fiji that one would not be justified, apart from questions of affinity, in regarding it as the parent form of inland species in the Pacific islands.
For food and other purposes Tacca pinnatifida is or was much valued by the Pacific islanders, and it grows so abundantly that cultivation is rarely practised. That the Polynesians have aided the currents in the distribution of the plant there can be no doubt, and this is particularly indicated by its occurrence in Hawaii. The genus contains ten or a dozen species, of which at least three are peculiar to America; but T. pinnatifida, the characteristic shore-plant of the Old World, and according to Schimper the only one that can be so designated, is not found in America, where, as far as I can gather, there is no widely-spread beach species dispersed by the currents from which the peculiar species could have been derived. In the case of the Pacific species, however, it should be noted that I am not endeavouring to prove the improbability of the inland species having been derived from the coast species in other regions, as in Australia, but that my point is to show there is no reason to suppose that this has taken place in the Pacific. There is no difficulty in attributing the dispersal of inland species to birds; and we are therefore not called on to connect them with the beach plants.
SECTION II
This division includes those genera where the littoral species has apparently given rise to one or more inland species and both still exist in the same group of islands. Two genera alone, Vigna and Premna, come into this category. The first-named seems to present a good case for the derivation of an inland from a coast species in Hawaii. Besides Vigna lutea, the beach species, which is found not only all over the Pacific islands but on the tropical beaches of the Old World, there are in Hawaii two endemic species (V. sandwicensis and V. oahuensis) that occur in the mountains, usually at elevations of from 1,500 to 5,000 feet; but I do not find any more inland species recorded from the other Polynesian archipelagoes. It may at first be noted that Vigna lutea, which in some parts of the world strays inland, displays considerable variety in its littoral station in the Pacific. Thus, in Hawaii, I found it sometimes on the sandy beach, sometimes on a rocky shore, and sometimes on the edge of old lava-cliffs overlooking the sea. In Fiji, though usually a trailer on the beach, it may become a climber hanging from the trees bordering the creeks in the mangrove-swamps. Though Hillebrand makes no mention of forms intermediate between coast and inland species in Hawaii, I found in one locality at the coast some specimens of Vigna lutea displaying the twisted pods and two callosities on the standard that are characteristic of V. sandwicensis, one of the inland species. The seeds of Vigna lutea float in sea-water unharmed for months, and they are to be found in the stranded drift of the Hawaiian and Fijian beaches, and floating in the drift of the Fijian rivers. I was unable to obtain the mature seeds of the inland species, and it has therefore yet to be determined whether they follow the rule in the loss of buoyancy. It may be added that a plant of Vigna lutea raised in Hawaii from seed displayed some small tubers of the size of a pea on its roots.
The case for Premna is stated in Note 32. In this genus, as with Vigna, the final test of experiment is needed; but the data at my disposal point to the probability that an inland species has here been derived from a littoral plant.
The summary of this chapter is given at the end of Chapter XVI.