V. PHYSIOLOGY…

V. PHYSIOLOGY

I. CELLS AND CELL PRODUCTS

Any study of cells or cell-membranes in lichens should naturally include those of both symbionts, but the algae though modified have not been profoundly changed, and their response to the influences of the symbiotic environment has been already described in the discussion of lichen gonidia. The description of cells and their contents refers therefore mainly to the fungal tissues which form the framework of the plant; they have been transformed by symbiosis to lichenoid hyphae in some respects differing from, in others resembling, the fungal hyphae from which they are derived.

A. CELL-MEMBRANES

a. CHITIN. It was recognized by workers in the early years of the nineteenth century that the substance forming the cell-walls of fungal hyphae differed very markedly from the cellulose of the membranes in other groups of plants, the blue colouration with iodine and sulphuric acid so characteristic of cellulose being absent in most fungi. Various explanations were suggested; but it was always held that the doubtful substance was a cellulose containing something peculiar to fungi, this view being strengthened by the fact that, after long treatment with potash, a blue reaction was obtained. It was called fungus-cellulose by De Bary[740] in order to distinguish it from true cellulose.

It was not till a much later date that any exact work was done on the fungal cell, and that Gilson[741] by his researches was able to prove that the membranes of fungi contained probably no cellulose, or, “if cellulose were present, it was in a different condition from the cellulose of other plants.” Winterstein[742] followed with the results of his examination of fungus-cellulose: he found that it contained nitrogen and therefore differed very considerably from typical plant cellulose. Gilson[743] published a second paper dealing entirely with fungal tissues in which he also established the presence of nitrogen, and added that this nitrogenous compound resembled in various ways the chitin[744] of animal cells. He further discovered that by heating it with potash a substance was obtained that took a reddish-violet stain when treated with iodine and weak sulphuric acid. This substance, called by him mycosin, was proved later to be similar to chitosan[744], a product of chitin.

Escombe[745] analysed the hyphal membranes of Cetraria and found that they consisted mainly of a body called by him lichenin and of a paragalactan. From Peltigera he extracted a substance with physical properties agreeing fairly well with those of chitosan, though analysis did not give percentages reconcilable with that substance; the yield however was very small. No lichenin was detected.

Van Wisselingh[746] examined the hyphae of lichens as well as of fungi and experimented with a considerable number of both types of plants. He succeeded in proving the presence of chitin in the higher fungi (Basidiomycetes and Ascomycetes) and in lichens with one or two exceptions (Cladonia and Cetraria). Though in some the quantity found was exceedingly small, in others, such as Peltigera, the walls of the hyphae were extremely chitinous. More recently Wester[747] has gone into the question as regards lichens, and he has practically confirmed all the results previously obtained by Wisselingh. In some species, as for instance in Cladonia rangiferina, Cl. squamosa, Cl. gracilis, Ramalina calicaris, Solorina crocea and others, he found that chitin existed in large quantities, while in Evernia prunastri, Usnea florida, U. articulata, Sticta damaecornis and Parmelia saxatilis very little was present. The variation in the amount present may be very great even in the species of one genus: none for instance has been detected in Cetraria islandica nor in C. nivalis while it is abundant in other Cetrariae. There is also considerable variation in quantity in different individuals of the same species, and even in different parts of the thallus of one lichen. Factors such as habitat, age of the plant, etc., may, however, account to a considerable extent for the differences in the results obtained.

b. LICHENIN AND ALLIED CARBOHYDRATES. It has been proved, as already stated, that chitin is present in the hyphal cell-walls of all the lichens examined except in those of Cetraria islandica (Iceland Moss), C. nivalis and, according to Wester[747], in those of Bryopogon (Alectoriae). In these lichens another substance of purely carbohydrate nature is the chief constituent of the cell-walls which swell up when soaked in water to a colourless gelatinous substance.

Berzelius[748] first drew attention to the peculiar qualities of this lichen product, and, recognizing its resemblance in many respects to ordinary starch, he called it “lichen-starch” or “moss-starch.” More exact observations were made later by Guérin-Varry[749] who described its properties and showed by his experiments that it contained no admixture of either starch or gum. He adopted the name lichenin for this organic soluble part of Iceland Moss. An analysis of lichenin was made by Mulder[750] who detected in addition to lichenin, which coloured yellow with iodine, small quantities of a blue-colouring substance which could be dissolved out from the lichenin and which he considered to be true starch. Berg[751] also demonstrated the compound nature of lichenin: he isolated two isomerous substances with the formula C₆H₁₀O₅. The name “isolichenin” was given to the second blue-colouring substance by Beilstein[752] in 1881.

More recently Escombe[753] has chemically analysed the cell-wall of Cetraria islandica: after the elimination of fat, oil, colouring matter and bitter constituents he found that there remained the compound lichenin, an anhydride of galactose with the formula C₆H₁₀O₅, which, as stated above, consists of two substances lichenin and isolichenin[754]; the latter is soluble in cold water and gives a blue reaction with iodine, lichenin is only soluble in hot water and is not coloured blue. Both are derivatives of galactose, a sugar found in a great number of organic tissues and substances, among others in gums.

Lichenin has also been obtained by Lacour[755] from Lecanora esculenta, an edible desert lichen supposed to be the manna of the Israelites. Wisselingh[756] tested the hymenium of thirteen different lichens for lichenin. He found it in the walls of the ascus of all those he examined except Graphis. Everniin, a constituent of Evernia prunastri, was isolated and described by Stüde[757]. It is soluble in water and, though considered by Czapek[758] to be identical with lichenin, it differs, according to Ulander[759], in being dextro-rotatory to polarized light; lichenin on the contrary is optically inactive. Escombe[753] also obtained a substance from Evernia which he considered to be comparable with chitosan. Usnein which has been extracted[756] from Usnea barbata may also be identical with lichenin, but that has not yet been established. Ulander[759] examined chemically the cell-walls of a fairly large number of lichens. Cetraria islandica, C. aculeata and Usnea barbata, designated as the “Cetraria group,” contained soluble mucilage-forming substances similar to lichenin. A second “Cladonia group” which included Cl. rangiferina with the variety alpestris, Stereocaulon paschale and Peltigera aphthosa yielded almost none. After the soluble carbohydrates were removed by hot water, the insoluble substances were hydrolysed and the “Cetraria group” was found to contain abundant d-glucose with small quantities of d-mannose and d-galactose; the “Cladonia group,” abundant d-mannose and d-galactose with but little d-glucose. Hydrolysis was easier and quicker with the former group than with the latter.

Besides these, which rank as hexosans, Ulander found small quantities of pentosans and methyl pentosans. All these substances which are such important constituents of the hyphal membranes of lichens are classed by Ulander as hemicelluloses of the same nature as mannan, galactan and dextran, or as substances between hemicellulose and the glucoses represented by lichenin, everniin, etc. They are doubtless reserve stores of food material, and they are chiefly located in the cell-walls of the medullary hyphae which are often so thick as almost to obliterate the lumen of the cells. Ulander made no test for chitin in his researches.

Ulander’s results have been confirmed by those obtained by K. Müller[760]. In Cladonia rangiferina, Müller found that the cell-membranes of the hyphae contained, as hemicelluloses, pentosans in small quantities and galactan, but no lichenin and very little chitin. In Evernia prunastri hemicelluloses formed the chief constituents of the thallus, and from it he was able to isolate galactan soluble in weak hot acid, and everniin soluble in hot water, the latter with the formula C₇H₁₅O₆, a result differing from that obtained by Stüde[761] who has given it as C₉H₁₄O₇; chitin was also present in small quantities. In Ramalina fraxinea, the soluble part of the thallus (in hot water) differed from everniin and might probably be lichenin. Cetraria islandica was also analysed and yielded various hemicelluloses, chiefly dextran and galactan, with less pentosan. No chitin has ever been found in this lichen. In testing minute quantities of material for chitin, Wisselingh[762] heated the tissue in potash to 160° C. The potash was then gradually replaced by glycerine and distilled water; the precipitate was placed on a slide and the preparation stained under the microscope by potassium-iodide-iodine and weak sulphuric acid. Chitin, if present, would have been changed by the potash to mycosin which gives a violet colour with the staining solution.

It has been stated by Schellenberg[763] that these lichen membranes may become lignified. He obtained a red reaction with phloroglucine test for lignin in Cetraria islandica and Cladonia furcata. Further research is required.

c. CELLULOSE. Several workers claim to have found true cellulose in the cell-walls of the hyphal tissues of a few lichens; but the more careful analyses of Escombe[764], Wisselingh[762] and Wester[765] have disproved their results. The cell-walls of all the gonidia, however, are formed of cellulose, or according to Escombe of glauco-cellulose, except those of Peltigera in which Wester found neither cellulose nor chitin. Czapek[766] suggests that the blue reaction with iodine characteristic of the cell-walls in some apothecia, of the asci and of the hyphae in cortex or medulla in a few instances, may be due to the presence of carbohydrates of the nature of galactose. Moreau[767] in a recent paper terms the substance that gives a blue reaction with iodine at the tips of the asci “amyloid.” In Peltigera the ascus tip is occupied by such a plug of amyloid which at maturity is projected like a cork from the ascus and may be found on the surface of the hymenium.

B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS

a. CELL-SUBSTANCES. The cells of lichen hyphae contain protoplasm and nucleus with glucoses. It is doubtful if starch has been found in fungal hyphae; it is replaced, in some of the tissues at least, by glycogen, a carbohydrate (C₆H₁₀O₅) very close to, if not identical with, animal glycogen, a substance which is soluble in water and colours reddish-brown (wine-red) with iodine. Errera[768] first detected its presence in Ascomycetes where it is associated with the epiplasm of the cells, more especially of the asci, and he considered it to be physiologically homologous with starch. He included lichens, as Ascomycetes, in his survey of fungi and quotes, in support of his view that lichen hyphae also contain glycogen, a statement made by Schwendener[769] that “the contents of the ascogenous hyphae of Coenogonium Linkii stain a deep-brown with iodine.” Errera also instances the red-brown reaction with iodine, described by de Bary[770], as characteristic of the large spores of Ochrolechia (Lecanora) pallescens, while the germinating tubes of these spores become yellow with iodine like ordinary protoplasm. Glycogen has been, so far, found only in the cells of the reproductive system.

Iodine was found by Gautier[771] in the gonidia of Parmelia and Peltigera, i.e. both in bright-green and blue-green algae. The amount was scarcely calculable.

Herissey[772] claims to have established the presence of emulsin in a large series of lichens belonging to such widely separated genera as Cladonia, Cetraria, Evernia, Peltigera, Pertusaria, Parmelia, Ramalina, and Usnea. It is a ferment which acts upon amygdalin, though its presence has been proved in plants such as lichens where no amygdalin has been found[773]. Diastase was demonstrated in the cells of Roccella tinctoria, R. Montagnei and of Dendrographa leucophaea by Ronceray[774] who states that, in conjunction with air and ammonia, it forms orchil, the well-known colouring substance of these lichens. Diastatic ferments have also been determined[775] in Usnea florida, Physcia parietina, Parmelia perlata and Peltigera canina.

b. CALCIUM OXALATE. Oxalic acid (C₂H₂O₄) is an oxidation product of alcohol and of most carbohydrates and in combination is a frequent constituent of plant cells. Knop[776] held that it was formed in lichens by the reduction and splitting of lichen acids, though, as Zopf[777] has pointed out, these are generally insoluble. Hamlet and Plowright[778] demonstrated the presence of free oxalic acid in many families of fungi including Pezizae and Sphaeriae. The acid combines with calcium to form the oxalate (CaC₂0₄), which in the crystalline form is very common in lichens. In the higher plants the crystals are formed within the cell, but in lichens they are always deposited on the outer surface of the hyphal membranes, mainly of the medulla and the cortex.

Calcium oxalate was first detected in lichens by Henri Braconnot[779], who extracted it by treating the powdered thallus of a number of species (Pertusaria communis, Diploschistes scruposus, etc.) with different reagents. The quantity present varies greatly in lichens: Zopf[780] found that it was abundant in all the species inhabiting limestone, and states that in such plants the more purely lichenic acids are relatively scarce. Errera[781] has calculated the amount of calcium oxalate in Lecanora esculenta, a desert lime-loving lichen, to be about 60 per cent. of the whole substance of the thallus. Euler[782] gives for the same lichen even a larger proportion, 66 per cent. of the dry weight. In Pertusaria communis, a corticolous species, the oxalate occurs as irregular crystalline masses in the medulla (Fig. 116) and has been calculated as 47 per cent. of the whole substance. Other crustaceous species such as Diploschistes scruposus, Haematomma coccineum, H. ventosum, Lecanora saxicola, Lecanora tartarea, etc., contain large amounts either in the form of octahedral crystals or as small granules.

Rosendahl[783] has recently made observations as to the presence of the oxalate in the thallus of the brown Parmeliae. Of the fourteen species examined by him, eleven contained calcium oxalate as octahedral crystals or as small prisms, often piled up in thick irregular masses. Usually the crystals were located in the medullary part of the thallus, but in two species, Parmelia verruculifera and P. papulosa, they were abundant on the surface cells of the upper cortex.

c. IMPORTANCE OF CALCIUM OXALATE TO THE LICHEN PLANT. It is natural to conclude that a substance of frequent occurrence in any group of plants is of some biological significance, and suggestions have not been lacking as to the value of oxalic acid or of calcium oxalate in the economy of the lichen thallus. Oxalic acid is known to be one of the most efficient solvents of argillaceous earth and of iron oxides likely to be in the soil. These materials are also conveyed to the thallus as air-borne dust, and would thus, with the aid of the acid, be easily dissolved and absorbed. As a direct proof of this, Knop[784] has stated that lichen-ash always contains argillaceous earth. According to Kratzmann[785], aluminium, a product of clay, is stored up in various lichens. He proved the amount in the ash of Umbilicaria pustulata to be 4·46 per cent., in Usnea barbata 1·79 per cent., in U. longissima considerable quantities while in Roccella tinctoria it occurred in great abundance. It was also abundant in Diploschistes scruposus, 28·17 per cent.; it declined in Variolaria (Pertusaria) dealbata to 7·77 per cent., in Cladonia rangiferina to 1·76-2·12 per cent. and in Ramalina fraxinea to 1·8 per cent.

Calcium oxalate is directly advantageous to the thallus by virtue of the capacity of the crystals to reduce or prevent evaporation, as has been pointed out by Zukal[786]. A like service afforded by crystals to the leaves of the higher plants in desert lands has been described by Kerner[787]. These are frequently encrusted with lime crystals which allow the copious night dews to soak underneath them to the underlying cells, while during the day they impede, if they do not altogether check, evaporation.

Calcium oxalate crystals are insoluble in acetic acid, soluble in hydrochloric acid without evolution of gas; they deposit gypsum crystals in a solution of sulphuric acid.

C. OIL-CELLS

a. OIL-CELLS OF ENDOLITHIC LICHENS. Calcicolous immersed lichens are able to dissolve the lime of the substratum, and their hyphae penetrate more or less deeply into the rock. In some forms the entire thallus may thus be immersed, the fruits alone being visible on the surface of the stone. In two such species, Verrucaria calciseda and Petractis (Gyalecta) exanthematica, Steiner[788] detected peculiar sphaeroid or barrel-shaped cells that differed from the other hyphal cells of the thallus, not only in their form, but in their greenish-coloured contents. Similar cells were found by Zukal[789] in another immersed (endolithic) lichen, Verrucaria rupestris f. rosea. He describes them as roundish organs crowded on the hyphae and filled with a greenish shimmering protoplasm. He[790] found the same types of sphaeroid and other swollen cells in the immersed thallus of several calcicolous lichens and he finally determined the contents as fat in the form of oil. He found also that these fat-cells, though very frequent, were not constantly present even in the same species. His observations were confirmed by Hulth[791] for a number of allied crustaceous lichens that grow not only on limestone but on volcanic rocks. In them he found a like variety of fat-cells—intercalary or torulose cells, terminal sphaeroid cells and hyphae containing scattered oil-drops. Bachmann[792] followed with a study of the thallus of purely calcicolous lichens. The specialized oil-cells were fairly constant in the species he examined, and, as a rule, they were formed either in the tissues immediately below, or at some distance from, the gonidial zone. Fünfstück[793] has also published an account of various oil-cells in a large series of calcicolous lichens (Fig. 117).

The occurrence of oil-(or fat-)cells is not dependent on the presence of any particular alga as the gonidium of the lichen. Fünfstück[794] has described the immersed thallus of Opegrapha saxicola as one of those richest in fat-cells. The gonidia belong to the filamentous alga Trentepohlia umbrina and form a comparatively thin layer about 160µ thick near the upper surface; isolated algal branches may grow down to 350µ into the rock, while the fungal elements descend to 11·5 mm., and though the very lowest hyphae were without oil—as were those immediately beneath the gonidia—the interlying filaments, he found, were crowded with oil-cells. Sphaeroid terminal cells were not present.

Fünfstück[794] has re-examined the thallus of Petractis exanthematica, an almost wholly immersed lichen with a gelatinous gonidium, a species of Scytonema. The thallus is homoiomerous: the alga forms no special zone, it intermingles with the hyphae down to the very base of the thallus; the hyphae are extremely slender and at the base they measure only about 1µ in width. Oil-cells are abundant in the form of intercalary cells about 3-5µ in thickness. Nearer the surface sphaeroid cells are formed on short lateral outgrowths; they measure 14-16µ in diameter and occur in groups of 15 to 20. The superficial part of the thallus is a mere film; the hyphae composing it are slightly stouter and more thickly interwoven.

Bachmann[795] and Lang[796] have further described the anatomy of endolithic thalli especially with reference to oil-cells, and have supplemented the researches of previous workers. New methods of cutting the rock in thin slices and of dissolving away the lime enabled them to see the tissues in their relative positions. In these immersed lichens, as described by them and by previous writers, and more especially in calcicolous species, the gonidial zone of Protococcaceous algae lies near the surface of the rock, and is mingled with delicate, thin-walled hyphae which usually do not contain oil. The more deeply immersed layer is formed of a weft of equally thin-walled hyphae, some of the cells of which are swollen and filled with fat globules. These oil-cells may occur at intervals along the hyphae or they may form an almost continuous row. In addition, strands or bundles of hyphae (Fig. 118) containing few or many oil globules traverse the tissue, and true sphaeroid cells are generally present. These latter arise in great numbers on short lateral branchlets, usually near the tip of a filament and the groups of cells are not unlike bunches of grapes. Sometimes the oil-cells are massed together into a complex tissue. Hyphae from this layer pierce still deeper into the rock and constitute the rhizoidal portion of the thallus. They also produce sphaeroid oil-cells in great abundance (Fig. 119). In the immersed thallus of Sarcogyne (Biatorella) pruinosa Lang[797] estimated the gonidial zone as 175-200µ in thickness, while the colourless hyphae penetrated the rock to a depth of quite 15 mm.

b. OIL-CELLS OF EPILITHIC LICHENS. The general arrangement of the tissues and the occurrence and form of the oil-cells vary in the different species according to the nature of the substratum. This has been clearly demonstrated by Bachmann[798] in Aspicilia (Lecanora) calcarea, an almost exclusively calcareous lichen as the name implies. On limestone, he found sphaeroid cells formed in great abundance on the deeply penetrating rhizoidal hyphae (Fig. 120). On a non-calcareous brick substratum[799], a specimen had grown which of necessity was epilithic. The cortex and gonidial zone together were 40µ thick; immediately below there were hyphae with irregular cells free from oil; lower still there was formed a compact tissue of globose fat-cells. In this case the calcareous lichen still retained the capacity to form oil-cells on the non-calcareous impenetrable substance.

Very little oil is formed, as a rule, in the cells of siliceous crustaceous lichens which are almost wholly epilithic, but Bachmann found a tissue of oil-cells in the thallus of Lecanora caesiocinerea, from Labrador, on a granite composed of quartz, orthoclase and traces of mica. A thallus of the same species collected in the Tyrol, though of a thicker texture, contained no oil. Bachmann[799] suggests no explanation of the variation.

On granite, rhizoidal hyphae penetrate the rock to a slight extent between the different crystals, but only in connection with the mica[800] are typical sphaeroid cells formed.

More or less specialized oil-cells have been demonstrated by Fünfstück[801] in several superficial (epilithic) lichens which grow on a calcareous substratum, as for instance Lecanora (Placodium) decipiens, Lecanora crassa and other similar species. The oil in these lichens is usually restricted to more or less swollen or globose cells; but it may also be present in the ordinary hyphae as globules. Zukal[802] found that the smooth little round granules sprinkled over the thallus of the soil-lichens, Baeomyces roseus and B. rufus, contained in the hyphae typical sphaeroid oil-cells and that they were specially well developed in specimens from Alpine situations. In still another soil-lichen, Lecidea granulosa, shimmering green oil was found in short-celled torulose hyphae.

Rosendahl’s[803] researches on the brown Parmeliae resulted in the unexpected discovery of specialized oil-cells situated in the cortices—upper and lower—of five species out of fourteen which he examined. In one of the species, P. papulosa, they also occurred in the cortex of the rhizoids. The oil-cells were thinner-walled and larger than the neighbouring cortical cells; they were clavate or ovate in form and sometimes formed irregular external processes. They were more or less completely filled with oil which coloured brown with osmic acid, left a fat stain on paper and, when extracted, burned with a shining reddish flame. These oil-cells were never formed in the medulla nor in the gonidial region.

c. SIGNIFICANCE OF OIL-FORMATION. Zukal[804] regarded the oil stored in these specialized cells as a reserve product of service to the plant in the strain of fruit-formation, or in times of prolonged drought or deprivation of light. According to his observations fat was most freely formed in lichens when periods of luxuriant growth alternated with periods of starvation. He cites, as proof of his view, the frequent presence of empty sphaeroid cells, and the varying production of oil affected by the condition, habitat, etc. of the plant. Fünfstück[805], on the other hand, considers the oil of the sphaeroid and swollen cells as an excretion, representing the waste products of metabolism in the active tissue, but due chiefly to the presence of an excess of carbonic acid which, being set free by the action of the lichen acids on the carbonate of lime, forms the basis of fat-formation. He points out that the development of fat-cells is always greater in endolithic species in which the gonidial layer—the assimilating tissue—is extremely reduced. In epilithic lichens with a wide gonidial zone, the formation of oil is insignificant. He states further that if the oil were a direct product of assimilation, the cells in which it is stored would be found in contact with the gonidia, and that is rarely the case, the maximum of fat production being always at some distance.

Fünfstück tested the correctness of his views by a prolonged series of growth experiments: he removed the gonidial layer in an endolithic lichen, and found that fat storage continued for some time afterwards, its production being apparently independent of assimilative activity. The correctness of his deductions was further proved by observations on lichens from glacier stones. In such unfavourable conditions the gonidia were scanty or absent, having died off, but the hyphae persisted and formed oil. He[806] also placed in the dark two quick-growing calcicolous lichens, Verrucaria calciseda and Opegrapha saxicola. At the end of the experiment, he found that they had increased in size without using up the fat. Lang[807] also is inclined to reject Zukal’s theory, seeing that the fat is formed at a distance from the tissues—reproductive and others—in need of food supply. He agrees with Fünfstück that the oil is an excretion and represents a waste-product of the plant.

Considerable light is thrown on the subject of oil-formation by the results of recent researches on the nutrition of algae and fungi. Beijerinck[808] made comparative cultures of diatoms taken from the soil, and he found that so long as culture conditions were favourable, any fat that might be formed was at once assimilated. If, however, some adverse influence checked the growth of the organism while carbonic acid assimilation was in full vigour, fat was at once accumulated. The adverse influence in this case was the lack of nitrogen, and Beijerinck considers it an almost universal rule in plants and animals, that where there is absence of nitrogen, in a culture otherwise suitable, fat-oils will be massed in those cells which are capable of forming oil. He observed that in two of the cultures of diatoms the one which alone was supplied with nitrogen grew normally, while the other, deprived of nitrogen, formed quantities of oil-drops. Wehmer[809] records the same experience in his cultural study of Aspergillus. Sphaeroid fat-cells, similar to those described by Zukal in calcicolous lichens, were formed in the hyphae of a culture containing an overplus of calcium carbonate, and he judged, entirely on morphological grounds, that these were not of the nature of reserve-storage cells.

Stahel[810] has definitely established the same results in cultures of other filamentous fungi. In an artificial culture medium in which nitrogen was almost wholly absent, the cells of the mycelium seemed to be entirely occupied by oil-drops, and this fatty condition he considered to be a symptom of degeneration due to the lack of nitrogen. These experiments enable us to understand how the hyphae of calcicolous lichens, buried deep in the substratum, deprived of nitrogen and overweighted with carbonic acid, may suffer from fatty degeneration as shown by the formation of “sphaeroid-cells.” The connection between cause and effect is more obscure in the case of lichens growing on the surface of the soil, such as Baeomyces roseus, or of tree lichens such as the brown Parmeliae, but the same influence—lack of sufficient nitrogenous food—may be at work in those as well as in the endolithic species, though to a less marked extent.

It seems probable that the capacity to form oil- or fat-cells has become part of the inherited development of certain lichen species and persists through changes of habitat as exemplified in Lecanora calcarea[811].

In considering the question of the formation and the function of fat in plant cells, it must be remembered that the service rendered to the life of the organism by this substance is a very variable one. In the higher plants (in seeds, etc.) fat undoubtedly functions in the same way as starch and other carbohydrates as a reserve food. It is evidently not so in lichens, and in one of his early researches, Pfeffer[812] proved that similarly oil was only an excretion in the cells of hepatics. He grew various species in which oil-cells occurred in the dark and then tested the cell contents. He found that after three months of conditions in which the formation of new carbohydrates was excluded, the oil in the cells, instead of having served as reserve material, was entirely unchanged and must in that instance be regarded as an excretion.

D. LICHEN-ACIDS

a. HISTORICAL. The most distinctive and most universal of lichen products are the so-called lichen-acids, peculiar substances found so far only in lichens. They occur in the form of crystals or minute granules deposited in greater or less abundance as excretory bodies on the outer surface of the hyphal cells. Though usually so minute as scarcely to be recognized as crystals, yet in a fairly large series their form can be clearly seen with a high magnification. Many of them are colourless; others are a bright yellow, orange or red, and give the clear pure tone of colour characteristic of some of our most familiar lichens.

The first definite discovery of a lichen-acid was made towards the beginning of the nineteenth century and is due to the researches of C. H. Pfaff[813]. He was engaged in an examination of Cetraria islandica, the Iceland Moss, which in his time was held in high repute, not only as a food but as a tonic. He wished to determine the chemical properties of the bitter principle contained in it, which was so much prized by the Medical Faculty of the period, though the bitterness had to be removed to render palatable the nutritious substance of the thallus. He succeeded in isolating an acid which he tested and compared with other organic acids and found that it was a new substance, nearest in chemical properties to succinic acid. In a final note, he states that the new “lichen-acid,” as he named it, approached still nearer to boletic acid, a constituent of a fungus, though it was distinct from that substance also in several particulars. The name “cetrarin” was proposed, at a later date, by Herberger[814] who described it as a “subalkaloidal substance, slightly soluble in cold water to which it gives a bitter taste; soluble in hot water, but, on continued boiling, throwing down a brown powder which is slightly soluble in alcohol and readily soluble in ether.” Knop and Schnederman[815] found that Herberger’s “cetrarin” was a compound substance and contained besides other substances “cetraric acid” and lichesterinic acid. It has now been determined by Hesse[816] as fumarprotocetraric acid (C₆₂H₅₀O₃₅), a derivative of which is cetraric acid or triaethylprotocetraric acid with the formula C₅₄H₃₉O₂₄(OC₂H₅)₃ and not C₂₀H₁₈O₉ as had been supposed. Cetraric acid has not yet been isolated with certainty from any lichen[817].

After this first isolation of a definite chemical substance, further research was undertaken, and gradually a number of these peculiar acids were recognized, the lichens examined being chiefly those that were of real or supposed economic value either in medicine or in the arts. In late years a wider chemical study of lichen products has been vigorously carried on, and the results gained have been recently arranged and published in book form by Zopf[818]. Many of the statements on the subject included here are taken from that work. Zopf gives a description of all the acids that had been discovered up to the date of publication, and the methods employed for extracting each substance. The structural formulae, the various affinities, derivatives and properties of the acids, with their crystalline form, are set forth along with a list of the lichens examined and the acids peculiar to each species. In many instances outline figures of the crystals obtained by extraction are given. For a fuller treatment of the subject, the student is referred to the book itself, as only a general account can be attempted here.

b. OCCURRENCE AND EXAMINATION OF LICHEN-ACIDS. Acids have been found, with few exceptions, in all the lichens examined. They are sometimes brightly coloured and are then easily visible under the microscope. Generally their presence can only be determined by reagents. Over 140 different kinds have been recognized and their formulae determined, though many are still imperfectly known. As a rule related lichen species contain the same acids, though in not a few cases one species may contain several different kinds. In growing lichens, they form 1 to 8 per cent. of the dry weight, and as they are practically, while unchanged, insoluble in water, they are not liable to be washed out by rain, snow or floods. Their production seems to depend largely on the presence of oxygen, as they are always found in greatest abundance on the more freely aerated parts of the thallus, such as the soredial hyphae, the outer rind or the loose medullary filaments. They are also often deposited on the exposed disc of the apothecium, on the tips of the paraphyses, and on the wall lining the pycnidia. They are absent from the thallus of the Collemaceae, these being extremely gelatinous lichens in which there can be little contact of the hyphae with the atmosphere. No free acids, so far as is known, are contained in Sticta fuliginosa, but a compound substance, trimethylamin, is present in the thallus of that lichen. It has also been affirmed that acids do not occur in any Peltigera nor in two species of Nephromium, but Zopf[818] has extracted a substance peltigerin both from species of Peltigera and from the section Peltidea.

For purposes of careful examination freshly gathered lichens are most serviceable, as the acids alter in herbarium or stored specimens. It is well, when possible, to use a fairly large bulk of material, as the acids are often present in small quantities. The lichens should be dried at a temperature not above 40°C. for fear of changing the character of the contained substances, and they should then be finely powdered. When only a small quantity of material is available, it has been recommended that reagents should be applied and the effect watched under the microscope with a low power magnification. This method is also of great service in determining the exact position of the acids in the thallus.

In micro-chemical examination, Senft[819] deprecates the use of chloroform, ether, etc., seeing that their too rapid evaporation leaves either an amorphous or crystalline mass of material which does not lend itself to further examination. He recommends as more serviceable some oil solution, preferably “bone oil” (neat’s-foot oil), in which a section of the thallus should be broken up under a cover-glass and subjected to a process of slow heating; some days must elapse before the extraction is complete. The surplus oil is then to be drained off, the section further bruised and the substance examined.

Acids in bulk should be extracted by ether, acetone, chloroform, benzole, petrol-ether and lignoin or by carbon bisulphide. Such solvents as alcohols, acetates and alkali solutions should not be used as they tend to split up or to alter the constitution of the acids. For the same reason, the use of chloroform is to a certain extent undesirable as it contains a percentage of alcohol. Ether and acetone, or a mixture of both, are the most efficient solvents, and all acids can be extracted by their use, if the material is left to soak a sufficient length of time, either in the cold or warmed. It is however advisable to follow with a second solvent in case any other acid should be present in the tissues. Concentrated sulphuric acid dissolves out all acids but often induces colour changes in the process.

All known lichen-acids form crystals, though the crystalline form may alter with the solution used. After filtering and distilling, the residue will be found to contain a mixture of these crystals along with other substances, which may be removed by washing, etc.

c. CHARACTER OF ACIDS. Many lichen-acids are more or less bitter to the taste; they are usually of an acid nature though certain of the substances are neutral, such as zeorin, a constituent of various Lecanoraceae, Physciaceae and Cladoniaceae, stictaurin, originally obtained from Sticta aurata, leiphemin, from Haematomma coccineum, and others.

A large proportion are esters or alkyl salts formed by the union of an alcohol and an acid; these are insoluble in alkaline carbonates. It is considered probable that the fungus generates the acid, while the alcohol arises in the metabolic processes in the alga. It has indeed been proved that the alcohol, erythrit, is formed in at least two algae, Protococcus vulgaris and Trentepohlia jolithus; and the lichen-acid, erythrin (C₂₀H₂₂O₁₀), obtained from species of Roccella in which the alga is Trentepohlia, is, according to Hesse, the erythrit ester of lecanoric acid (C₁₆H₁₄O₇), a very frequent constituent of lichen thalli. It is certain that the interaction of both symbionts is necessary for acid production. This was strikingly demonstrated by Tobler[820] in his cultural study of the lichen thallus. He succeeded in growing, to a limited extent, the hyphal part of the thallus of Xanthoria parietina on artificial media; but the filaments remained persistently colourless until he added green algal cells to the culture. Almost immediately thereafter the characteristic yellow colour appeared, proving the presence of parietin, formerly known as chrysophanic acid. Tobler’s observation may easily be verified in plants from natural habitats. A depauperate form of Placodium citrinum consisting mainly of a hypothallus of felted hyphae, with minute scattered granules containing algae, was tested with potash, and only the hyphae immediately covering the algal granules took the stain; the hypothallus gave no reaction.

It has been suggested[821] that when a decrease of albumenoids takes place, the quantity of lichen-acid increases, so that the excreted substance should be regarded as a sort of waste product of the living plant, “rather than as a product of deassimilation.” The subject is not yet wholly understood.

d. CAUSES OF VARIATION IN QUANTITY AND QUALITY OF LICHEN-ACIDS. Though it has been proved that lichen-acids are formed freely all the year round on any soil or in any region, it happens occasionally that they are almost or entirely lacking in growing plants. Schwarz[822] found this to be the case in certain plants of Lecanora tartarea, and he suggests that the gyrophoric acid contained in the outer cortex of that lichen had been broken up by the ammonia of the atmosphere into carbonic acid and orcin which is soluble in water, and would thus be washed away by rain. It has also been shown by Schwendener[823] and others that the outer layers of the older thallus in many lichens slowly perish, first breaking up and then peeling off; the denuded areas would therefore have lost, for some time at least, their particular acids. Fünfstück[824] considers that the difference in the presence and amount of acid in the same species of lichen may be due very often to variation in the chemical character of the substratum, and this view tallies with the results noted by Heber Howe[825] in his study of American Ramalinae. He observed that, though all showed a pale-yellow reaction with potash, those growing on mineral substrata gave a more pronouncedly yellow colour.

M. C. Knowles[826] found that in Ramalina scopulorum the colour reaction to potash varied extremely, being more rapid and more intense, the more the plants were subject to the influence of the sea-spray.

Lichen-acids are peculiarly abundant in soredia, and as, in some species, the thallus forms these outgrowths, or even becomes leprose more freely in damp weather, the amount of acids produced may depend on the amount of moisture in the atmosphere.

Their formation is also strongly influenced by light, as is well shown by the varying intensity of colour in some yellow thalli. Placodium elegans, always a brightly coloured lichen, changes from yellow to sealing-wax red in situations exposed to the full blaze of the sun. _Haematomma ventosum,_though greenish-yellow in lowland situations is intensely yellow in the high Alps. The same variation of colour is characteristic of Rhizocarpon geographicum which is a bright citron-yellow at high altitudes, and becomes more greenish in hue as it nears the plains. The familiar foliose lichen Xanthoria parietina is a brilliant orange-yellow in sunny situations, but grey-green in the shade, and then yielding only minute quantities of parietin. West[827] and others have noted its more luxuriant growth and brighter colour when it grows in positions where nitrogenous food is plentiful, such as the roofs of farm-buildings, which are supplied with manure-laden dust, and boulders by the sea-shore frequented by birds.

e. DISTRIBUTION OF ACIDS. Some acids, so far as is known, are only to be found in one or at most in very few lichens, as for instance cuspidatic acid which is present in Ramalina cuspidata, and scopuloric acid, a constituent of Ramalina scopulorum, the acids having been held to distinguish by their reactions the one plant from the other.

Others of these peculiar products are abundant and widely distributed. Usninic acid, one of the commonest, has been determined in some 70 species belonging to widely diverse genera, and atranorin, a substance first discovered in Lecanora atra, has been found again many times; Zopf gives a list of about 73 species or varieties from which it has been extracted. Another widely distributed acid is salazinic acid which has been found by Lettau[828] in a very large number of lichens.

E. CHEMICAL GROUPING OF LICHEN-ACIDS

Most of these acids have been provisionally arranged by Zopf in groups under the two great organic series: I. The Fat series; and II. The Benzole or Aromatic series.

I. LICHEN-ACIDS OF THE FAT SERIES

=Group 1.= Colourless substances soluble in alkali, the solution not coloured by iron chloride. Exs. protolichesterinic acid (C₁₉H₃₄O₄) obtained from species of Cetraria, and roccellic acid (C₁₇H₃₂O₄) from species of Roccella, from Lecanora tartarea, etc.

=Group 2.= Neutral colourless substances insoluble in alkalies, but soluble in alcohol, the solution not coloured by iron chloride. Exs. zeorin (C₅₂H₈₈O₄), a product of widely diverse lichens, such as Lecanora (Zeora) sulphurea, Haematomma coccineum, Physcia caesia, Cladonia deformis, etc. and barbatin (C₉H₁₄O), a product of Usnea barbata.

=Group 3.= Brightly coloured acids, yellow, orange or red, all derivatives of pulvinic acid (C₁₈H₁₂O₅), a laboratory compound which has not been found in nature. The group includes among others vulpinic acid (C₁₉H₁₄O₅) from the brilliant yellow Evernia (Letharia) vulpina, stictaurin (C₃₆H₂₂O₉) deposited in orange-red crystals on the hyphae of Sticta aurata, and rhizocarpic acid (C₂₆H₂₀O₆) chiefly obtained from Rhizocarpon geographicum: it crystallizes out in slender citron-yellow prisms.

=Group 4.= Only one acid, usninic (C₁₈H₁₆O₇), a derivative of acetylacetic acid, is placed in this group. It is of very wide-spread occurrence, having been found in at least 70 species belonging to very different genera and families of crustaceous shrubby and leafy lichens. Zopf himself isolated it from 48 species.

=Group 5.= The thiophaninic acid (C₁₂H₆O₉) group representing only a small number. They are all sulphur-yellow in colour and soluble in alcohol, the solution becoming blackish-green or dirty blue on the addition of iron chloride, with one exception, that of subauriferin obtained from the yellow-coloured medulla of Parmelia subaurifera which stains faintly wine-red in an iron solution. Thiophaninic acid, which gives its name to this group, occurs in Pertusaria lutescens and P. Wulfenii, both of which are yellowish crustaceous lichens growing mostly on the trunks of trees.

II. LICHEN-ACIDS OF THE BENZOLE SERIES

The larger number of lichen-acids belong to this series, of which 94 at least are already known. They are divided into two subseries: I. Orcine derivatives, and II. Anthracene derivatives.

SUBSERIES I. ORCINE DERIVATIVES

Zopf specially insists that the grouping of this series must be regarded as only a provisional arrangement of the many lichen-acids that are included therein. All of them are split up into orcine and carbonic acid by ammonia and other alkalies. On exposure to air, the ammoniacal or alkaline solution changes gradually into orceine, the colouring principle and chief constituent of commercial orchil. Orcine is not found free in nature. The orcine subseries includes five groups:

=Group 1.= The substances in this group form, with hypochlorite of lime (“CaCl”), red-coloured compounds which yield, on splitting, orsellinic acid. Zopf enumerates seven acids as belonging to this group, among which is lecanoric acid (C₁₆H₁₄O₇), found in many different lichens, e.g. Roccella tinctoria, Lecanora tartarea, etc.: whenever there is a differentiated pith and cortex it occurs in the pith alone. Erythrin (C₂₀H₂₂O₁₀), a constituent of the British marine lichen Roccella fuciformis, also belongs to this orsellinic group.

=Group 2.= Substances which also form red products with CaCl, but do not break up into orsellinic acid. Among the most noteworthy are olivetoric acid (C₂₁H₂₆O₇), a constituent of Evernia furfuracea, perlatic acid (C₂₈H₃₀O₁₀) and glabratic acid (C₂₄H₂₆O₁₁), which are obtained from species of Parmelia.

=Group 3.= Contains three acids of somewhat restricted occurrence. They do not form red products with CaCl, and they yield on splitting everninic acid. They are: evernic acid (C₁₇H₁₆O₇), found in Evernia prunastri var. vulgaris, ramalic acid (C₁₇H₁₆O₇) in Ramalina pollinaria, and umbilicaric acid (C₂₅H₂₂O₁₁) in species of Gyrophora.

=Group 4.= The numerous acids of this group are not easily soluble and have a very bitter taste. They are not coloured by CaCl; when extracted with concentrated sulphuric acid, the solution obtained is reddish-yellow or deep red. Among the most frequent are fumarprotocetraric acid (C₆₂H₅₀O₃₅), the bitter principle of Cetraria islandica, Cladonia rangiferina, etc., psoromic acid (C₂₀H₁₄O₉), obtained from Alectoria implexa, Lecanora varia, Cladonia pyxidata and many other lichens, and salazinic acid (C₁₉H₁₄O₁₀), recorded by Zopf as occurring in Stereocaulon salazinum and in several Parmeliae, but now found by Lettau[829] to be very wide-spread. He used micro-chemical methods and detected its presence in 72 species from twelve different families. The distribution of the acid in the thallus varies considerably.

=Group 5.= This is called the atranorin group from one of the most important members. They are colourless substances and, like the preceding group, are not affected by CaCl, but when split they form bodies that colour a more or less deep red with that reagent. Atranorin (C₁₉H₁₈O₈) is one of the most widely spread of all lichen-acids; it occurs in Lecanoraceae, Parmeliaceae, Physciaceae and Lecideaceae. Barbatinic acid (C₁₉H₂₀O₇), another member, is found in Usnea ceratina, Alectoria ochroleuca and in a variety of Rhizocarpon geographicum. A very large number of acids more or less fully studied belong to this group.

SUBSERIES II. ANTHRACENE DERIVATIVES

The constituents of this subseries are derived from the carbohydrate anthracene, and are characterized by their brilliant colours, yellow, red, brown, red-brown or violet-brown. So far, only ten different kinds have been isolated and studied. Parietin[830] (C₁₆H₁₂O₅), one of the best known, has been extracted from Xanthoria parietina, Placodium murorum and several other bright-yellow lichens; solorinic acid (C₁₅H₁₄O₅) occurs in orange-red crystals on the hyphae of the pith and under surface of Solorina crocea; nephromin (C₁₆H₁₂O₆) is found in the yellow medulla of Nephromium lusitanicum; rhodocladonic acid (C₁₂H₈O₆ or C₁₄H₁₀O₇) is the red substance in the apothecia of the red-fruited Cladoniae.

There are, in addition, a short series of coloured substances which are of uncertain position. They are imperfectly known and are of rare occurrence. An acid containing nitrogen has been extracted from Roccella fuciformis, and named picroroccellin[831] (C₂₇H₂₉N₃O₅). It crystallizes in comparatively large prisms, has an exceedingly bitter taste, and is very sparingly soluble. It is the only lichen-acid in which nitrogen has been detected.

One acid at least, belonging to the Fat series, vulpinic acid, which gives the greenish-yellow colour to Letharia vulpina, has been prepared synthetically by Volkard[832].

F. CHEMICAL REAGENTS AS TESTS FOR LICHENS

The employment of chemical reagents as colour tests in the determination of lichen species was recommended by Nylander[833] in a paper published by him in 1866. Many acids had already been extracted and examined, and as they were proved to be constant in the different species where they occurred, he perceived their systematic importance. As an example of the new tests, he cited the use of hypochlorite of lime, a solution of which, applied directly to the thallus of species of Roccella, produced a bright-red “erythrinic” reaction. Caustic potash was also found to be of service in demonstrating the presence of parietin in lichens by a beautiful purple stain. Many lichenologists eagerly adopted the new method, as a sure and ready means of distinguishing doubtful species; but others have rejected the tests as unnecessary and not always to be relied on, seeing that the acids are not always produced in sufficient abundance to give the desired reaction, and that they tend to alter in time.

The reagents most commonly in use are caustic potash, generally indicated by K; hypochlorite of calcium or bleaching powder by CaCl; and a solution of iodine by I. The sign + signifies a colour reaction, while- indicates that no change has followed the application of the test solution. Double signs ⁺₊ or any similar variation indicate the upper or lower parts of the thallus affected by the reagent. In some instances the reaction only follows after the employment of two reagents represented thus: K(CaCl)+. In such a case the potash breaks up the particular acid and compounds are formed which become red, orange, etc., on the subsequent application of hypochlorite of lime.

As an instance of the value of chemical tests, Zopf cites the reaction of hypochlorite of lime on the thallus of four different species of Gyrophora, the “tripe de roche”:—

Gyrophora torrefacta CaCl ⁻₊. ” polyrhiza CaCl ⁺₊. ” proboscidea CaCl ⁺₋. ” erosa CaCl ⁻₋.

It must however be borne in mind that these species are well differentiated and can be recognized, without difficulty, by their morphological characters. Experienced systematists like Weddell refuse to accept the tests unless they are supported by true morphological distinctions, as the reactions are not sufficiently constant.

G. CHEMICAL REACTIONS IN NATURE

Similar colour changes may often be observed in nature. The acids of the exposed thallus cortex are not unfrequently split up by the gradual action of the ammonia in the atmosphere, one of the compounds thus set free being at the same time coloured by the alkali. Thus salazinic acid, a constituent of several of our native Parmeliae, is broken up into carbonic acid and salazininic acid, the latter taking a red colour. Fumarprotocetraric acid is acted on somewhat similarly, and the red colour may be seen in Cetraria at the base of the thallus where contact with soil containing ammonia has affected the outer cortex of the plant. The same results are produced still more effectively when the lichen comes into contact with animal excrement.

Gummy exudations from trees which are more or less ammoniacal may also act on the thallus and form red-coloured products on contact with the acids present. Lecanora (Aspicilia) cinerea is so easily affected by alkalies that a thin section left exposed may become red in time owing to the ammonia in the atmosphere.

II. GENERAL NUTRITION

A. ABSORPTION OF WATER

Lichens are capable of enduring almost complete desiccation, but though they can exist with little injury through long periods of drought, water is essential to active metabolism. They possess no special organs for water conduction, but absorb moisture over their whole surface. Several interdependent factors must therefore be taken into account in considering the question of absorption: the type of thallus, whether gelatinous or non-gelatinous, crustaceous, foliose or fruticose, as also the nature of the substratum and the prevailing condition of the atmosphere.

a. GELATINOUS LICHENS. The algal constituent of these lichens is some member of the Myxophyceae and is provided with thick gelatinous walls which have great power of imbibition and swell up enormously in damp surroundings, becoming reservoirs of water. Species of Collema, for instance, when thoroughly wet, weigh thirty-five times more than when dry[834]. There are no interstices in the thallus and frequently no cortex in these lichens, but the gelatinous substance itself forms on drying an outer skin that checks evaporation so that water is retained within the thallus for a longer period than in non-gelatinous forms. They probably always retain some amount of moisture, as they share with gelatinous algae the power of revival after long desiccation.

Gelatinous lichens are entirely dependent on a surface supply of water: their hyphae—or rhizinae when present—rarely penetrate the substratum.

b. CRUSTACEOUS NON-GELATINOUS LICHENS. The lichens with this type of thallus are in intimate contact with the substratum whether it be soil, rock, tree or dead wood. The hyphae on the under surface of the thallus function primarily as hold-fasts, but if water be retained in the substratum, the lichen will undoubtedly benefit, and water, to some extent, will be absorbed by the walls of the hyphae or will be drawn up by capillary attraction. In any case, it could only be surface water that would be available, as lichens have no means of tapping any deeper sources of supply.

Lichens are, however, largely independent of the substratum for their supply of water. Sievers[835], who gave attention to the subject, found that though some few crustaceous lichens took up water from below, most of them absorbed the necessary moisture on the surface or at the edges of the thallus or areolae, where the tissue is looser and more permeable. The swollen gelatinous walls of the hyphae forming the upper layers of such lichens are admirably adapted for the reception and storage of water, though, according to Zukal[836], less hygroscopic generally than in the larger forms. Beckmann[837] proved this power of absorption, possessed by the upper cortex, by placing a crustaceous lichen, Haematomma sp., in a damp chamber: he found after a while that water had been taken up by the cortex and by the gonidial zone, while the lower medullary hyphae had remained dry.

Herre[838] has recorded an astonishing abundance of lichens from the desert of Reno, Nevada, and these are mostly crustaceous forms, belonging to a limited number of species. The yearly rainfall of the region is only about eight or ten inches, and occurs during the winter months, chiefly as snow. It is during that period that active vegetation goes on; but the plants still manage to exist during the long arid summer, when their only possible water supply is that obtained from the moisture of the atmosphere during the night, or from the surface deposit of dews.

c. FOLIOSE LICHENS. Though many of the leafy lichens are provided with a tomentum of single hyphae, or with rhizinae on the under surface, the principal function of these structures is that of attaching the thallus. Sievers[839] tested the areas of absorption by placing pieces of the thallus of Parmeliae, of Evernia furfuracea, and of Cetraria glauca in a staining solution. After washing and cutting sections, it was seen that the coloured fluid had penetrated by the upper surface and by the edge of the thallus, as in crustaceous forms, but not through the lower cortex.

By the same methods of testing, he proved that water penetrates not only by capillarity between the closely packed hyphae, but also within the cells. A considerable number of lichens were used for experiment, and great variations were found to exist in the way in which water was taken up. It has been proved that in some species of Gyrophora water is absorbed from below: in those in which rhizinae are abundant, water is held by them and so gradually drawn up into the thallus; the upper cortex in this genus is very thick and checks transpiration. Certain other northern lichens such as Cetraria islandica, Cladonia rangiferina, etc., imbibe water very slowly, and they, as well as Gyrophora, are able to endure prolonged wet periods.

That foliose lichens do not normally contain much water was proved by Jumelle[840] who compared the weight of seven different species when freshly gathered, and after being dried; he found that the proportion of fresh weight to dry weight showed least variation in Parmelia acetabulum, as 1·14 to 1; in Xanthoria parietina it was as 1·21 to 1.

d. FRUTICOSE LICHENS. There is no water-conducting tissue in the elongate thallus of the shrubby or filamentous lichens, as can easily be tested by placing the base in water: it will then be seen that the submerged parts alone are affected. Many lichens are hygroscopic and become water-logged when placed simply in damp surroundings. The thallus of Usnea, for instance, can absorb many times its weight of water: a mass of Usnea filaments that weighed 3·8 grms. when dry increased to 13·3 grms. after having been soaked in water for twelve hours. Schrenk[841], who made the experiment, records in a second instance an increase in weight from 3·97 grms. to 11·18 grms. The Cladoniae retain large quantities of water in their upright hollow podetia. The Australian species, Cladonia retepora, the podetium of which is a regular network of holes, competes with the Sphagnum moss in its capacity to take up water.

To conclude: as a rule, heteromerous, non-gelatinous lichens do not contain large quantities of water, the weight of fresh plants being generally about three times only that of the dry weight. Their ordinary water content is indeed smaller than that of most other plants, though it varies at once with a change in external conditions. It is noteworthy that a number of lichens have their habitat on the sea-shore, constantly subject to spray from the waves, but scarcely any can exist within the spray of a waterfall, possibly because the latter is never-ceasing.

B. STORAGE OF WATER

The gonidial algae Gloeocapsa, Scytonema, Nostoc, etc. among Myxophyceae, Palmella and occasionally Trentepohlia among Chlorophyceae, have more or less gelatinous walls which act as a natural reservoir of water for the lichens with which they are associated. In these lichens the hyphae for the most part have thin walls, and the plectenchyma when formed—as below the apothecium in Collema granuliferum, or as a cortical layer in Leptogium—is a thin-walled tissue. In lichens where, on the contrary, the alga is non-gelatinous—as generally in Chlorophyceae—or where the gelatinous sheath is not formed as in the altered Nostoc of the Peltigera thallus, the fungal hyphae have swollen gelatinous walls both in the pith and the cortex, and not only imbibe but store up water.

Bonnier[842] had his attention directed to this thickening of the cell-walls as he followed the development of the lichen thallus. He made cultures from the ascospore of Physcia (Xanthoria) parietina and obtained a fair amount of hyphal tissue, the cell-walls of which became thickened, but more slowly and to a much less extent than when associated with the gonidia.

He noted also that when his cultures were kept in a continuously moist atmosphere there was much less thickening, scarcely more than in fungi ordinarily; it was only when they were grown under drier conditions with necessity for storage, that any considerable swelling of the walls took place. Further he found that the thallus of forms cultivated in an abundance of moisture could not resist desiccation as could those with the thicker membranes. These latter survived drying up and resumed activity when moisture was supplied.

C. SUPPLY OF INORGANIC FOOD

As in the higher plants, mineral substances can only be taken up when they are in a state of solution. Lichens are therefore dependent on the substances that are contained in the water of absorption: they must receive their inorganic nutriment by the same channels that water is conveyed to them.

a. FOLIOSE AND FRUTICOSE LICHENS. These larger lichens are provided with rhizinae or with hold-fasts, which are only absorptive to a very limited extent; the main source of water supply is from the atmosphere and the salts required in the metabolism of the cell must be obtained there also—from atmospheric dust dissolved in rain, or from wind-borne particles deposited on the surface of the thallus which may be gradually dissolved and absorbed by the cortical and growing hyphae. That substances received from the atmospheric environment may be all important is shown by the exclusive habitat of some marine lichens; the Roccellae, Lichinae, some species of Ramalina and others which grow only on rocky shores are almost as dependent on sea-water as are the submerged algae. Other lichens, such as Hydrothyria venosa and Lecanora lacustris, grow in streams, or on boulders that are subject to constant inundation, and they obtain their inorganic food mainly, if not entirely, from an aqueous medium.

Though lichens cannot live in an atmosphere polluted by smoke, they thrive on trees and walls by the road-side where they are liable to be almost smothered by soil-dust. West[843] has observed that they flourish in valleys that are swept by moisture laden winds more especially if near to a highway, where animal excreta are mingled with the dust. The favourite habitats of Xanthoria parietina are the walls and roofs of farm-buildings where the dust must contain a large percentage of nitrogenous material; or stones by the sea-shore that are the haunts of sea-birds. Sandstede[844] found on the island of Rügen that while the perpendicular faces of the cliffs were quite bare, the tops bore a plentiful crop of Lecanora saxicola, Xanthoria lychnea and Candellariella vitellina. He attributed their selection of habitat to the presence of the excreta of sea-birds. As already stated the connection of foliose and fruticose lichens with the substratum is mainly mechanical but occasionally a kind of semiparasitism may arise. Friedrich[845] gives an instance in a species of Usnea of unusually vigorous development. It grew on bark and the strands of hyphae, branching from the root-base of the lichen, had reached down to the living tissue of the tree-trunk and had penetrated between the cells by dissolving the middle lamella. It was possible to find holes pierced in the cell-walls of the host, but it was difficult to decide if the hyphae had attacked living cells or were merely preying on dead material. Lindau[846] held very strongly that lichen hyphae were non-parasitic, and merely split apart the tissues already dead, and the instance recorded by Friedrich is of rare occurrence[847].

That the substratum does have some indirect influence on these larger lichens has been proved once and again. Uloth[848], a chemist as well as a botanist, made analyses of plants of Evernia prunastri taken from birch bark and from sandstone. Qualitatively the composition of the lichen substances was the same, but the quantities varied considerably. Zopf[849] has, more recently, compared the acid content of a form of Evernia furfuracea on rock with that of the same species growing on the bark of a tree. In the case of the latter, the thallus produced 4 per cent. of physodic acid and 2·2 per cent. of atranorin. In the rock specimen, which, he adds, was a more graceful plant than the other, the quantities were 6 per cent. of physodic acid, and 2·75 per cent. of atranorin. In both cases there was a slight formation of furfuracinnic acid. He found also that specimens of Evernia prunastri on dead wood contained 8·4 per cent. of lichen-acids, while in those from living trees there was only 4·4 per cent. or even less. Other conditions, however, might have contributed to this result, as Zopf[850] found later that this lichen when very sorediate yielded an increased supply of atranoric acid.

Ohlert[851], who made a study of lichens in relation to their habitat, found that though a certain number grew more or less freely on either tree, rock or soil, none of them was entirely unaffected. Usnea barbata, Evernia prunastri and Parmelia physodes were the most indifferent to habitat; normally they are corticolous species, but Usnea on soil formed more slender filaments, and Evernia on the same substratum showed a tendency to horizontal growth, and became attached at various points instead of by the usual single base.

b. CRUSTACEOUS LICHENS. The crustaceous forms on rocks are in a more favourable position for obtaining inorganic salts, the lower medullary hyphae being in direct contact with mineral substances and able to act directly on them. Many species are largely or even exclusively calcicolous, and there must be something in the lime that is especially conducive to their growth. The hyphae have been traced into the limestone to a depth of 15 mm.[852] and small depressions are frequently scooped out of the rock by the action of the lichen, thus giving a lodgement to the foveolate fruit.

On rocks mainly composed of silica, the lichen has a much harder substance to deal with, and one less easily affected by acids, though even silica may be dissolved in time. Uloth[853] concluded from his observations that the relation of plants to the substratum was chemical even more than physical, so far as crustaceous species were concerned. He found that the surface of the area of rock inhabited was distinctly marked: even such a hard substance as chalcedony was corroded by a very luxuriant lichen flora, the border of growth being quite clearly outlined. The corrosive action is due he considered to the carbon dioxide liberated by the plant, though oxalic acid, so frequent a constituent of lichens, may also share in the corrosion. Egeling[854] made similar observations in regard to the effect of lichen growth on granite rocks; and he further noticed that pieces of glass, over which lichens had spread, had become clouded, the dulness of the surface being due to a multitude of small cracks eaten out by the hyphae. Buchet[855] also gives an instance of glass which had been corroded by the action of lichen hyphae. It formed part of an old stained window in a chapel that was obscured by a lichen growth which adhered tenaciously. When the window was taken down and cleaned, it was found that the surface of the glass was covered with small, more or less hemispherical pits which were often confluent. The different colours in the picture were unequally attacked, some of the figures or draperies being covered with the minute excavations, while other parts were intact. It happened also, occasionally, that a colour while slightly corroded in one pane would be uninjured in another, but the suggestion is made that there might in that case have been a difference in the length of attack by the lichen. The selection of colours by the lichens might also be influenced by some chemical or physical characters.

Bachmann[856] found that on granite there is equally a selection of material by the hyphae: as a rule they avoid the acid silica constituents; while they penetrate and traverse the grains of mica which are dissolved by them exactly as are lime granules.

On another rock consisting mainly of muscovite and quartz he[857] found that crystals of garnet embedded in the rock were reduced to a powder by the action of the lichen. He concludes that the destroying action of the hyphae is accelerated by the presence of carbon dioxide given off by the lichen, and dissolved in the surrounding moisture. Lang[858] and Stahlecker[859] have both come to the conclusion that even the quartz grains are corroded by the lichen hyphae. Stahlecker finds that they change the quartz into amorphous silicic acid, and thus bring it into the cycle of organic life. Chalk and magnesia are extracted from the silicates where no other plant could procure them. Lichens are generally rare on pure quartz rocks, chiefly, however, for the mechanical reason that the structure is of too close a grain to afford a foothold.

D. SUPPLY OF ORGANIC FOOD

a. FROM THE SUBSTRATUM. The Ascomycetous fungi, from which so many of the lichens are descended, are mainly saprophytes, obtaining their carbohydrates from dead plant material, and lichen hyphae have in some instances undoubtedly retained their saprophytic capacity. It has been proved that lichen hyphae, which naturally could not exist without the algal symbiont, may be artificially cultivated on nutrient media without the presence of gonidia, though the chief and often the only source of carbon supply is normally through the alga with which the hyphae are associated in symbiotic union.

A large number of crustaceous lichens grow on the bark of trees, and their hyphae burrow among the dead cells of the outer bark using up the material with which they come in contact. Others live on dead wood, palings, etc. where the supply of disintegrated organic substance is even greater; or they spread over withered mosses and soil rich in humus.

b. FROM OTHER LICHENS. Bitter[860] has recorded several instances observed by him of lichens growing over other lichens and using up their substance as food material. Some lichens are naturally more vigorous than others, and the weaker or more slow growing succumb when an encounter takes place. Pertusaria globulifera is one of these marauding species; its habitat is among mosses on the bark of trees, and, being a quick grower, it easily overspreads its more sluggish neighbours. It can scarcely be considered a parasite, as the thallus of the victim is first killed, probably by the action of an enzyme.

Lecanora subfusca and allied species which have a thin thallus are frequently overgrown by this Pertusaria and a dark line generally precedes the invading lichen; the hyphae and the gonidia of the Lecanorae are first killed and changed to a brown structureless mass which is then split up by the advancing hyphae of the Pertusaria into small portions. A little way back from the edge of the predatory thallus the dead particles are no longer visible, having been dissolved and completely used up. Pertusaria amara also may overgrow Lecanorae, though, generally, its onward course is checked and deflected towards a lateral direction; if however it is in a young and vigorous condition, it attacks the thallus in its path, and ahead of it appears the rather broad blackish line marking the fatal effect of the enzyme, the rest of the host thallus being unaffected. Neither Pertusaria seems to profit much, and does not grow either faster or thicker; the thallus appears indeed to be hindered rather than helped by the encounter. Biatora (Lecidea) quernea with a looser, more furfuraceous thallus is also killed and dissolved by Pertusariae; but if the Biatora is growing near to a withering or dead lichen it, also, profits by the food material at hand, grows over it and uses it up. Bitter has also observed lichens overgrown by Haematomma sp.; the growth of that lichen is indeed so rapid that few others can withstand its approach.

Another common rock species, Lecanora sordida (L. glaucoma), has a vigorous thallus that easily ousts its neighbours. Rhizocarpon geographicum, a slow-growing species, is especially liable to be attacked; from the thallus of L. sordida the hyphae in strands push directly into the other lichen in a horizontal direction and split up the tissues, the algae persist unharmed for some time, but eventually they succumb and are used up; the apothecia, though more resistant than the thallus, are also gradually undermined and hoisted up by the new growth, till finally no trace of the original lichen is left. Lecanora sordida is however in turn invaded by Lecidea insularis (L. intumescens) which is found forming small orbicular areas on the Lecanora thallus. It kills its host in patches and the dead material mostly drifts away. On any strands that are left Candellariella vitellina generally settles and evidently profits by the dead nutriment. It does not spread to the living thallus. Lecanora polytropa also forms colonies on these vacant patches, with advantage to its growth.

Even the larger lichens are attacked by these quick-growing crusts. Pertusaria globulifera spreads over Parmelia perlata and P. physodes, gradually dissolving and consuming the different thalline layers; the lower cortex of the victim holds out longest and can be seen as an undigested black substance within the Pertusaria thallus for some time. As a rule, however, the lichens with large lobes grow over the smaller thalli in a purely mechanical fashion.

c. FROM OTHER VEGETATION. Zukal[861] has given instances of association between mosses and lichens in which the latter seemed to play the part of parasite. The terricolous species Baeomyces rufus (Sphyridium) and Biatora decolorans, as well as forms of Lepraria and Variolaria, he found growing over mosses and killing them. Stems and leaves of the moss Plagiothecium sylvaticum were grown through and through by the hyphae of a Pertusaria, and he observed a leaf of Polytrichum commune pierced by the rhizinae of a minute Cladonia squamule. The cells had been invaded and the neighbouring tissue was brown and dead.

Perhaps the most voracious consumer of organic remains is Lecanora tartarea, more especially the northern form frigida. It is the well-known cudbear lichen of West Scotland, and is normally a rock species. It has an extremely vigorous thickly crustaceous and quick-growing thallus, and spreads over everything that lies in its path—decaying mosses, dead leaves, other lichens, etc. Kihlman[862] has furnished a graphic description of the way it covers up the vegetation on the high altitudes of Russian Lapland. More than any other plant it is able to withstand the effect of the cold winds that sweep across these inhospitable plains. Other plant groups at certain seasons or in certain stages of growth are weakened or killed by the extreme cold of the wind, and, immediately, a growth of the more hardy grey crust of Lecanora tartarea begins to spread over and take possession of the area affected—very frequently a bank of mosses, of which the tips have been destroyed, is thus covered up. In the same way the moorland Cladoniae, C. rangiferina (the reindeer moss) and some allied species, are attacked. They have no continuous cortex, the outer covering of the long branching podetia being a loose felt of hyphae; they are thus sensitive to cold and liable to be destroyed by a high wind, and their stems, which are blackened as decay advances, become very soon dotted with the whitish-grey crust of the more vigorous and resistant Lecanora.

III. ASSIMILATION AND RESPIRATION

A. INFLUENCE OF TEMPERATURE

a. HIGH TEMPERATURE. It has been proved that plants without chlorophyll are less affected by great heat than those that contain chlorophyll. Lichens in which both types are present are more capable of enduring high temperatures than the higher plants, but with undue heat the alga succumbs first. In consequence, respiration, by the fungus alone, can go on after assimilation (photosynthesis) and respiration in the alga have ceased.

Most Phanerogams cease assimilation and respiration after being subjected for ten minutes to a temperature of 50° C. Jumelle[863] made a series of experiments with lichens, chiefly of the larger fruticose or foliaceous types, with species of Ramalina, Physcia and Parmelia, also with Evernia prunastri and Cladonia rangiferina. He found that as regards respiration, plants which had been kept for three days at 45° C., fifteen hours at 50°, then five hours at 60°, showed an intensity of respiration almost equal to untreated specimens, gaseous interchange being manifested by an absorption of oxygen and a giving up of carbon dioxide.

The power of assimilation was more quickly destroyed: as a rule it failed after the plants had been subjected successively to a temperature of one day at 45° C., then three hours at 50° and half-an-hour at 60°. The assimilating green alga, being less able to resist extreme heat, as already stated, succumbed more quickly than the fungus. Jumelle also gives the record of an experiment with a crustaceous lichen, Lecidea (Lecanora) sulphurea, a rock species. It was kept in a chamber heated to 50° for three hours and when subsequently placed in the sunlight respiration took place but no assimilation.

Very high temperatures may be endured by lichen plants in quite natural conditions, when the rock or stone on which they grow becomes heated by the sun. Zopf[864] tested the thalli of crustaceous lichens in a hot June, under direct sunlight, and found that the thermometer registered 55° C.

b. LOW TEMPERATURE. Lichens support extreme cold even better than extreme heat. In both cases it is the power of drying up and entering at any season into a condition of lowered or latent vitality that enables them to do so. In winter during a spell of severe cold they are generally in a state of desiccation, though that is not always the case, and resistance to cold is not due to their dry condition. The water of imbibition is stored in the cell-walls and it has been found that lichens when thus charged with moisture are able to resist low temperatures, even down to -40° C. or -50° as well as when they are dry. Respiration in that case was proved by Jumelle[865] to continue to -10°, but assimilation was still possible at a temperature of -40°: Evernia prunastri exposed to that extreme degree of cold, but in the presence of light, decomposed carbon dioxide and gave off oxygen.

B. INFLUENCE OF MOISTURE

a. ON VITAL FUNCTIONS. Gaseous interchange has been found to vary according to the degree of humidity present[865]. In lichens growing in sheltered positions, or on soil, there is less complete desiccation, and assimilation and respiration may be only enfeebled. Lichens more exposed to the air—those growing on trees, etc.—dry almost completely and gaseous interchange may be no longer appreciable. In severe cold any water present would become frozen and the same effect of desiccation would be produced. At normal temperatures, on the addition of even a small amount of moisture the respiratory and assimilative functions at once become active, and to an increasing degree as the plant is further supplied with water until a certain optimum is reached, after which the vital processes begin somewhat to diminish.

Though able to exist with very little moisture, lichens do not endure desiccation indefinitely, and both assimilation and respiration probably cease entirely during very dry seasons. A specimen of Cladonia rangiferina was kept dry for three months, and then moistened: respiration followed but it was very feeble and assimilation had almost entirely ceased. Somewhat similar results were obtained with Ramalina farinacea and Usnea barbata.

In normal conditions of moisture, and with normal illumination, assimilation in lichens predominates over respiration, more carbon dioxide being decomposed than is given forth; and Jumelle has argued from that fact, that the alga is well able to secure from the atmosphere all the carbon required for the nutrition of the whole plant. The intensity of assimilation, however, varies enormously in different lichens and is generally more powerful in the larger forms than in the crustaceous: the latter have often an extremely scanty thallus and they are also more in contact with the substratum—rock, humus or wood—on which they may be partly saprophytic, thus obtaining carbohydrates already formed, and demanding less from the alga.

An interesting comparison might be made with fungi in regard to which many records have been taken as to their possible duration in a dry state, more especially on the viability of spores, i.e. their persistent capacity of germination. A striking instance is reported by Weir[866] of the regeneration of the sporophores of Polystictus sanguineus, a common fungus of warm countries. The plant was collected in Brazil and sent to Munich. After about two years in the mycological collection of the University, the branch on which it grew was exposed in the open among other branches in a wood while snow still lay on the ground. In a short time the fungus revived and before the end of spring not only had produced a new hymenium, but enlarged its hymenial surface to about one-fourth of its original size and had also formed one entirely new, though small, sporophore.

b. ON GENERAL DEVELOPMENT. Lichens are very strongly influenced by abundance or by lack of moisture. The contour of the large majority of species is concentric, but they become excentric owing to a more vigorous development towards the side of damper exposure, hence the frequent one-sided increase of monophyllous species such as Umbilicaria pustulata. Wainio[867] observed that species of Cladonia growing in dry places, and exposed to full sunlight, showed a tendency not to develop scyphi, the dry conditions hindering the full formation of the secondary thallus. As an instance may be cited Cl. foliacea, in which the primary thallus is much the most abundantly developed, its favourite habitat being the exposed sandy soil of sea-dunes.

Too great moisture is however harmful: Nienburg[868] has recorded his observations on Sphyridium (Baeomyces rufus): on clay soil the thallus was pulverulent, while on stones or other dryer substratum it was granular—warted or even somewhat squamulose.

Parmelia physodes rarely forms fruits, but when growing in an atmosphere constantly charged with moisture[869], apothecia are more readily developed, and the same observation has been made in connection with other usually barren lichens. It has been suggested that, in these lichens, the abrupt change from moist to dry conditions may have a harmful effect on the developing ascogonium.

The perithecia of Pyrenula nitida are smaller on smooth bark[870] such as that of Corylus, _Carpinu_s, etc., probably because the even surface does not retain water.

IV. ILLUMINATION OF LICHENS

A. EFFECT OF LIGHT ON THE THALLUS

As fungi possess no chlorophyll, their vegetative body has little or no use for light and often develops in partial or total darkness. In lichens the alga requires more or less direct illumination; the lichen fungus, therefore, in response to that requirement has come out into the open: it is an adaptation to the symbiotic life, though some lichens, such as those immersed in the substratum, grow with very little light. Like other plants they are sensitive to changes of illumination: some species are shade plants, while others are as truly sun plants, and others again are able to adapt themselves to varying degrees of light.

Wiesner[871] made a series of exact observations on what he has termed the “light-use” of various plants. He took as his standard of unity for the higher plants the amount of light required to darken photographic paper in one second. When dealing with lichens he adopted a more arbitrary standard, calculating as the unit the average amount of light that lichens would receive in entirely unshaded positions. He does not take account of the strength or duration of the light, and the conclusions he draws, though interesting and instructive, are only comparative.

a. SUN LICHENS. The illumination of the Tundra lichens is reckoned by Wiesner as representing his unit of standard illumination. In the same category as these are included many of our most familiar lichens, which grow on rocks subject to the direct incidence of the sun’s rays, such as, for instance, Parmelia conspersa, P. prolixa, etc. Physcia tenella (hispida) is also extremely dependent on light, and was never found by Wiesner under 1/8 of full illumination. Dermatocarpon miniatum, a rock lichen with a peltate foliose thallus, is at its best from 1/3 to 1/8 of illumination, but it grows well in situations where the light varies in amount from 1 to 1/24. Psora (Lecidea) lurida, with dark-coloured crowded squamules, grows on calcareous soil among rocks well exposed to the sun and has an illumination from 1 to 1/30, but with a poorer development at the lower figure. Many crustaceous rock lichens are also by preference sun-plants as, for instance, Verrucaria calciseda which grows immersed in calcareous rocks but with an illumination of 1 to 1/3; in more shady situations, where the light had declined to 1/29, it was found to be less luxuriant and less healthy.

Sun lichens continue to grow in the shade, but the thallus is then reduced and the plant is sterile. Zukal has made a list of those which grow best with a light-use of 1 to 1/10, though they are also found not unfrequently in habitats where the light cannot be more than 1/50. Among these light-loving plants are the Northern Tundra species of Cladonia, Stereocaulon, Cetraria, Parmelia, Umbilicaria, and Gyrophora, as also Xanthoria parietina, Placodium elegans, P. murorum, etc., with some crustaceous species such as Lecanora atra, Haematomma ventosum, Diploschistes scruposus, many species of Lecideaceae, some Collemaceae and some Pyrenolichens.

Wiesner’s conclusion is that the need of light increases with the lowering of the temperature, and that full illumination is of still more importance in the life of the plants when they grow in cold regions and are deprived of warmth: sun lichens are, therefore, to be looked for in northern or Alpine regions rather than in the tropics.

b. COLOUR-CHANGES DUE TO LIGHT. Lichens growing in full sunlight frequently take on a darker hue. Cetraria islandica for instance in an open situation is darker than when growing in woods; C. aculeata on bare sand-dunes is a deeper shade of brown than when growing entangled among heath plants. Parmelia saxatilis when growing on exposed rocks is frequently a deep brown colour, while on shaded trees it is normally a light bluish-grey.

An example of colour-change due directly to light influences is given by Bitter[872]. He noted that the thallus of Parmelia obscurata on pine trees, and therefore subject only to diffuse light, grew to a large size and was of a light greyish-green colour marked by lighter-coloured lines, the more exposed lobes being always the most deeply tinted. In a less shaded habitat or in full sunlight the lichen was distinguished by a much darker colour, and the lobes were seamed and marked by blackish lines and spots. Bruce Fink[873] noted a similar development of dark lines on the thallus of certain rock lichens growing in the desert, more especially on Parmelia conspersa, Acarospora xanthophana and Lecanora muralis. He attributes a protective function to the dark colour and observes that it seemingly spreads from centres of continued exposure, and is thus more abundant in older parts of the thallus. He contrasts this colouration with the browning of the tips of the fronds of fruticose lichens by which the delicate growing hyphae are protected from intense light.

Galløe[874] finds that protection against too strong illumination is afforded both by white and dark colourations, the latter because the pigments catch the light rays, the former because it throws them back. The white colour is also often due to interspaces filled with air which prevent the penetration of the heat rays.

A deepening of colour due to light effect often visible on exposed rock lichens such as Parmelia saxatilis is more pronounced still in Alpine and tropical species: the cortex becomes thicker and more opaque through the cuticularizing and browning of the hyphal membranes, and the massing of crystals on the lighted areas. The gonidial layer becomes, in consequence, more reduced, and may disappear altogether. Zukal[875] found instances of this in species of Cladonia, Parmelia, Roccella, etc. The thickened cortex acts also as a check to transpiration and is characteristic of desert species exposed to strong light and a dry atmosphere.

Bitter[876] remarked the same difference of development in plants of Parmelia physodes: he found that the better lighted had a thicker cortex, about 20-30 µ in depth, as compared with 15-22 µ or even only 12 µ in the greener shade-plants, and also that there was a greater deposit of acids in the more highly illuminated cortices, thus giving rise to the deeper shades of colour.

Many lichens owe their bright tints to the presence of coloured lichen-acids, the production of which is strongly influenced by light and by clear air. Xanthoria parietina becomes a brilliant yellow in the sunlight: in the shade it assumes a grey-green hue and yields only small quantities of parietin. Placodium elegans, normally a brightly coloured yellow lichen, becomes, in the strong light of the high Alps, a deep orange-red. Rhizocarpon geographicum is a vivid citrine-yellow on high mountains, but is almost green at lesser elevations.

c. SHADE LICHENS. Many species grow where the light is abundant though diffuse. Those on tree-trunks rarely receive direct illumination and may be generally included among shade-plants. Wiesner found that corticolous forms of Parmelia saxatilis grew best with an illumination between 1/8 and 1/17 of full light, and Pertusaria amara from 1/12 to 1/21; both of them could thrive from 1/3 to 1/56, but were never observed on trees in direct light. Physcia ciliaris, which inhabits the trunks of old trees, is also a plant that prefers diffuse light. In warm tropical regions, lichens are mostly shade-plants: Wiesner records an instance of a species found on the aerial roots of a tree with an illumination of only 1/250.

In a study of subterranean plants, Maheu[877] takes note of the lichens that he found growing in limestone caves, in hollows and clefts of the rocks, etc. A fair number grew well just within the opening of the caves; but species such as Cl. cervicornis, Placodium murorum and Xanthoria parietina ceased abruptly where the solar rays failed. Only a few individuals of one or two species were found to remain normal in semi-darkness: Opegrapha hapalea and Verrucaria muralis were found at the bottom of a cave with the thallus only slightly reduced. The nature of the substratum in these cases must however also be taken into account, as well as the light influences: limestone for instance is a more favourable habitat than gypsum; the latter, being more readily soluble, provides a less permanent support.

Maheu has recorded observations on growth in its relation to light in the case of a number of lichens growing in caves.

Physcia obscura grew in almost total darkness; Placodium murorum within the cave had lost nearly all colour; Placodium variabile var. deep within the cave, sterile; Opegrapha endoleuca in partial obscurity; Verrucaria rupestris f. in total obscurity, the thallus much reduced and sterile; Verrucaria rupestris in partial obscurity, the asci empty; Homodium (Collema) granuliferum in the inmost recess of the cave, sterile, and the hyphae more spongy than in the open.

Siliceous rocks in darkness were still more barren, but a few odd lichens were collected from sandstone in various caves: Cladonia squamosa, Parmelia perlata var. ciliata, Diploschistes scruposus, Lecidea grisella, Collema nigrescens and Leptogium lacerum.

d. VARYING SHADE CONDITIONS. It has been frequently observed that on the trees of open park lands lichens are more abundant on the side of the trunk that faces the prevailing winds. Wiesner[878] remarks that spores and soredia would more naturally be conveyed to that side; but there are other factors that would come into play: the tree and the branches frequently lean away from the wind, giving more light and also an inclined surface that would retain water for a longer period on the windward side[879]. Spores and soredia would also develop more readily in those favourable conditions.

In forests there are other and different conditions: on the outskirts, whether northern or southern, the plants requiring more light are to be found on the side of the trunk towards the outside; in the depths of the forest, light may be reduced from 1/200 to 1/300, and any lichens present tend to become mere leprose crusts. Krempelhuber[880] has recorded among his Bavarian lichens those species that he found constantly growing in the shade: they are in general species of Collemaceae and Caliciaceae, several species of Peltigera (P. venosa, P. horizontalis and P. polydactyla); Solorina saccata; Gyalecta Flotovii, G. cupularis; Pannaria microphylla, P. triptophylla, P. brunnea; Icmadophila aeruginosa, etc.

B. EFFECT ON REPRODUCTIVE ORGANS

In the higher plants, it is recognized that a certain light-intensity is necessary for the production of flowers and fruit. In the lower plants, such as lichens, light is also necessary for reproduction; it is a common observation that well-lighted individuals are the most abundantly fruited. In the higher fungi also, the fruiting body is more or less formed in the light.

a. POSITION AND ORIENTATION OF FRUITS WITH REGARD TO LIGHT. There is an optimum of light for the fruits as well as for the thallus in each species of lichen: in most cases it is the fullest light that can be secured.

Zukal[881] finds an exception to that rule in species of Peltigera: when exposed to strong sunlight, the lobes, fertile at the tips, curve over so that to some extent the back of the apothecium is turned to the light; with diffuse light, the horizontal position is retained and the apothecia face upwards. In the closely allied genera Nephroma, Nephromium and Nephromopsis, the apothecia are produced on the back of the lobe at the extreme tip, but as they approach maturity the fertile lobes turn right back and they become exposed to direct illumination. In a well-developed specimen the full-grown fruits may thus become so prominent all over the thallus, that it is difficult to realize they are on reversed lobes. In one species of Cetraria (C. cucullata) the rarely formed apothecia are adnate to the back of the lobe; but in that case the margins of the strap-shaped fronds are incurved and connivent, and the back is more exposed than the front.

In Ramalina the frond frequently turns at a sharp angle at the point of insertion of the apothecium which is thus well exposed and prominent; but Zukal[882] sees in this formation an adaptation to enable the frond to avoid the shade cast by the apothecium which may exceed it in width. In most lichens, however, and especially in shade or semi-shade species, the reproductive organs are to be found in the best-lighted positions.

b. INFLUENCE OF LIGHT ON COLOUR OF FRUITS. Lichen-acids are secreted freely in the apothecium from the tips of the paraphyses which give the colour to the disc, and as acid-formation is furthered by the sun’s rays, the well-lighted fruits are always deeper in hue. The most familiar examples are the bright-yellow species that are rich in chrysophanic acid (parietin). Hedlund[883] has recorded several instances of varying colour in species of Micarea (Biatorina, etc.) in which very dark apothecia became paler in the shade. He also cites the case of two crustaceous species, Lecidea helvola and L. sulphurella, which have white apothecia in the shade, but are darker in colour when strongly lighted.

V. COLOUR OF LICHENS

The thalli of many lichens, more especially of those associated with blue-green gonidia, are hygroscopic, and it frequently happens that any addition of moisture affects the colour by causing the gelatinous cell-walls to swell, thus rendering the tissues more transparent and the green colour of the gonidia more evident. As a general rule it is the dry state of the plant that is referred to in any discussion of colour.

In the large majority of species the colouring is of a subdued tone—soft bluish-grey or ash-grey predominating. There are, however, striking exceptions, and brilliant yellow and white thalli frequently form a conspicuous feature of vegetation. Black lichens are rare, but occasionally the very dark brown of foliaceous species such as Gyrophora or of crustaceous species such as Verrucaria maura or Buellia atrata deepens to the more sombre hue.

A. ORIGIN OF LICHEN-COLOURING

The colours of lichens may be traced to several different causes.

a. COLOUR GIVEN BY THE ALGAL CONSTITUENT. As examples may be cited most of the gelatinous lichens, Ephebaceae, Collemaceae, etc. which owe, as in Collema, their dark olivaceous-green appearance, when somewhat moist, to the enclosed dark-green gonidia, and their black colour, when dry, to the loss of transparency. When the thallus is of a thin texture as in Collema nigrescens, the olivaceous hue may remain constant. Leptogium Burgessii, another thin plant of the same family, is frequently of a purplish hue owing to the purple colour of the gonidial Nostoc cells. The dull-grey crustaceous thallus of the Pannariaceae becomes more or less blue-green when moistened, and the same change has been observed in the Hymenolichens, Cora, etc.

In Coenogonium, the alga is some species of Trentepohlia, a filamentous genus mostly yellow, which often gives its colour to the slender lichen filaments, the covering hyphae being very scanty. Other filamentous species, such as Usnea barbata, etc., are persistently greenish from the bright-green Protococcaceous cells lying near the surface of the thalline strands. Many of the furfuraceous lichens are greenish from the same cause, especially when moist, as are also the larger lichens, Physcia ciliaris, Stereocaulons, Cladonias and others.

b. COLOUR DUE TO LICHEN-ACIDS. These substances, so characteristic of lichens, are excreted from the hyphae, and lie in crystals on the outer walls; they are generally most plentiful on exposed tissues such as the cortex of the upper surface or the discs of the apothecia. Many of these crystals are colourless and are without visible effect, except in sometimes whitening the surface, strikingly exemplified in Thamnolia vermicularis[884]; but others are very brightly coloured. These latter belong to two chemical groups and are found in widely separated lichens[885]:

1. Derivatives of pulvinic acid which are usually of a bright-yellow colour. They are the colouring substance of Letharia vulpina, a northern species, not found in our islands, of Cetraria pinastri and C. juniperina[886] which inhabit mountainous or hilly regions. The crustaceous species, Lecidea lucida and Rhizocarpon geographicum, owe their colour to rhizocarpic acid.

The brilliant yellow of the crusts of some species of Caliciaceae is due to the presence of the substance calycin, while coniocybic acid gives the greenish sulphur-yellow hue to Coniocybe furfuracea. Epanorin colours the hyphae and soredia of Lecanora epanora a citrine-yellow and stictaurin is the deep-yellow substance found in the medulla and under surface of Sticta aurata and S. crocata.

2. The second series of yellow acids are derivatives of anthracene. They include parietin, formerly described as chrysophanic acid, which gives the conspicuous colour to Xanthoriae and to various wall lichens; solorinic acid, the crystals of which cover the medullary hyphae and give a reddish-grey tone to the upper cortex of Solorina crocea, and nephromin which similarly colours the medulla of Nephromium lusitanicum a deep yellow, the colour of the general thallus being, however, scarcely affected. In this group must also be included the acids that cause the yellow colouring of the medulla in Parmelia subaurifera and the yellowish thallus of some Pertusariae.

In many cases, changes in the normal colouring[887] are caused by the breaking up of the acids on contact with atmospheric or soil ammonia. Alkaline salts are thus formed which may be oxidized by the oxygen in the air to yellow, red, brown, violet-brown or even to entirely black humus-like products which are insoluble in water. These latter substances are frequently to be found at the base of shrubby lichens or on the under surface of leafy forms that are closely appressed to the substratum.

c. COLOUR DUE TO AMORPHOUS SUBSTANCES. These are the various pigments which are deposited in the cell-walls of the hyphae. The only instance, so far as is known, of colours within the cell occurs in Baeomyces roseus, in which species the apothecia owe their rose-colour to oil-drops in the cells of the paraphyses, and in Lecidea coarctata where the spores are rose-coloured when young. In a few instances the colouring matter is excreted (Arthonia gregaria and Diploschistes ocellatus); but Bachmann[888], who has made an extended study of this subject and has examined 120 widely diversified lichens, found that with few exceptions the pigment was in the membranes.

Bachmann was unable to determine whether the pigments were laid down by the protoplasm or were due to changes in the cell-wall. The middle layer, he found, was generally more deeply coloured than the inner one, though that was not universal. In other cases the outer sheath was the darkest, especially in cortices one to two cells thick such as those of Parmelia olivacea, P. fuliginosa and P. revoluta, and in the brown thick-walled spores of Physcia stellaris and of Rhizocarpon geographicum. Still another variation occurs in Parmelia tristis in which the dark cortical cells show an outer colourless membrane over the inner dark wall.

The coloured pigments are mainly to be found in the superficial tissues, but if the thallus is split by areolation, as in crustaceous lichens, the internal hyphae may be coloured like those of the outer cortex wherever they are exposed. The hyphae of the gonidial layer are persistently colourless, but the lower surface and the rhizoids of many foliose lichens are frequently very deeply stained, as are the hypothalli of crustaceous species.

The fruiting bodies in many different families of lichens have dark coloured discs owing to the abundance of dark-brown pigment in the paraphyses. In these the walls, as determined by Bachmann, are composed generally of an inner wall, a second outer wall, and the outermost sheath which forms the middle lamella between adjacent cells. In some species the second wall is pigmented, in others the middle lamella is the one deeply coloured. The hymenium of many apothecia and the hyphae forming the amphithecium are often deeply impregnated with colour. The wall hyphae of the pycnidia are also coloured in some forms; more frequently the cells round the opening pore are more or less brown.

The presence of these coloured substances enables the cell-wall to resist chemical reactions induced by the harmful influences of the atmosphere or of the substratum. The darker the cell-wall and the more abundant the pigment, the less easily is the plant injured either by acids or alkalies. The coloured tips of the paraphyses thus give much needed protection to the long lived sporiferous asci, and the dark thalline tissues prevent premature rotting and decay.

d. ENUMERATION OF AMORPHOUS PIGMENTS:

=1. Green.= Bachmann found several different green pigments: “Lecidea-green,” colouring red with nitric acid, is the dark blue-green or olive-green (smaragdine) of the paraphyses of many apothecia in the Lecideaceae, and may vary to a lighter blue; it appears almost black in thalline cells[889]. “Aspicilia-green” occurs in the thalline margin and sometimes in the epithecium of the fruits of species of Aspicilia; it becomes a brighter green on the application of nitric acid. “Bacidia-green,” also a rare pigment, becomes violet with the same acid; it is found in the epithecium of Bacidia muscorum and Bacidia acclinis (Lecideaceae). “Thalloidima-green” in the apothecia of some species of Biatorina is changed to a dirty-red by nitric acid and to violet by potash. Still another termed “rhizoid-green” gives the dark greenish colour to the rhizoids of Physcia pulverulenta and P. aipolia and to the spores of some species of Physcia and Rhizocarpon. It becomes more olive-green with potash.

=2. Blue.= A very rare colour in lichens, so far found in only a few species, Biatora (Lecidea) atrofusca, Lecidea sanguinaria and Aspicilia flavida f. coerulescens. It forms a layer of amorphous granules embedded in the outer wall of the paraphyses, becoming more dense towards the epithecium. A few granules are also present in the hymenium.

=3. Violet.= “Arthonia-violet” as it is called by Bachmann is a constituent of the tissues of Arthonia gregaria, occurring in minute masses always near the cortical cells; it is distinct from the bright cinnabarine granules present in every part of the thallus.

=4. Red.= Several different kinds of red have been distinguished: “Urceolaria-red,” visible as an interrupted layer on the upper side of the medulla in the thallus of Diploschistes ocellatus, a continental species with a massive, crustaceous, whitish thallus that shows a faint rose tinge when wetted. “Phialopsis-red” is confined to the epithecium of the brightly coloured apothecia of Phialopsis rubra. “Lecanora-red,” by which Bachmann designates the purplish colour of the hymenium, is an unfailing character of Lecanora atra; the colouring substance is lodged in the middle lamella of the paraphysis cells; it occurs also in Rhizocarpon geographicum and in Rh. viridiatrum; it becomes more deeply violet with potash. M. C. Knowles[890] noted the blue colouring of Rh. geographicum growing in W. Ireland near the sea and she ascribed it to an alkaline reaction. Two more rare pigments, “Sagedia-red” and “Verrucaria-red,” are found in species of Verrucariaceae. These tinge the calcareous rocks in which the lichens are embedded a beautiful rose-pink. They are scarcely represented in our country.

=5. Brown.= A frequent colouring substance, but also presenting several different kinds of pigment which may be arranged in two groups:

=(1) Substances with some characteristic chemical reaction.= These are of somewhat rare occurrence: “Bacidia-brown” in the middle lamella of the paraphyses of Bacidia fuscorubella stains a clear yellow with acids or a violet colour with potash; “Sphaeromphale-brown,” which occurs in the perithecia and in the cortex of Staurothele clopismoides, becomes deep olive-green with potash, changing to yellow-brown on the application of sulphuric acid; “Segestria-brown” in Porina lectissima changes to a beautiful violet colour with sulphuric acid, while “Glomellifera-brown,” which is confined to the outer cortical cells of the upper surface of Parmelia glomellifera, becomes blue with nitric and sulphuric acids, but gives no reaction with potash. Rosendahl[891] confirmed Bachmann’s discovery of this colour and further located it in corresponding cells of Parmelia prolixa and P. locarensis.

=(2) Substances with little or no chemical reaction.= There is only one such to be noted: “Parmelia-brown,” usually a very dark pigment, which is lodged in the outer membranes of the cells. It becomes a clearer colour with nitric acid, and if the reagent be sufficiently concentrated, some of the pigment is dissolved out. Some tissues, such as the lower cortex of some Parmeliae, may be so impregnated and hardened, that nothing short of boiling acid has any effect on the cells; membranes less deeply coloured and changed, such as the cortex of the Gyrophorae, become disintegrated with such drastic treatment. With potash the colour becomes darker, changing from a clear brown to olivaceous-brown or-green, or in some cases, as in a more faintly coloured epithecium, to a dirty-yellow, but the lighter colour produced there is largely due to the swelling up of the underlying tissues to which the potash penetrates readily between the paraphyses.

“Parmelia-brown” is a colouring substance present in the dark epithecium and hypothecium of the fruits of many widely diverse lichens, and in the cortical cells and rhizoids of many thalli. In some plants the thallus is brown both above and below, in others, as in Parmelia revoluta, etc. only the under surface is dark-coloured.

e. COLOUR DUE TO INFILTRATION. There are several crustaceous lichens that are rusty-red, the colour being due to the presence of iron. These lichens occur on siliceous rocks of gneiss, granite, etc., and more especially on rocks rich in iron. Iron as a constituent of lichens was first demonstrated by John[892] in Ramalina fraxinea and R. calicaris. Grimbel[893] proved that the colour of rust lichens was due to an iron salt, and Molisch[894] by microscopic examination located minute granules of ferrous oxide as incrustations on the hyphae of the upper surface of the thallus. Molisch held that the rhizoids or penetrating hyphae dissolved the iron from the rocks by acid secretions. Rust lichens however grow on rocks that are frequently under water in which the iron is already present.

Among “rusty” lichens are the British forms, Lecanora lacustris, the thallus of which is normally white, though generally more or less tinged with iron; it inhabits rocks liable to inundation. L. Dicksonii owes its ferruginous colour to the same influences. Lecidea contigua var. flavicunda and L. confluens f. oxydata are rusty conditions of whitish-grey lichens.

Nilson[895] found rusty lichens occurring frequently in the Sarak-Gebirge, more especially on glacier moraines where they were liable, even when uncovered by snow, to be flooded by water from the higher reaches. It is the thallus that is affected by the iron, rarely if ever are apothecia altered in colour.

BACHMANN’S PIGMENT REACTIONS

----------------+---------------+-------------+-------------+----------+ Name of Pigment | Colour | KOH | NH₃ | Ba(OH)₂ | or Lichen | | | | | ----------------+---------------+-------------+-------------+----------+ Lecidea-green | green | | | | | | | | | Aspicilia-green | green | | | | | | | | | Bacidia-green | green | | | | | | | | | Thalloidima- | green | violet | | | green | | | | | | | | | | Rhizoid-green | bluish-green | olive-green | | | | | to brown | | | | | | | | Biatora | blue | dissolves | | | atrofusca | | with | | | | |greenish-blue| | | | | colour | | | | | | | | Phialopsis | brick-red | | dirty | | rubra | | | purple-red | | | | | | | Lecanora-red | purple-red | | deep violet | | | | | | | Sagedia | bluish-red | blue |greenish-blue| blue | declivum | | (green) | then | | | | | grey-black | | | | | | | Verrucaria | rose-red | dark-green | |dark-green| Hoffmanni f. | | | | | purpurascens | | | | | | | | | | Bacidia |yellowish-brown| violet | violet | violet | fuscorubella | | | | | | | | | | Sphaeromphale | leather-brown | deep | | | clopismoides | | olive-green | | | | | | | | Segestria | yellow-brown | rose-red | | | lectissima— | | | | | perithecia | | | | | | | | | | Segestria | brown and | | | | lectissima— | colourless | | | | entire tissue | | | | | | | | | | Parmelia | leather-brown | | | | glomellifera | | | | | | | | | | Parmelia-brown | yellow to |dirty- to | | | |blackish-brown |olive-brown | | | ----------------+---------------+-------------+-------------+----------+

----------------+---------------+-------------+------------------------- Name of Pigment | HNO₃ | H₂SO₄ | Special or Lichen | | | Reactions ----------------+---------------+-------------+------------------------- Lecidea-green | copper or | | KOH then HCl: blue | brick-red | | | | | Aspicilia-green | | | HNO₃: brighter green | | | Bacidia-green | violet | violet | HCl: violet | | | Thalloidima- | indistinctly | | HCl: indistinctly green | purple-red | | purple-red | | | | | | Rhizoid-green | olive-green | | | | | _Biatora | violet, then | dissolves | H₂O insoluble atrofusca_ | yellow, then | | | decolourized | | | | | _Phialopsis | violet | | rubra_ | | | | | | Lecanora-red | | | | | | _Sagedia | | | declivum_ | | | | | | _Verrucaria | | | KOH then HNO₃ then Hoffmanni_ f. | | | H₂SO₄: violet purpurascens | | | crystals | | | _Bacidia | | | fuscorubella_ | | | | | | _Sphaeromphale | | | KOH, then H₂SO₄, clopismoides_ | | | then HNO₃: blackish | | | _Segestria | bright yellow | | dilute H₂SO₄: lectissima_— | | | bright yellow perithecia | | | | | | _Segestria | | | Strong H₂SO₄: deep lectissima_— | | | violet, then grey entire tissue | | | | | | _Parmelia | blue, | | CaCl₂O₂: blue, then glomellifera_ | then violet, | | grey; finally | at last grey | | decolourized | | | Parmelia-brown | bright | | | red-brown | | ----------------+---------------+-------------+-------------------------

Spotted a typo or a problem in this chapter? Tell us

VI. BIONOMICS

A. GROWTH AND DURATION

Lichens are perennial plants mostly of slow growth and of long continuance; there can therefore only be approximate calculations either as to their rate of increase in dimensions or as to their duration in time. A series of somewhat disconnected observations have however been made that bear directly on the question, and they are of considerable interest.

Meyer[896] was among the first to be attracted by this aspect of lichen life, and after long study he came to the conclusion that growth varied in rapidity according to the prevailing conditions of the atmosphere and the nature of the substratum; but that nearly all species were very slow growers. He enumerates several,—Lichen (Xanthoria) parietinus, L. (Parmelia) tiliaceus, L. (Rhizocarpon) geographicus, L. (Haematomma) ventosus, and L. (Lecanora) saxicolus,—all species with a well-defined outline, which, after having attained some considerable size, remained practically unchanged for six and a half years, though, in some small specimens of foliose lichens, he noted, during the same period, an increase of one-fourth to one-third of their size in diameter. In one of the above crustaceous species, L. ventosus, the specimen had not perceptibly enlarged in sixteen years, though during that time the centre of the thallus had been broken up by weathering and had again been regenerated.

Meyer also records the results of culture experiments made in the open, possibly with soredia or with thalline scraps: he obtained a growth of Xanthoria parietina (on wrought iron kept well moistened), which fruited in the second year, and in five years had attained a width of 5-6 lines (about 1 cm.); Lecanora saxicola growing on a moist rock facing south grew 4-7 lines in six and a half years, and bore very minute apothecia.

Lindsay[897] quotes a statement that a specimen of Lobaria pulmonaria had been observed to occupy the same area of a tree after the lapse of half a century. Berkeley[898] records that a plant of Rhizocarpon geographicum remained in much the same condition of development during a period of twenty-five years. The latter is a slow grower and, in ordinary circumstances, it does not fruit till about fifteen years after the thallus has begun to form. Weddell[899], also commenting on the long continuance of lichens, says there are crustaceous species occupying on the rock a space that might be covered by a five-franc piece, that have taken a century to attain that size.

Phillips[900] on the other hand argues against the very great age of lichens, and suggests 20 years as a sufficient time for small plants to establish themselves on hard rocks and attain full development. He had observed a small vigorous plant of Xanthoria parietina that in the course of five years had extended outwards to double its original size. The centre then began to break up and the whole plant finally disappeared.

Exact measurements of growth have been made by several observers. Scott Elliot[901] found that a Pertusaria had increased about half a millimetre from the 1st February to the end of September. Vallot[902] kept under observation at first three then five different plants of Parmelia saxatilis during a period of eight years: the yearly increase of the thallus was half a centimetre, so that specimens of twenty centimetres in breadth must have been growing from forty to fifty years.

Bitter’s[903] observations on Parmelia physodes agree in the main with those of Vallot: the increase of the upper lobes during the year was 3-4 mm. In a more favourable climate Heere found that Parmelia caperata (Fig. 49) on a trunk of Aesculus in California had grown longitudinally 1·5 cm. and transversely 1 cm. The measurements extended over a period of seven winter months, five of them being wet and therefore the most favourable season of growth. In warm regions lichens attain a much greater size than in temperate or northern countries, and growth must be more rapid.

A series of measurements was also made by Heere[904] on Ramalina reticulata (Fig. 64), a rapid growing tree-lichen, and one of the largest American species. The shorter lobes were selected for observation, and were tested during a period of seven months from September to May, five of the months being in the wet season. There was great variation between the different lobes but the average increase during that period was 41 per cent.

Krabbe[905] took notes of the colonization of Cladonia rangiferina (Fig. 127) on burnt soil: in ten years the podetia had reached a height of 3 to 5 cm., giving an annual growth of about 3-5 mm. It is not unusual to find specimens in northern latitudes 18 inches long (50 cm.), which, on that computation, must have been 100 to 160 years old; but while increase goes on at the apex of the podetia, there is constant perishing at the base of at least as much as half the added length and these plants would therefore be 200 or 300 years old. Reinke[906] indeed has declared that apical growth in these Cladina species may go on for centuries, given the necessary conditions of good light and undisturbed habitat.

Other data as to rate of growth are furnished by Bonnier[907] in the account of his synthetic cultures which developed apothecia only after two to three years. The culture experiments of Darbishire[908] and Tobler[909] with Cladonia soredia are also instructive, the former with synthetic spore- and alga-cultures having obtained a growth of soredia in about seven months; the latter, starting with soredia, had a growth of well-formed squamules in nine months.

It has been frequently observed that abundance of moisture facilitates growth, and this is nowhere better exemplified than in crustaceous soil-lichens. Meyer found that on lime-clay soil which had been thrown up from a ditch in autumn, lichens such as Gyalecta geoica were fully developed the following summer. He gives an account also of another soil species, Verrucaria (Thrombium) epigaea, which attained maturity during the winter half of the year. Stahl[910] tells us that Thelidium minutulum, a pyrenocarpous soil-lichen, with a primitive and scanty thallus, was cultivated by him from spore to spore in the space of three months. Such lichens retain more of the characteristics of fungi than do those with a better developed thallus. Rapid colonization by a soil-lichen was also observed in Epping Forest by Paulson[911]. In autumn an extensive growth of Lecidea uliginosa covered as if with a dark stain patches of soil that had been worn bare during the previous spring. The lichen had reached full development and was well fruited.

These facts are quite in harmony with other observations on growth made on Epping Forest lichens. The writers[912] of the report record the finding of “fruiting lichens overspreading decaying leaves which can scarcely have lain on the ground more than two or three years; others growing on old boots or on dung and fruiting freely; others overspreading growing mosses.” They also cite a definite instance of a mass of concrete laid down in 1903 round a surface-water drain which in 1910—seven years later—was covered with Lecanora galactina in abundant fruit; and of another case of a Portland stone garden-ornament, new in 1904, and, in 1910, covered with patches of a fruiting Verrucaria (probably V. nigrescens). Both these species, they add, have a scanty thallus and generally fruit very freely.

A series of observations referring to growth and “ecesis” or the spreading of lichens have been made by Bruce Fink[913] over a period of eight years. His aim was mainly to determine the time required for a lichen to re-establish itself on areas from which it had been previously removed. Thus a quadrat of limestone was scraped bare of moss and of Leptogium lacerum, except for bits of the moss and particles of the lichen which adhered to the rock, especially in depressions of the surface. After four years, the moss was colonizing many small areas on which grew patches of the lichen 2 to 10 mm. across. Very little change occurred during the next four years.

Numerous results are also recorded as to the rate of growth, the average being 1 cm. per year or somewhat under. The greatest rate seems to have been recorded for a plant of Peltigera canina growing on “a mossy rock along a brook in a low moist wood, well-shaded.” A plant, measuring 10 by 14 cm., was deprived of several large apothecia. The lobes all pointed in the same direction, and the plant increased 1·75 cm. in one year. Two other plants, deprived of their lobes, regenerated and increased from 2 and 5 cm. respectively to 3·5 and 6 cm. No other measurements are quite so high as these, though a plant of Parmelia caperata (sterile), measuring from 1 to 2 cm. across, reached in eight years a dimension of 10 by 13 cm. Other plants of the same species gave much slower rates of increase. A section of railing was marked bearing minute scattered squamules of Cladonia pityrea. After two years the squamules had attained normal size and podetia were formed 2 to 4 mm. long.

Several areas of Verrucaria muralis were marked and after ten months were again measured; the largest plants, measuring 2·12 by 2·4 cm. across, had somewhat altered in dimensions and gave the measurements 2·2 by 3 cm. Some crustose species became established and produced thalli and apothecia in two to eight years. Foliose lichens increased in diameter from 0·3 to 3·5 cm. per year. So far as external appearance goes, apothecia are produced in one to eight years; it is concluded that they require four to eight years to attain maturity in their natural habitats.

B. SEASON OF FRUIT FORMATION

The presence of apothecia (or perithecia) in lichens does not always imply the presence of spores. In many instances they are barren, the spores having been scattered or not yet matured; the disc in these cases is composed of paraphyses only, with possible traces of asci. In any month of the year, however, some lichens may be found in fruit.

Baur[914] found, for instance, that Parmelia acetabulum developed carpogonia the whole year round, though somewhat more abundantly in spring and autumn. Pertusaria communis similarly has a maximum period of fruit-formation at these two seasons. This is probably true of tree-lichens generally: in summer the shade of the foliage would inhibit the formation of fruits, as would the extreme cold of winter; but were these conditions relaxed spore-bearing fruits might be expected at any season though perhaps not continuously on the same specimen.

An exception has been noted by Baur in Pyrenula nitida, a crustaceous tree Pyrenolichen. He found carpogonia only in February and April, and the perithecia matured in a few weeks, presumably at a date before the trees were in full leaf; but even specimens of Pyrenula are not unusual in full spore-bearing conditions in the autumn of the year.

To arrive at any true knowledge as to the date and duration of spore production, it would be necessary to keep under observation a series of one species, examining them microscopically at intervals of a few weeks or months and noting any conditions that might affect favourably or unfavourably the reproductive organs. A comparison between corticolous and saxicolous species would also be of great interest to determine the influence of the substratum as well as of light and shade. But in any case it is profitable to collect and examine lichens at all seasons of the year, as even when the bulk of the spores is shed, there may remain belated apothecia with a few asci still intact.

C. DISPERSAL AND INCREASE

The natural increase of lichen plants may primarily be sought for in the dispersal of the spores produced in the fruiting-bodies. These are ejected, as in fungi, by the pressure of the paraphyses on the mature ascus. The spores are then carried away by wind, water, insects, etc. In a few lichens gonidia are enclosed in the hymenium and are ejected along with the spores, but, in most, the necessary encounter with the alga is as fortuitous, and generally as certain, as the pollination of anemophilous flowers. A case of dispersal in Sagedia microspora has been described by Miyoshi[915] in which entire fruits, small round perithecia, were dislodged and carried away by the wind. The addition of water caused them to swell enormously and brought about the ejection of the spores. Areas covered by the thallus are also being continually enlarged by the spreading growth of the hypothallus.

a. DISPERSAL OF CRUSTACEOUS LICHENS. These lichens are distributed fairly equally on trees or wood (corticolous) and on rocks (saxicolous). Some species inhabit both substrata. As regards corticolous lichens that live on smooth bark such as hazel or mountain-ash, the vegetative body or thallus is generally embedded beneath the epidermis of the host. Soredia are absent and the thallus is protected from dispersal. In these lichens there is rather an abundant and constant formation of apothecia or perithecia.

Other species that affect rugged bark and are more superficial are less dependent on spore production. The thallus is either loosely granular, or is broken up into areolae. The areolae are each a centre of growth, and with an accession of moisture they swell up and exert pressure on each other. Parts of the thallus thus become loosened and are dislodged and carried away. If anchored on a suitable substratum they grow again to a complete lichen plant. Sorediate lichens are dependent almost wholly on these bud-like portions for increase in number; soredia are easily separated from the parent plant, and easily scattered. Darbishire[916] noted frequently that small Poduridae in moving over the surface of Pertusaria amara became powdered with soredia and very evidently took a considerable part in the dissemination of the species.

Crustaceous rock lichens are rarely sorediate, but they secure vegetative propagation[917] by the dispersal of small portions of the thallus. The thalli most securely attached are cracked into small areolae which, by unequal growth, become very soon lop-sided, or, by intercalary increase, form little warts and excrescences on their surface. These irregularities of development give rise to more or less tension which induces a loosening of the thallus from the substratum. Weather changes act similarly and gradually the areolae are broken off. Loosening influence is also exercised by the developing fruits, the expanding growth of which pushes aside the neighbouring tissues. Wind or water then carries away the thalline particles which become new centres of growth if a suitable substratum is reached.

b. DISPERSAL OF FOLIOSE LICHENS. It is a matter of common observation that, in foliose lichens where fruits are abundant, there are few or no soredia and vice versa. In either case propagation is ensured. In addition to these obvious methods of increase many lichens form isidia, outgrowths from the thallus which are easily detached. Bitter[918] considers for instance that the coralloid branchlets, which occur in compact tufts on the thallus of Umbilicaria pustulata, are of immense service as organs of propagation. Apothecia and pycnidia are rarely present in that species, and the plant thus falls back on vegetative production. Slender crisp thalline outgrowths, easily separable, occur also on the edges of lobes, as in species of Peltigera, Platysma, etc.

Owing to the gelatinous character of lichen hyphae, the thallus quickly becomes soft with moisture and is then easily torn and distributed by wind, animals, etc. The action of lichens on rocks has been shown to be of a constantly disintegrating character, and the destruction of the supporting rock finally entails the scattering of the plant. This cause of dispersal is common to both crustaceous and foliose species. The older central parts of a lichen may thus have disappeared while the areolae on lobes of the circumference are still intact and in full vigour.

As in crustaceous lichens the increase in the area of growth may take place by means of the lichen mycelium which, originating from the rhizinae in contact with the substratum, spreads as a hypothallus under the shelter of the lobes and far beyond them. When algae are encountered a new lobe begins to form. The process can be seen perhaps most favourably in lichens on decaying wood which harbours moisture and thus enables the wandering hyphae to retain life.

c. DISPERSAL OF FRUTICOSE LICHENS. Many of these lichens are abundantly fruited; in others soralia are as constantly developed. Species of Usnea, Alectoria, Ramalina and many Cladoniae are mainly propagated by soredia. They are all peculiarly liable to be broken and portions of the thallus scattered by the combined action of wind and rain.

Peirce[919] found that Ramalina reticulata (Fig. 65), of which the fronds are an open network, was mainly distributed by the tearing of the lichen in high wind. This takes place during the winter rains, when not only the lichen is wet and soft in texture, but when the deciduous trees are bare of leaves, at a season, therefore, when the drifting thalline scraps can again catch on to branch or stem. A series of observations on the dispersal of forms of long pendulous Usneas was made by Schrenk[920]. In the Middle and North Atlantic States of America these filamentous species rarely bear apothecia. The high winds break and disperse them when they are in a wet condition. They generally grow on Spruces and Firs, because the drifting filaments are more easily caught and entangled on short needles. The successive wetting and drying causes them to coil and uncoil, resulting in a tangle impossible to unravel, which holds them securely anchored to the support.

D. ERRATIC LICHENS

In certain lichens, there is a tendency for the thallus to develop excrescences of nodular form which easily become free and drift about in the wind while still living and growing. They are carried sometimes very long distances, and fall in thick deposits over localities far from their place of origin. The most famous instance is the “manna lichen,” Lecanora esculenta, which has been scientifically examined and described by Elenkin[921]. He distinguishes seven different forms of the species: f. esculenta, f. affinis, f. alpina, and f. fruticulosa-foliacea which are Alpine lichens, the remainder, f. desertoides, f. foliacea and f. esculenta-tarquina, grow on the steppes or in the desert[922].

Elenkin[921] adds to the list of erratic lichens a variety of Parmelia molliuscula along with P. ryssolea from S. Russia, from the Asiatic steppes and from Alpine regions. Mereschkovsky[923] has also recorded from the Crimea Parmelia vagans, probably derived from P. conspersa f. vaga (f. nov.). It drifts about in small rather flattened bits, and, like other erratics, it never fruits.

Meyer[924] long ago described the development of wandering lichens: scraps that were torn from the parent thallus continued to grow if there were sufficient moisture, but at the same time undergoing considerable change in appearance. The dark colour of the under surface disappears in the frequently altered position, as the lobes grow out into narrow intermingling fronds forming a more or less compact spherical mass; the rhizoids also become modified and, if near the edge, grow out into thread-like structures which bind the mass together. Meyer says that “wanderers” have been noted as belonging to Parmelia acetabulum, Platysma glaucum and Anaptychia ciliaris.

The most notable instance in Britain of the “erratic” habit is that of Parmelia revoluta var. concentrica (Fig. 121), first found on Melbury Hill near Shaftesbury, Dorset, and described as “a spherical unattached lichen which rolls on the exposed downs.” It has recently been observed on the downs near Seaford in Sussex, where, however, it seems to be confined to a small area about eight acres in extent which is exposed to south-west winds. The lichen is freely distributed over this locality. To R. Paulson and Somerville Hastings[925] we owe an account of the occurrence and origin of the revoluta wanderers. The specimens vary considerably in shape and size, and measure from 1 to 7 cm. in longest diameter. Very few are truly spherical, some are more or less flattened and many are quite irregular. The revolute edges of the overlapping lobes give a rough exterior to the balls, which thereby become entangled amongst the grass, etc., and movement is impeded or prevented, except in very high winds. Crombie[926] had suggested that the concentric plant originated from a corticolous habitat, but no trees are near the Seaford locality. Eventually specimens were found growing on flints in the immediate neighbourhood. While still on the stone the lichen tends to become panniform, a felt of intermingling imbricate lobes is formed, portions of which, in time, become crowded out and dislodged. When scattered over the ground, these are liable to be trampled on by sheep or other animals and so are broken up; each separate piece then forms the nucleus of new concentric growth.

Crombie[926] observed at Braemar, drifting about on the detritus of Morrone, an analogous structure in Parmelia omphalodes. He concluded that nodular excrescences of the thallus had become detached from the rocks on which the lichen grew; while still attached to the substratum Parmelia omphalodes and the allied species, P. saxatilis, form dense cushion-like masses.

E. PARASITISM

a. GENERAL STATEMENT. The parasitism of Strigula complanata, an exotic lichen found on the leaves of evergreen trees, has been already described[927]; Dufrenoy[928] records an instance of hyphae from a Parmelia thallus piercing pine-needles through the stomata and causing considerable injury. Lichen hyphae have attacked and destroyed the protonemata of mosses. Cases have also been recorded of Usnea and Ramalina penetrating to the living tissue of the tree on which they grew, and there may be other similar parasitisms; but these exceptions serve to emphasize the independent symbiotic growth of lichens.

There are however some lichens belonging to widely diverse genera that have retained, or reverted to, the saprophytic or parasitic habit of their fungal ancestors, though the cases that occur are generally of lichens preying on other lichens. The conditions have been described as those of “antagonistic symbiosis” when one lichen is hurtful or fatal in its action on the other, and as “parasymbiosis” when the association does little or no injury to the host. The parasitism of fungi on lichens, though falling under a different category, in many instances exhibits features akin to parasymbiosis.

The parasitism of fungus on fungus is not unusual; there are instances of its occurrence in all the different classes. In the Phycomycetes there are genera wholly parasitic on other fungi such as Woronina and other Chytridiaceae; Piptocephalus, one of the Mucorini, is another instance. Cicinnobolus, one of the Sphaeropsideae, preys on Perisporiae; a species of Cordyceps is found on Elaphomyces, and Orbilia coccinella on Polyporus; while among Basidiomycetes, Nyctalis, an agaric, grows always on Russula.

There are few instances of lichens finding a foothold on fungi, for the simple reason that the latter are too short lived. On the perennial Polyporeae a few have been recorded by Arnold[929], but these are not described as doing damage to the host. They are mostly species of Lecidea or of allied genera. Kupfer[930] has also listed some 15 different lichens that he found on Lenzites sp.

b. ANTAGONISTIC SYMBIOSIS. In discussing the nutrition of lichens[931] note has been taken of the extent to which some species by means of enzymes destroy the thallus of other lichens in their vicinity and then prey on the dead tissues. A constantly cited[932] example is that of Lecanora atriseda which in its early stages lives on the thallus of Rhizocarpon geographicum inhabiting mountain rocks. A detailed examination of the relationship between these two plants was made by Malme and later by Bitter[933]. Both writers found that the Lecanora thallus as it advanced caused a blackening of the Rhizocarpon areolae, the tissues of which were killed by the burrowing slender filaments of the Lecanora, easily recognized by their longer cells. The invader thereafter gradually formed its own medulla, gonidial layer and cortex right over the surface of the destroyed thallus. Lecidea insularis (L. intumescens) similarly takes possession of and destroys the thallus of Lecanora glaucoma and Malme[932] strongly suspects that Buellia verruculosa and B. aethalea may be living on the thallus of Rhizocarpon distinctum with which they are constantly associated.

Other cases of facultative parasitism have been studied by Hofmann[934], more especially three different species, Lecanora dispersa, Lecanora sp. and Parmelia hyperopta, which were found growing on the thick foliose thallus of Dermatocarpon miniatum. These grew, at first independently, on a wall along with many examples of Endocarpon on to which they spread as opportunity offered. The thallus of the latter was in all cases distorted, the area occupied by the invaders being finally killed. The attacking lichens had benefited materially by the more nutritive substratum: their apothecia were more abundant and their thallus more luxuriant. The gonidia especially had profited; they were larger, more brightly coloured, and they increased more freely. Hoffmann offers the explanation that the strain on the algae of providing organic food for the hyphal symbiont was relaxed for the time, hence their more vigorous appearance.

Arthonia subvarians is always parasitic on the apothecia of Lecanora galactina, and Almquist[935] discovered that the hymenium of the host alone is injured, the hypothecium and excipulum being left intact.

The “parasitism” of Pertusaria globulifera on Parmelia perlata and P. physodes, as described by Bitter[936], may also be included under antagonistic symbiosis. The hyphae pierce the Parmelia thallus, break it up and gradually absorb it. Chemical as well as mechanical influences are concerned in the work of destruction as both the fungus and the alga of the victim are dissolved. Lecanora tartarea already dealt with as a marauding lichen[937] over decaying vegetation may spread also to living lichens. Fruticose soil species, such as Cetraria aculeata and others, die from the base and the Lecanora gains entrance to their tissues at the decaying end which is open.

Arnold[938] speaks of these facultative parasites that have merely changed their substratum as pseudo-parasites, and he gives a list of instances of such change. In many cases it is rather the older thalli that are taken possession of, and, in nearly every case, the invader is some crustaceous species. The plants attacked are generally ground lichens or more particularly those that inhabit damp localities, such as Peltigera or Cladonia or certain bark lichens. Drifting soredia or particles of a lichen would easily take hold of the host thallus and develop in suitable conditions. To give a few of the instances observed, there have been found, by Arnold, Crombie and others:

on Peltigera canina: Callopisma cerina, Rinodina turfacea var., Bilimbia obscurata and Lecanora aurella;

on Peltigera aphthosa: Lecidea decolorans;

on Cladoniae: Bilimbia microcarpa, Bacidia Beckhausii and Urceolaria scruposa, etc.

Urceolaria (Diploschistes) has a somewhat bulky crustaceous thallus which may be almost evanescent in its semi-parasitic condition, the only gonidia retained being in the margin of the apothecia. Nylander[939] found isolated apothecia growing vigorously on Cladonia squamules.

Hue[940] describes Lecanora aspidophora f. errabunda, an Antarctic lichen, as not only a wanderer but as a “shameless robber.” It is to be seen everywhere on and about other lichens, settling small glomeruli of apothecia here and there on the thallus of Umbilicariae or between the areolae of Buelliae, and always too vigorous to be ousted from its position.

Bacidia flavovirescens has been regarded by some lichenologists[941] as a parasite on Baeomyces, but recent work by Tobler[942] seems to have proved that the bright green thallus is that of the Bacidia.

c. PARASYMBIOSIS. There are certain lichens that are obligative parasites and pass their whole existence on an alien thallus. They may possibly have degenerated from the condition of facultative parasitism as the universal history of parasitism is one of increased dependence on the host, and of growing atrophy of the parasite, but, in the case of lichens, there is always the peculiar symbiotic condition to be considered: the parasite produces its own vigorous hyphae and normal healthy fruits, it often claims only a share of the carbohydrates manufactured by the gonidia. The host lichen is not destroyed by this parasymbiosis though the tissues are very often excited to abnormal growth by the presence of the invading organism.

Lauder Lindsay[943] was one of the first to study these “microlichens” as he called them, and he published descriptions of those he had himself observed on various hosts. He failed however to discriminate between lichens and parasitic fungi. It is only by careful research in each case that the affinity to fungi or to lichens can be determined; very frequently the whole of them, as possessing no visible thallus, have been classified with fungi, but that view ignores the symbiosis that exists between the hyphae of the parasite and the gonidia of the host.

Parasitic lichens are rather rare on gelatinous thalli; but even among these, a few instances have been recorded. Winter[944] has described a species of Leptoraphis, the perithecia of which are immersed in the thallus of Physma franconicum. The host is wholly unaffected by the presence of the parasite except for a swelling where it is situated. The foreign hyphae are easily distinguishable; they wander through the thallus of the host with their free ends in the mucilage of the gonidial groups from which they evidently extract nourishment. Species of the lichen genus Obryzum are also parasitic on gelatinous lichens.

The parasitic genus Abrothallus[945] has been the subject of frequent study. There are a number of species which occur as little black discs on various thalli of the large foliose lichens. They were first of all described as parasitic fungi, later Tulasne[946] affirmed their lichenoid nature as proved by the structure, consistence and long duration of the apothecia. Lindsay[947] wrote a monograph of the genus dealing chiefly with Abrothallus Smithii (Buellia Parmeliarum) and A. oxysporus, with their varieties and forms that occur on several different hosts. In some instances the thallus is apparently quite unaffected by the presence of Abrothallus, in others, as in Cetraria glauca, there is considerable hypertrophy produced, the portion of the thallus on which the parasites are situated showing abnormal growth in the form of swellings or pustules which may be regarded as gall-formations. Crombie[948] points this out in a note on C. glauca var. ampullacea, figured first by Dillenius, which is merely a swollen condition due to the presence of Abrothallus.

The internal structure and behaviour of Abrothallus has more recently been followed in detail by Kotte[949]. He recognized a number of different species growing on various thalli of Parmelia and Cetraria, but Abrothallus Cetrariae was the only one that produced gall-formation. The mycelium of the parasite in this instance penetrates to the medulla of the host lichen as a loose weft of hyphae which are divided into more or less elongate cells. These send out side branches, which grow towards the algal cells, and by their short-celled filaments clasp them exactly in the same way as do the normal lichen hyphae. Thus in the neighbourhood of the parasite an algal cell may be surrounded by the hyphae not only of the host, but also by those of Abrothallus. The two different hyphae can generally be distinguished by their reaction to iodine: in some cases Abrothallus hyphae take the stain, in others the host hyphae. In addition to apothecia, spermogonia or pycnidia are produced, but in one of the species examined by Kotte, Abrothallus Peyritschii on Cetraria caperata, there was no spermogonial wall formed. The hyphae also penetrate the host soredia or isidia, so that on the dispersal of these vegetative bodies the perpetuation of both organisms is secured in the new growth.

Abrothallus draws its organic food from the gonidia in the same way as the host species, and possibly the parasitic hyphae obtain also water and inorganic food along with the host hyphae. They have been traced down to the rhizinae and may even reach the hypothallus, but no injury to the host has been detected. It is a case of joint symbiosis and not of parasitism. Microscopic research has therefore justified the inclusion of these and other forms among lichens.

d. PARASYMBIOSIS OF FUNGI. There occur on lichens, certain parasites classed as fungi which at an early stage are more or less parasymbionts of the host; as growth advances they may become parasitic and cause serious damage, killing the tissues on which they have settled.

Zopf[950] found several instances of such parasymbiosis in his study of fungal parasites, such as Rhymbocarpus punctiformis, a minute Discomycete which inhabits the thallus of Rhizocarpon geographicum. By means of staining reagents he was able to trace the course of the parasitic hyphae, and found that they travelled towards the gonidia and clasped them lichen-wise without damaging them, since these remained green and capable of division. At no stage was any harm caused to the host by the alien organism. Another instance he observed was that of Conida rubescens on the thallus of Rhizocarpon epipolium. By means of fine sections through the apothecia of Conida and the thallus of the host, he proved the presence of numerous gonidia in the subhymenial tissue, these being closely surrounded by the hyphae of the parasite, and entirely undamaged: they retained their green colour, and in size and form were unchanged. Zopf[951] at first described these parasites as fungi though later[951] he allows that they may represent lower forms of lichens.

Tobler[952] has added two more of these parasymbiotic species on the border line between lichens and fungi, similar to those described by Zopf. One of these, Phacopsis vulpina, belonging to the fungus family Celidiaceae, is parasitic on Letharia vulpina. The fronds of the host plant are considerably altered in form by its presence, being more branched and curly. Where the parasite settles a swelling arises filled with its hyphae, and the host gonidia almost disappear from the immediate neighbourhood, only a few “nests” being found and these very mucilaginous. These nests as well as single gonidia are surrounded by Phacopsis hyphae which have gradually displaced those of the Letharia thallus. The gonidia are excited to division and increase in number on contact with either lichen or fungus hyphae, but in the latter case the increase is more abundant owing doubtless to a more powerful chemical irritant in the fungus. As development advances, the Phacopsis hyphae multiply to the exclusion of both lichen hyphae and gonidia from the area of invasion. Finally the host cortex is split, the fungus bursts through, and the tissue beneath the parasite becomes brown and dead. Phacopsis begins as a “parasymbiont,” then becomes parasitic, and is at last saprophytic on the dead cells. The hyphae travel down into the medulla of the host and also into the soredial outgrowths, and are dispersed along with the host. The effect of Verrucula on the host thallus may also be cited[953].

Tobler gives the results of his examination of still another fungus, Karschia destructans. It becomes established on the thallus of Chaenotheca chrysocephala and its hyphae gradually penetrate down to the underlying bark (larch). The lichen thallus beneath the fungus is killed, but gonidia in the vicinity are sometimes clasped: Karschia also is thus a parasymbiont, then a parasite, and finally a saprophyte.

Elenkin[954] describes certain fungi which to some extent are parasymbionts. One of these, Conidella urceolata n. sp., grew on forms of Lecanora esculenta. The other, a stroma-forming species, had invaded the thallus of Parmelia molliuscula, where it caused gall-formation. As the growth of the gall was due to the co-operation of the lichen gonidia, the fungus must at first have been a parasymbiont. Only dead gonidia were present in the stroma; probably they had been digested by the parasite. Because of the stroma Elenkin placed the fungus in a new genus, Trematosphaeriopsis.

e. FUNGI PARASITIC ON LICHENS. A solution or extract of lichen thallus is a very advantageous medium in which to grow fungi. It is therefore not surprising that lichens are a favourite habitat for parasitic fungi. Stahl[955] has noted that the lichens themselves flourish best where there is frequent moistening by rain or dew with equally frequent drying which effectively prevents the growth of fungi. Species of Peltigera are however able to live in damp conditions: without being injured, they have been observed to maintain their vigour when cultivated in a very moist hothouse while all the other forms experimented with were attacked and finally destroyed by various fungi.

Lindsay[956] devoted a great deal of attention to the microscopic study of the minute fruiting bodies so frequently present on lichen thalli and published descriptions of microlichens, microfungi and spermogonia. He and others naturally considered these parasitic organisms to be in many cases either the spermogonia or pycnidia of the lichen itself. It is often not easy to determine their relationship or their exact systematic position; many of them are still doubtful forms.

There exists however a very large number of fully recognized parasitic microfungi belonging to various genera. Lindsay discovered many of them. Zopf[957] has given exact descriptions of a series of forms, with special reference to their effect on the host thallus. In an early paper he described a species, Pleospora collematum, that he found on Physma compactum and other Collemaceae. The hyphae of the parasite differed from those of the host in being of a yellow colour; they did not penetrate or spread far, being restricted to rhizoid-like filaments at the base of their fruiting bodies (perithecia and pycnidia). Their presence caused a slight protuberance but otherwise did no harm to the host; the Nostoc cells in their immediate vicinity were even more brightly coloured than in other parts of the thallus. In another paper[958] he gives an instance of gall-formation in Collema pulposum induced by the presence of the fungus Didymosphaeria pulposi. Small protuberances were formed on the margins of the apothecia, more rarely on the lobes of the thallus, each one the seat of a perithecium of the fungus. No damage was done to either constituent of the thallus.

Agyrium flavescens grows parasitically on the under surface of Peltigera polydactyla. M. and Mme Moreau[959] found that the hyphae of the fungus spread between the medullary filaments of the lichen; no haustoria were observed. The mature fruiting body had no distinct excipulum, but was surrounded by a layer of dead lichen cells.

It is not easy to determine the difference between parasites that are of fungal nature and those that are lichenoid; but as a general rule the fungi may be recognized by their more transient character, very frequently by their effect on the host thallus, which is more harmful than that produced by lichens, and generally by their affinity to fungi rather than to lichens. Opinions differ and will continue to differ on this very difficult question.

The number of such fungi determined and classified has gradually increased, and now extends to a very long list. Even as far back as 1896 Zopf reckoned up 800 instances of parasitism of 400 species of fungi on about 350 different lichens and many more have been added. Abbé Vouaux[960] is the latest writer on the subject, but his work is mostly a compilation of species already known. He finds representatives of these parasites in nine families of Pyrenomycetes and six of Discomycetes. He leaves out of account the much debated Coniocarps, but he includes with fungi all those that have been proved to be parasymbiotic, such as Abrothallus.

A number of fungus genera, such as Conida, etc., are parasitic only on lichens. Most of them have one host only; others, such as Tichothecium pygmaeum, live on a number of different thalli. Crustaceous species are often selected by the parasites, and no great damage, if any, is caused to these hosts, except when the fungus is seated on the disc of the apothecium, so that the spore-bearing capacity is lessened or destroyed.

In some of the larger lichens, however, harmful effects are more visible. In Lobaria pulmonaria, the fruits of which are attacked by the Discomycete, Celidium Stictarum[961], there is at first induced an increased and unusual formation of lichen apothecia. These apothecia are normally seated for the most part on the margins of the lobes or pustules, but when they are invaded by the fungus, they appear also in the hollows between the pustules and even on the under surface of the thallus. In the large majority of cases the fungus is partly or entirely embedded in the thallus; the gonidia in the vicinity may remain green and healthy, or all the tissues in the immediate neighbourhood of the parasite may be killed.

f. MYCETOZOA PARASITIC ON LICHENS. Mycetozoa live mostly on decayed wood, leaves, humus, etc. One minute species, Listerella paradoxa, always inhabits the podetia of Cladonia rangiferina. Another species, Hymenobolina parasitica, was first detected and described by Zukal[962] as a true parasite on the thallus of Physciaceae; it has since been recorded in the British Islands on Parmeliae[963]. This peculiar organism differs from other mycetozoa in that the spores on germination produce amoebae. These unite to form a rose-red plasmodium which slowly burrows into the lichen thallus and feeds on the living hyphae. It is a minute species, but when abundant the plasmodia can just be detected with the naked eye as rosy specks scattered over the surface of the lichen. Later the grey sporangia are produced on the same areas.

F. DISEASES OF LICHENS

a. CAUSED BY PARASITISM. Zopf[964] has stated that of all plants, lichens are the most subject to disease, reckoning as diseases all the instances of parasitism by fungi or by other lichens. There are however only rare instances in which total destruction or indeed any permanent harm to the host is the result of such parasitism. At worst the trouble is localized and does not affect the organism as a whole. Some of these cases have been already noted under antagonistic symbiosis or parasymbiosis. Several instances have however been recorded where real injury has been caused by the penetration of some undetermined fungus mycelium. Zukal[965] records two such observed by him in Parmelia encausta and Physcia villosa: the thallus of the former was dwarfed and deformed by the presence of the alien mycelium, the latter was excited to abnormal proliferation.

b. CAUSED BY CROWDING. Lichens suffer frequently from being overgrown by other lichens; they may also be crowded out by other plants. My attention was called by Mr P. Thompson to a burnt plot of ground in Epping Forest, which, after the fire, had been colonized by Peltigera spuria. In the course of a few years, other vegetation had followed, depriving the lichen of space and light and gradually driving it out. When last examined only a few miserable specimens remained, and these were reduced in vitality by an attack of the lichen parasite Illosporium carneum.

c. CAUSED BY ADVERSE CONDITIONS. Zukal considers as pathological, at least in origin, the cracking of the thallus so frequent in crustaceous lichens as well as in the more highly developed forms. As the cracks are beneficial in the aeration of the plant, they can hardly be regarded as symptoms of a diseased condition. The more evident ringed breaks in the cortex of Usneae, due probably to wind action, have more reason to be so regarded; they are most pronounced in Usnea articulata, where the portions bounded by the rings are contracted and swollen, and a hollow space is formed between the cortex and the central axis. The swellings that are produced on lichen thalli, such as those of Umbilicaria and some species of Gyrophora, due to intercalary growth are normal to the plant, though occasionally the swollen weaker portions may become ruptured and the cortex be thrown off. As pathological also must be regarded the loss of cortex sometimes occasioned by excessive soredial formation at the margins of the lobes: the upper cortex may be rolled back and eventually torn away; the gonidial layer is exposed and transformed into soredia which are swept away by the wind and rain, till finally only traces of the lower cortex are left.

Zukal[966] has instanced, as a case of diseased condition observed by him, the undue thickening of the cortex in Pertusaria communis whereby the formation of the fruiting bodies is inhibited and even vegetative development is rendered impossible. There arrives finally a stage when splitting takes place and the whole thallus breaks down and disappears. As a rule however there need be no limit to the age of the lichen plant. There is no vital point or area in the thallus; injury of one part leaves the rest unhurt, and any fragment in growing condition, if it combines both symbionts, can carry on the life of the plant, the constant renewal of gonidia preventing either decay or death. Barring accidents many lichens might exist as long as the world endures.

G. HARMFUL EFFECT OF LICHENS

One lichen only, Strigula complanata, a tropical species, has been proved to be truly and constantly parasitic. It grows on the surface of thick leathery leaves such as those of Camellia[967], etc. and the alga and fungus both penetrate the epidermis and burrow beneath the cuticle and outer cells, causing them to become brown. It undoubtedly injures the leaves.

Friedrich[968] has given an isolated instance of the hold-fast hyphae of Usnea piercing through the cortex to the living tissue of the host, and not only destroying the middle lamella by absorption, but entering the cells. The Usnea plant was characterized by exceptionally vigorous growth. Practically all corticolous lichens are epiphytic and the injury they cause is of an accidental nature. Crustaceous species on the outer bark occupy the dead cortical layers and seem to be entirely harmless[969]. The larger foliose and fruticose forms are not so innocuous: by their abundant enveloping growth they hinder the entrance of air and moisture, and thus impede the life of the higher plant. Gleditsch[970], one of the earliest writers on Forestry, first indicated the possibly harmful effect of lichens especially on young trees and “in addition,” he says, “they serve as cover for large numbers of small insects which are hurtful in many ways to the trees.” Lindau[971] pointed out the damage done to pine-needles by Xanthoria parietina which grew round them like a cuff and probably choked the stomata, the leaves so clothed being mostly withered. Dufrenoy[972] states that he found the hyphae of a Parmelia entering a pine-needle by the stomata, and that the starch disappeared from the neighbouring parenchyma the cells of which tended to disintegrate.

It is no uncommon sight to see neglected fruit trees with their branches crowded with various lichens, Evernia prunastri, Ramalina farinacea, etc. Such lichens often find the lenticels a convenient opening for their hold-fasts and exercise a smothering effect on the trees. Lilian Porter[973] distinctly states that Ramalinae by their penetrating bases damage the tissues of the trees. The presence of lichens is however generally due to unhealthy conditions already at work. Friedrich[974] reported of a forest which he examined, in which the atmospheric moisture was very high, with the soil water scarce, that those trees that were best supplied with soil water were free from lichens, while those with little water at the base bore dead branches which gave foothold to a rich growth of the epiphytes.

Experiments to free fruit trees from their coating of lichens were made by Waite[975]. With a whitewash brush he painted over the infested branches with solutions of Bordeaux mixture of varying strength, and found that this solution, commonly in use as a fungicide, was entirely successful. The trees were washed down about the middle of March, and some three weeks later the lichens were all dead, the fruticose and foliose forms had changed in colour to a yellowish or brownish tint and were drooping and shrivelled.

Waite was of opinion that the lichens did considerable damage to the trees, but it has been held by others that in very cold climates they may provide protection against severe frost. Instances of damage are however asserted by Bouly de Lesdain[976]. The bark of willows he found was a favourite habitat of numerous lichens: certain species, such as Xanthoria parietina, completely surrounded the branches, closing the stomata; others, such as Physcia ascendens, by the mechanical strain of the rhizoids, first wet and then dry, gradually loosened the outer bark and gave entry to fungi which completed the work of destruction.

H. GALL-FORMATION

Several instances of gall-formation to a limited extent have been already noted as caused by parasitic fungi or lichens. Greater abnormality of development is induced in a few species by the presence of minute animals, mites, wood-lice, etc. Zopf[977] noted these deformations of the thallus in specimens of Ramalina Kullensis collected on the coasts of Sweden. The fronds were frequently swollen in a sausage-like manner, and branching was hindered or altogether prevented; apothecia were rarely formed, though pycnidia were abundant. Here and there, on the swollen portions of the thallus, small holes could be detected and other larger openings of elliptical outline, about 1-1-1/2 mm. in diameter, the margins of which had a nibbled appearance. Three types of small articulated animals were found within the openings: species of mites, spiders and wood-lice. Mites were the most constant and were more or less abundant in all the deformations; frequently a minute Diplopodon belonging to the genus Polyxenus was also met with.

Zopf came to the conclusion that the gall-formation was mainly due to the mites: they eat out the medulla and possibly through some chemical irritation excite the algal zone and cortex to more active growth, so that an extensive tangential development takes place. The small spiders may exercise the same power; evidently the larger holes were formed by them.

Later Zopf added to gall-deformed plants Ramalina scopulorum var. incrassata and R. cuspidata var. crassa. He found in the hollow swollen fronds abundant evidence of mites, but whether identical with those that attacked R. Kullensis could not be determined. These two Ramalinae are maritime species; they are morphologically identical, as are also the deformed varieties, and the presence of mites, excreta, etc., are plainly visible in our British specimens.

Bouly de Lesdain[978] found evidence of mite action in Ramalina farinacea collected from Pinus sylvestris on the dunes near Dunkirk. The cortex had been eaten off either by mites or by a small mollusc (Pupa muscorum) and the fronds had collapsed to a more or less convex compact mass. Somewhat similar deformations, though less pronounced, were observed in other Ramalinae.

In Cladonia sylvatica and also in Cl. rangiformis Lesdain has indicated ff. abortiva Harm. as evidently the result of insect attack. In both cases the tips of the podetia are swollen, brown, bent and shrivelled.

One of the most curious and constant effects, also worked out by Lesdain, occurs in Physcia hispida (Ph. stellaris var. tenella). In that lichen the gonidia at the tips of the fronds are scooped out and eaten by mites, so that the upper cortex becomes separated from the lower part of the thallus. As the hyphae of the cortex continue to develop, an arched hood is formed of a whitish shell-like appearance and powdery inside. Sometimes the mites penetrate at one point only, at other times the attack is at several places which may ultimately coalesce into one large cavity. In a crustaceous species, Caloplaca (Placodium) citrina he found constant evidence of the disturbing effect of the small creatures, which by their action caused the areolae of the thallus to grow into minute adherent squamules. A pathological variety, which he calls var. sorediosa, is distinguished by the presence of cup-like hollows which are scooped out by Acarinae and are filled by yellowish soredia. In another form, var. maritima, the margins of the areolae, occasionally the whole surface, become powdery with a citrine yellow efflorescence as a result of their nibbling.

Zukal[979] adds to the deformations due to organic agents, the hypertrophies and abnormalities caused by climatic conditions. He finds such irregularities of structure more especially developed in countries with a very limited rainfall, as in certain districts of Chili, Australia and Africa, where changes in cortex and rhizoids and proliferations of the thallus testify to the disturbance of normal development.

Spotted a typo or a problem in this chapter? Tell us