CHAPTER V. Physiology

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 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 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 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 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 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, a product of chitin.

Escombe 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 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 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, 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 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 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 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 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 in 1881.

More recently Escombe 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; 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 from Lecanora esculenta, an edible desert lichen supposed to be the manna of the Israelites. Wisselingh 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. It is soluble in water and, though considered by Czapek to be identical with lichenin, it differs, according to Ulander, in being dextro-rotatory to polarized light; lichenin on the contrary is optically inactive. Escombe also obtained a substance from Evernia which he considered to be comparable with chitosan. Usnein which has been extracted from Usnea barbata may also be identical with lichenin, but that has not yet been established. Ulander 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. 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 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 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 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, Wisselingh and Wester 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 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 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 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 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, 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 in the gonidia of Parmelia and Peltigera, i.e. both in bright-green and blue-green algae. The amount was scarcely calculable.

Herissey 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. Diastase was demonstrated in the cells of Roccella tinctoria, R. Montagnei and of Dendrographa leucophaea by Ronceray 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 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 held that it was formed in lichens by the reduction and splitting of lichen acids, though, as Zopf has pointed out, these are generally insoluble. Hamlet and Plowright 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, 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 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 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 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 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 has stated that lichen-ash always contains argillaceous earth. According to Kratzmann, 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. A like service afforded by crystals to the leaves of the higher plants in desert lands has been described by Kerner. 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 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 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 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 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 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 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 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 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 and Lang 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 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 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, 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 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 are typical sphaeroid cells formed.

More or less specialized oil-cells have been demonstrated by Fünfstück 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 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 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 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, 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 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 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 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 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 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.

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 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. 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 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 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 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.

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. 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 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 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 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 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 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 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 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 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 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 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 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 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 (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 (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.

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 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. 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, 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, less hygroscopic generally than in the larger forms. Beckmann 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 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 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 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, 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 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 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 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 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 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.

That the substratum does have some indirect influence on these larger lichens has been proved once and again. Uloth, 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 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 found later that this lichen when very sorediate yielded an increased supply of atranoric acid.

Ohlert, 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. 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 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 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 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 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 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 and Stahlecker 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 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 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 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 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 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 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. 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 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 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 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, 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 such as that of Corylus, Carpinus, 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 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. 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 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 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 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 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 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 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. 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 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 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 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 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; but others are very brightly coloured. These latter belong to two chemical groups and are found in widely separated lichens:

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 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 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, 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. “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 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 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 in Ramalina fraxinea and R. calicaris. Grimbel proved that the colour of rust lichens was due to an iron salt, and Molisch 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 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 | | ----------------+---------------+-------------+-------------------------