Chapter V: Physiology (2)

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

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

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

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

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

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

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

B. STORAGE OF WATER

The gonidial algae _Gloeocapsa_, _Scytonema_, _Nostoc_, etc. among Myxophyceae, _Palmella_ and occasionally _Trentepohlia_ among Chlorophyceae, have more or less gelatinous walls which act as a natural reservoir of water for the lichens with which they are associated. In these lichens the hyphae for the most part have thin walls, and the plectenchyma when formed—as below the apothecium in _Collema granuliferum_, or as a cortical layer in _Leptogium_—is a thin-walled tissue. In lichens where, on the contrary, the alga is non-gelatinous—as generally in Chlorophyceae—or where the gelatinous sheath is not formed as in the altered _Nostoc_ of the _Peltigera_ thallus, the fungal hyphae have swollen gelatinous walls both in the pith and the cortex, and not only imbibe but store up water.

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

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

C. SUPPLY OF INORGANIC FOOD

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

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

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

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

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

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

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

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

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

D. SUPPLY OF ORGANIC FOOD

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

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

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

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

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

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

_c._ FROM OTHER VEGETATION. Zukal[861] has given instances of association between mosses and lichens in which the latter seemed to play the part of parasite. The terricolous species _Baeomyces rufus_ (_Sphyridium_) and _Biatora decolorans_, as well as forms of _Lepraria_ and _Variolaria_, he found growing over mosses and killing them. Stems and leaves of the moss _Plagiothecium sylvaticum_ were grown through and through by the hyphae of a _Pertusaria_, and he observed a leaf of _Polytrichum commune_ pierced by the rhizinae of a minute _Cladonia_ squamule. The cells had been invaded and the neighbouring tissue was brown and dead.

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

III. ASSIMILATION AND RESPIRATION

A. INFLUENCE OF TEMPERATURE

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

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

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

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

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

B. INFLUENCE OF MOISTURE

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

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

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

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

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

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

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

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

IV. ILLUMINATION OF LICHENS

A. EFFECT OF LIGHT ON THE THALLUS

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

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

_a._ SUN LICHENS. The illumination of the Tundra lichens is reckoned by Wiesner as representing his unit of standard illumination. In the same category as these are included many of our most familiar lichens, which grow on rocks subject to the direct incidence of the sun’s rays, such as, for instance, _Parmelia conspersa_, _P. prolixa_, etc. _Physcia tenella_ (_hispida_) is also extremely dependent on light, and was never found by Wiesner under 1/8 of full illumination. _Dermatocarpon miniatum_, a rock lichen with a peltate foliose thallus, is at its best from 1/3 to 1/8 of illumination, but it grows well in situations where the light varies in amount from 1 to 1/24. _Psora_ (_Lecidea_) _lurida_, with dark-coloured crowded squamules, grows on calcareous soil among rocks well exposed to the sun and has an illumination from 1 to 1/30, but with a poorer development at the lower figure. Many crustaceous rock lichens are also by preference sun-plants as, for instance, _Verrucaria calciseda_ which grows immersed in calcareous rocks but with an illumination of 1 to 1/3; in more shady situations, where the light had declined to 1/29, it was found to be less luxuriant and less healthy.

Sun lichens continue to grow in the shade, but the thallus is then reduced and the plant is sterile. Zukal has made a list of those which grow best with a light-use of 1 to 1/10, though they are also found not unfrequently in habitats where the light cannot be more than 1/50. Among these light-loving plants are the Northern Tundra species of _Cladonia_, _Stereocaulon_, _Cetraria_, _Parmelia_, _Umbilicaria_, and _Gyrophora_, as also _Xanthoria parietina_, _Placodium elegans_, _P. murorum_, etc., with some crustaceous species such as _Lecanora atra_, _Haematomma ventosum_, _Diploschistes scruposus_, many species of Lecideaceae, some Collemaceae and some Pyrenolichens.

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

_b._ COLOUR-CHANGES DUE TO LIGHT. Lichens growing in full sunlight frequently take on a darker hue. _Cetraria islandica_ for instance in an open situation is darker than when growing in woods; _C. aculeata_ on bare sand-dunes is a deeper shade of brown than when growing entangled among heath plants. _Parmelia saxatilis_ when growing on exposed rocks is frequently a deep brown colour, while on shaded trees it is normally a light bluish-grey.

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

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

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

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

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

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

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

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

_Physcia obscura_ grew in almost total darkness; _Placodium murorum_ within the cave had lost nearly all colour; _Placodium variabile_ var. deep within the cave, sterile; _Opegrapha endoleuca_ in partial obscurity; _Verrucaria rupestris_ f. in total obscurity, the thallus much reduced and sterile; _Verrucaria rupestris_ in partial obscurity, the asci empty; _Homodium_ (_Collema_) _granuliferum_ in the inmost recess of the cave, sterile, and the hyphae more spongy than in the open.

Siliceous rocks in darkness were still more barren, but a few odd lichens were collected from sandstone in various caves: _Cladonia squamosa_, _Parmelia perlata_ var. _ciliata_, _Diploschistes scruposus_, _Lecidea grisella_, _Collema nigrescens_ and _Leptogium lacerum_.

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

In forests there are other and different conditions: on the outskirts, whether northern or southern, the plants requiring more light are to be found on the side of the trunk towards the outside; in the depths of the forest, light may be reduced from 1/200 to 1/300, and any lichens present tend to become mere leprose crusts. Krempelhuber[880] has recorded among his Bavarian lichens those species that he found constantly growing in the shade: they are in general species of Collemaceae and Caliciaceae, several species of _Peltigera_ (_P. venosa_, _P. horizontalis_ and _P. polydactyla_); _Solorina saccata_; _Gyalecta Flotovii_, _G. cupularis_; _Pannaria microphylla_, _P. triptophylla_, _P. brunnea_; _Icmadophila aeruginosa_, etc.

B. EFFECT ON REPRODUCTIVE ORGANS

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

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

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

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

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

V. COLOUR OF LICHENS

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

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

A. ORIGIN OF LICHEN-COLOURING

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

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

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

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

1. Derivatives of pulvinic acid which are usually of a bright-yellow colour. They are the colouring substance of _Letharia vulpina_, a northern species, not found in our islands, of _Cetraria pinastri_ and _C. juniperina_[886] which inhabit mountainous or hilly regions. The crustaceous species, _Lecidea lucida_ and _Rhizocarpon geographicum_, owe their colour to rhizocarpic acid.

The brilliant yellow of the crusts of some species of Caliciaceae is due to the presence of the substance calycin, while coniocybic acid gives the greenish sulphur-yellow hue to _Coniocybe furfuracea_. Epanorin colours the hyphae and soredia of _Lecanora epanora_ a citrine-yellow and stictaurin is the deep-yellow substance found in the medulla and under surface of _Sticta aurata_ and _S. crocata_.

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

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

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

Bachmann was unable to determine whether the pigments were laid down by the protoplasm or were due to changes in the cell-wall. The middle layer, he found, was generally more deeply coloured than the inner one, though that was not universal. In other cases the outer sheath was the darkest, especially in cortices one to two cells thick such as those of _Parmelia olivacea_, _P. fuliginosa_ and _P. revoluta_, and in the brown thick-walled spores of _Physcia stellaris_ and of _Rhizocarpon geographicum_. Still another variation occurs in _Parmelia tristis_ in which the dark cortical cells show an outer colourless membrane over the inner dark wall.

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

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

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

_d._ ENUMERATION OF AMORPHOUS PIGMENTS:

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

=2. Blue.= A very rare colour in lichens, so far found in only a few species, _Biatora_ (_Lecidea_) _atrofusca_, _Lecidea sanguinaria_ and _Aspicilia flavida_ f. _coerulescens_. It forms a layer of amorphous granules embedded in the outer wall of the paraphyses, becoming more dense towards the epithecium. A few granules are also present in the hymenium.

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

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

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

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

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

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

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