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.