the study of lichen physiology attention must be given to the activities of the symbionts as well as to those of the symbiotic plant. Gonidia do not greatly differ from the allied algae growing in the open : they possess chloro plasts, and form starch by photosynthesis. Mameli (1920) and Tobler (1923) demonstrated minute granules of starch on the outside of the gonidia—a ready food for the fungus. The hyphal cells have been more affected by symbiosis, and a much slower growth than in fungi has become a fixed character as proved by artificial cultures. The cell-walls, as in fungal tissues, are formed of hemi-celluloses, chitin being present in nearly all lichens. There is no true cellulose, but a substance, lichenin (C6111005), allied to starch, has been demonstrated as well as a slightly different sub stance, isolichenin, the latter proved by Ziegenspeck (1924) to be a reserve material. Amyloid hyphae giving a blue reaction with iodine are present in the medulla of several species. Swollen cells filled with oil, probably an excretory substance, occur in many lichens especially in limestone species. Oxalic acid is also fre quently found in lichen tissues in the form of crystals, small gran ules, or in large clear masses as in Pertusaria communis.
These are the most interesting and character istic of lichen products. They are deposited on the outside of the hyphal cells as minute coloured specks or as colourless substances, and show a wide range of chemical formulae and a great variety of crystalline form. They are the product of the symbiotic plant as was proved by Tobler (1909) in his cultures of lichen tissues. Many of them are bright yellow, orange or red, and give the clear pure tone of colour to many familiar lichens. They are strongly influenced by light: Xanthoria parietina, a brilliant yellow plant in full exposure, becomes grey green in the shade, with a small acid. content. Some of these acids are rare, others are widely distrib uted, e.g.—usninic acid, found in some 7o widely diverse species; atranorin, first discovered in Lecanora atra, in about 7o species; salazinnic acid is equally common. They are abundant chiefly on well-aerated portions of the thallus—the soredial hyphae, the outer cortex, the loose medullary tissue, and on the disc of the apothecia.
Chemical Grouping. The acids have been arranged by Zopf (1907) in I, the fat series and 2, the benzole series.
I. The fat series. Zopf includes five groups in this series : three of the series are colourless substances; the coloured include vul pinic acid from the yellow lichen, Letharia vulpina, stictaurin deposited in orange-red crystals on the hyphae of Sticta aurata, and rhizocarpic acid obtained from the yellow lichen Rhizocarpon geographicum.
The benzole series. with two subseries—orceine and anthra cine derivatives. The colourless orceine contains the colouring principle of commercial orchil. In the anthracine derivatives some of the acids are also coloured, such as parietin from Xanthoria parietina and solorinic acid from Solorina crocea.
The question has been debated as to the service rendered by the acids : to some extent they protect the plants from wholesale destruction by snails, insects, etc., as they render the thallus more or less unpalatable. Goebel (1927) has demonstrated that they are also a protection against water-logging. He found that out growths of hyphal hairs, cilia, etc., formed efficient water conduc tors, but if acids were abundant they remained dry : when the acids were removed by chemical means saturation was easily achieved. As acids are present on all aerated portions, they must be a powerful aid in keeping the air-channels open and thus serve a useful purpose.
Water is supplied by rain, mist or dew, mist being the most favourable for lichen requirements (Stocker 1927). Dew is important in extremely dry localities such as des erts. Inorganic substances are obtained to some extent from the substratum but mainly from air borne particles. Organic food is provided by the algae or may be procured by the hyphae from humus, etc.
Lichens show marvellous resistance as regards heat or cold. They survive the high temperatures of direct illumination and they endure seasons of extreme cold on mountains or in the polar zone. It is to their power of drying up to a condition of latent vitality that they owe this resistance. Light that can penetrate the thickened cortex and reach the gonidial zone is essential, but the same dense cortex protects the gonidia from too intense sunlight as do the acids and pigments. Light is of first importance in fruit formation, and the fruit bodies are therefore situated on well lighted portions of the thallus.
Soft grey colours predominate, the thick cortex and the underlying gonidia combining to produce this effect; when wetted the cortex becomes transparent and the green colour is more evident. Acids and pigments, the latter usually some shade of brown, give various colours from yellow to brown or almost black. Strong sunlight induces the formation of both acids and pigments, and intensifies colour as seen in exposed situations. Blue, violet or red colours occur more rarely, and generally in con nection with the fruiting bodies. Some lichens become rust-col oured by infiltration from an iron soil. It is only when we com pare untouched nature with the ugly gash of recent quarrying that we realize the beauty given to the rocks by the variety of lichen colouring.