d. Value in Diagnosis. Lichen spermogonia have once and again been found of value in deciding the affinity of related plants, and though there are a number of lichens in which we have no record of their occurrence, they are so constant in others, that they cannot be ignored in any true estimation of species. Nylander laid undue stress on spermogonial characters, considering them of almost higher diagnostic value than the much more important ascosporous fruit. They are, after all, subsidiary organs, and often—especially in crustaceous species—they are absent, or their relation to the species under examination is doubtful.
CHAPTER V
PHYSIOLOGY
I. CELLS AND CELL PRODUCTS
Any study of cells or cell-membranes in lichens should naturally include those of both symbionts, but the algae though modified have not been profoundly changed, and their response to the influences of the symbiotic environment has been already described in the discussion of lichen gonidia. The description of cells and their contents refers therefore mainly to the fungal tissues which form the framework of the plant; they have been transformed by symbiosis to lichenoid hyphae in some respects differing from, in others resembling, the fungal hyphae from which they are derived.
A. Cell-Membranes
a. Chitin. It was recognized by workers in the early years of the nineteenth century that the substance forming the cell-walls of fungal hyphae differed very markedly from the cellulose of the membranes in other groups of plants, the blue colouration with iodine and sulphuric acid so characteristic of cellulose being absent in most fungi. Various explanations were suggested; but it was always held that the doubtful substance was a cellulose containing something peculiar to fungi, this view being strengthened by the fact that, after long treatment with potash, a blue reaction was obtained. It was called fungus-cellulose by De Bary[740] in order to distinguish it from true cellulose.
It was not till a much later date that any exact work was done on the fungal cell, and that Gilson[741] by his researches was able to prove that the membranes of fungi contained probably no cellulose, or, “if cellulose were present, it was in a different condition from the cellulose of other plants.” Winterstein[742] followed with the results of his examination of fungus-cellulose: he found that it contained nitrogen and therefore differed very considerably from typical plant cellulose. Gilson[743] published a second paper dealing entirely with fungal tissues in which he also established the presence of nitrogen, and added that this nitrogenous compound resembled in various ways the chitin[744] of animal cells. He further discovered that by heating it with potash a substance was obtained that took a reddish-violet stain when treated with iodine and weak sulphuric acid. This substance, called by him mycosin, was proved later to be similar to chitosan[744], a product of chitin.