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 1 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 2 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 3 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 4 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 5 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 6 , a product of chitin. 



1 De Bary 1866, p. 7. 2 Gilson 1893. 3 Winterstein 1893. 4 Gilson 1894. 



5 The chemical formula of chitin is given as CeoHiooNgOsg, that of chitosan as CuHge^Oio. 



S. L. 14 



