CELLS AND CELL PRODUCTS 229 



As an instance of the value of chemical tests, Zopf cites the reaction of 

 hypochlorite of lime on the thallus of four different species of Gyrophora, 

 the "tripe de roche": — 



Gyrophora torrefacta CaCl +. 



„ polyrhiza CaCl X- 



„ proboscidea CaCl ±. 



„ erosa CaCl z. 



It must however be borne in mind that these species are well differentiated 

 and can be recognized, without difficulty, by their morphological characters. 

 Experienced systematists like Weddell refuse to accept the tests unless 

 they are supported by true morphological distinctions, as the reactions are 

 not sufficiently constant. 



G. Chemical Reactions in Nature 



Similar colour changes may often be observed in nature. The acids of 

 the exposed thallus cortex are not unfrequently split up by the gradual 

 action of the ammonia in the atmosphere, one of the compounds thus set 

 free being at the same time coloured by the alkali. Thus salazinic acid, a 

 constituent of several of our native Parmeliae, is broken up into carbonic 

 acid and salazininic acid, the latter taking a red colour. Fumarprotocetraric 

 acid is acted on somewhat similarly, and the red colour may be seen in 

 Cetraria at the base of the thallus where contact with soil containing 

 ammonia has affected the outer cortex of the plant. The same results are 

 produced still more effectively when the lichen comes into contact with 

 animal excrement. 



Gummy exudations from trees which are more or less ammoniacal may 

 also act on the thallus and form red-coloured products on contact with the 

 acids present. Lecanora {Aspicilia) cinerea is so easily affected by alkalies 

 that a thin section left exposed may become red in time owing to the 

 ammonia in the atmosphere. 



II. GENERAL NUTRITION 



A. Absorption of Water 



Lichens are capable of enduring almost complete desiccation, but though 

 they can exist with little injury through long periods of drought, water is 

 essential to active metabolism. They possess no special organs for water 

 conduction, but absorb moisture over their whole surface. Several inter- 

 dependent factors must therefore be taken into account in considering the 

 question of absorption : the type of thallus, whether gelatinous or non- 

 gelatinous, crustaceous,foliose or fruticose,as also the nature of the substratum 

 and the prevailing condition of the atmosphere. 



