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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 +.
proboscidea CaCl .
erosa CaCl I.
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 (Aspicilta) 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
water is
they can exist with little injury through long periods of drought,
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.