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250
PHYSIOLOGY
in the cortical cells and rhizoids of many thalli. In some plants the thallus
is brown both above and below, in others, as in Parmelia revoluta, etc. only
the under surface is dark-coloured.
e. COLOUR DUE TO INFILTRATION. There are several crustaceous lichens
that are rusty-red, the colour being due to the presence
of iron. These
lichens occur on siliceous rocks of gneiss, granite, etc., and more especially
on rocks rich in iron. Iron as a constituent of lichens was first demonstrated
by John 1 in Ramalina fraxinea and R. calicaris. Grimbel 2
colour of rust lichens was due to an iron salt, and Molisch 3
proved that the
by microscopic
examination located minute granules of ferrous oxide as incrustations on
the hyphae of the upper surface of the thallus. Molisch held that the rhizoids
or penetrating hyphae dissolved the iron from the rocks by acid secretions.
Rust lichens however grow on rocks that are frequently under water in which
the iron is already present.
Among " rusty " lichens are the British forms, Lecanora lacustris, the
thallus of which is normally white, though generally more or less tinged
with iron;
it inhabits rocks liable to inundation. L. Dicksonii owes its fer-
ruginous colour to the same influences. Lecidea contigua vax.flavicunda and
L. confluens f. oxydata are rusty conditions of whitish-grey lichens.
Nilson 4 found rusty lichens occurring frequently in the Sarak-Gebirge,
more especially on glacier moraines where they were liable, even when un-
covered by snow, to be flooded by water from the higher reaches. It is the
thallus that is affected by the iron, rarely if ever are apothecia altered in
colour.
1
John 1819.
2 Grimbel 1856.
3 Molisch 1892.
4 Nilson 1907.