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GENERAL NUTRITION 233
from atmospheric dust dissolved in rain, or from wind -borne particles deposited
on the surface of the thallus which may be gradually dissolved and
absorbed by the cortical and growing hyphae. That substances received
from the atmospheric environment may be all important is shown by the
exclusive habitat of some marine lichens; the Roccellae, Lichinae, some
species of Ramalina and others which grow only on rocky shores are almost
as dependent on sea-water as are the submerged algae. Other lichens, such
as Hydrothyria venosa and Lecanora lacustris, grow in streams, or on boulders
that are subject to constant inundation, and they obtain their inorganic food
mainly, if not entirely, from an aqueous medium.
Though lichens cannot live in an atmosphere polluted by smoke, they
thrive on trees and walls by the road-side where they are liable to be almost
smothered by soil-dust. West 1 has observed that they flourish in valleys
that are swept by moisture laden winds more especially if near to a high-
with the dust. The favourite habitats
way, where animal excreta are mingled
of Xanthoria parietina are the walls and roofs of farm-buildings where the
dust must contain a large percentage of nitrogenous material ; or stones by
the sea-shore that are the haunts of sea-birds. Sandstede 2 found on the
island of Riigen that while the perpendicular faces of the cliffs were quite
bare, the tops bore a plentiful crop of Lecanora saxicola, Xanthoria lychnea
and Candellariella mtellina. He attributed their selection of habitat to the
presence of the excreta of sea-birds. As already stated the connection of
foliose and fruticose lichens with the substratum is mainly mechanical but
occasionally a kind of semiparasitism may arise.
3
Friedrich gives
in a species of Usnea of unusually vigorous development. It grew
an instance
on bark
and the strands of hyphae, branching from the root-base of the lichen,
had reached down to the living tissue of the tree-trunk and had penetrated
between the cells by dissolving the middle lamella. It was possible to find
holes pierced in the cell-walls of the host, but it was difficult to decide if
the hyphae had attacked living cells or were merely preying on dead material.
Lindau 4 held very strongly that lichen hyphae were non-parasitic, and merely
split apart the tissues already dead, and the instance recorded by Friedrich
is of rare occurrence 5 .
That the substratum does have some indirect influence on these larger
lichens has been proved once and again. Uloth 6
, a chemist as well as a
botanist, made analyses of plants of Evernia prunastri taken from birch bark
and from sandstone. Qualitatively the composition of the lichen substances
was the same, but the quantities varied considerably. Zopf 7
has, more
recently, compared the acid content of a form of Evernia furfuracea on rock
with that of the same species growing on the bark of a tree. In the case of
1 2 3 4 West 1905.
Sandstede 1904.
Friedrich 1906.
Lindau 1895*.
"
6 6 See p. 109.
Uloth 1861.
Zopf 1903.