LICHENS, lī'kĕnz (Lat. lichen, from Gk.
λειχήν, leichēn, λιχήν, lichēn, lichen). A very
large group of plants, mostly northern and arctic
in their distribution, some of the forms living
at the very limits of vegetation. They have
generally flat, membranous, or crustaceous bodies,
but some forms have branching stalks. Lichens
are exceedingly interesting in their structure and
life activities, for they are known to be
composed of two plants living together (smyymbiosis,
q.v.). Every lichen is made up of two elements:
(1) colorless threads (filaments); and (2) cells
(gonidia) containing green or blue-green
pigment. These latter are blue-green (Cyanophyceæ)
or green algæ (Chlorophyceæ), and the colorless
thready portion is a fungus. The fruit of a lichen
is that of an ascomycete fungus, generally a
flattened or cup-shaped structure with variously
colored lining (black, brown, red, etc.). The
colored lining of the cup or disk consists chiefly
of the little sacs (asci) which contain the spores.
There is a small group of tropical lichens whose
fungus constituent is related to the mushrooms
(Basidiomycetes). Lichens generally live in
exposed situations, where their only source of
water is that from rain and fog. This water is
used by the algal cells which manufacture organic
food in the manner characteristic of all green
plants (photosynthesis, q.v.). When the lichen
dries up in the sunshine all vegetative activities
cease for the time, to be resumed when the plants
are again moistened. However, lichens are
especially adapted to retain moisture by the gelatinous
character of the thallus. There are also
present thread-like outgrowths (rhizines) from
the fungal part of the plant, which may draw up
some water from the substratum, but probably
serve chiefly as organs of attachment. The algal
portion of a lichen manufactures the organic
food that is absorbed by the fungal elements,
which bear the same relation to the algal cells
as does any parasitic fungus to its host. The
fungal portion makes no return at all
commensurate with the benefits it receives; indeed, it is
well known that the fungus may injure and even
destroy the alga. The algæ are really slaves of
the fungi, and under such conditions the latter
are absolutely dependent upon the former. The
algæ are of course perfectly able to take care of
themselves, and do actually grow wild on many
of the surfaces frequented by lichens. Some of
the lichens are conspicuous in the landscape, e.g.
many forms in barren highlands among the
mountains, and on the rocky shores in the Arctic
regions. The lichen called ‘reindeer moss’
(Cladonia rangiferino) supports imniense herds of
reindeer and cattle in the highlands of Norway,
and the ‘Iceland moss’ (Cetraria islandica) is
eaten by man. Some typical lichens are shown
on the accompanying plate.
A lichen-spore can produce a new plant only if it germinates among the cells of a suitable alga. It then proceeds to inclose the algal cells in a web of threads (mycelium) and holds them ever afterwards in slavery. This hehavior has been well established by a number of investigators who have produced artificial lichens by bringing the spores in contact with the proper wild algæ. Groups of algal cells with a portion of the thready body (mycelium) may form scales (‘soredia’) on the surface of the lichen body, and these are capable of reproduction.
For American species, consult Tuckerman, A Synopsis of North American Lichens. See Ascomycetes and Colored Plate of Mosses and Lichens with Musci.
LICHENS
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