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39 2
ECOLOGY
though in dry weather the lichens may be exposed, and can withstand
desiccation for a considerable time :
Pterygium Kenmorensis. Lecidea contigua.
Collema fluviatile.
Lecidea albocoerulescens.
Lecanora lacustris. Dermatocarpon miniatum var. complicaium.
Lecanora epulotica. Dermatocarpon aquaticutil.
Bacidia inundata. Verrncaria laevata.
Rhizocarpum obscuratum. Verrucaria aethiobola.
Rhizocarpum petraeum.
Verrucaria margacea.
The second group of species usually inhabits damp, shaded rocks of
ravines or large boulders by streams or near waterfalls. It includes species
of Collema, Sticta, Peltigera, Solorina, Pannaria, etc., with Opegrapha zonata,
Porina lectissima and Verrucaria nigrescens.
The last-mentioned lichen grows by preference on limestone, but in
excessive moisture 1
importance.
,
as by the sea-side, the substratum seems to be of minor
D. LICHENS AS PIONEERS
a. SOIL-FORMERS. The part played by lichens in the "Economy of
Nature" is of very real importance: to them is allotted the pioneer work
of breaking down the hard rock surfaces and preparing a soil on which
more highly developed plants can grow. This was pointed out by Linnaeus 2
who thus describes the succession of : plants "Crustaceous lichens," he
writes, "are the first foundation of vegetation. Though hitherto we have
considered theirs a trifling place among plants, nevertheless they are of
great importance at that first stage in the economy of nature. When the
rocks emerge from the seas, they are so polished by the force of the waves,
that scarcely any kind of plant could settle on them, seen more especially
near the sea. But very soon, in truth, the smallest crustaceous lichens begin
to cover those arid rocks, and are sustained by minute quantities of soil and
by imperceptible particles brought to them by rain and by the atmosphere.
These lichens in time become converted by decay into a. thin layer of
humus, so that at length imbricate lichens are able to thrust their rhizoids
into it. As these in turn change to humus by natural decay, various mosses
such as Hypnum, BryuDi and Polytrichum follow, and find suitable place
and nourishment. In time there is produced by the dying down of the
mosses such a quantity of soil that herbs and shrubs are able to establish
themselves and maintain their existence."
Similar observations have been made since Linnaeus's day, among others
by Guembel 3 in his account of Lecanora ventosa. Either by the excretion of
carbon dioxide which acidifies the surrounding moisture, or by the mechanical
1 Wheldon and Wilson 1913.
2 Linnaeus 1762.
3 Guembel 1856.