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EXTERNAL INFLUENCES 359
observed the gradual colonization of the depressions, first by a growth of
hepatics and mosses and by such ground lichens as Peltigera canina, P.
aphthosa and Nephromium arcticum ; they cover the soil and in time the
hollow becomes filled with a mass of vegetation consisting of Cladonias,
mosses, etc. On reaching a certain more exposed level these begin to wither
and die off at the tips, killed by the high cold winds. Then arrives Lecanora
tartarea, one of the commonest Arctic lichens, and one which is readily
a saprophyte on decayed vegetation. It covers completely the mound of
weakened plants which are thus smothered and finally killed. The collapse
of the substratum entails in turn the breaking of the Lecanora crust, and
the next high wind sweeps away the whole crumbling mass. How long
recolonization takes, it was impossible to find out.
Upright fruticose lichens are necessarily more liable to damage by wind,
but maritime Ramalinae and Roccellae do not seem to suffer in temperate
climates, though in regions of extreme cold fruticose forms are dwarfed and
stunted. The highest development of filamentous lichens is to be found in
more or less sheltered woods, but the effect of wind on these lichens is not
wholly unfavourable. Observations have been made by Peirce 1 on two
American pendulous lichens which are dependent on wind for dissemina-
tion. On the Californian coasts a very large and very frequent species,
Ramalina reticulata (Fig. 64), is seldom found undamaged by wind. In
Northern California the deciduous oaks Quercus alba and Q. Douglasii are
festooned with the lichen, while the evergreen " live oak," Q. chrysolepis,
with persistent foliage, only bears scraps that have been blown on to it.
Nearer the coast and southward the lichen grows on all kinds of trees and
shrubs. The fronds of this Ramalina form a delicate reticulation and when
moist are easily torn. In the winter season, when the leaves are off the
trees, wind- and rain-storms are frequent ; the lichen is then exposed to
the full force of the elements and fragments and shreds are blown to other
trees, becoming coiled and entangled round the naked branches and barky
excrescences, on which they continue to grow and fruit perfectly well.
A succeeding storm may loosen them and carry them still further. Peirce
noted that only plants developed from the spore formed hold-fasts and
they were always small, the largest formed measuring seven inches in length.
Both the hold-fast and the primary stalk were too slight to resist the tearing
action of the wind.
Schrenk 2 made a series of observations and experiments
with the lichens
Usneaplicata and U. dasypoga, long hanging forms common on short-leaved
conifers such as spruce and juniper. The branches of these trees are often
covered with tangled masses of the lichens not due to local growth, but to
wind-borne strands and to coiling and intertwining of the filaments owing
1 Peirce 1898.
2 Schrenk 1898.