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GENERAL NUTRITION 237
Lecanora thallus. It kills its host in patches and the dead material mostly
drifts away. On any strands that are left Candellariella vitellina generally
settles and evidently profits by the dead nutriment. It does not spread to
the living thallus. Lecanora polytropa also forms colonies on these vacant
patches, with advantage to its growth.
Even the larger lichens are attacked by these quick-growing crusts.
Pertitsaria globulifera spreads over Parmelia perlata and P. physodes,
gradually dissolving and consuming the different thalline layers; the lower
cortex of the victim holds out longest and can be seen as an undigested
black substance within the Pertitsaria thallus for some time. As a rule,
however, the lichens with large lobes grow over the smaller thalli in a purely
mechanical fashion.
c. FROM OTHER VEGETATION. Zukal 1 has given instances of association
between mosses and lichens in which the latter seemed to play the part of
parasite. The terricolous species Baeomyces rufus (Sphyridium) and Biatora
decolorans, as well as forms of Lepraria and Variolarta, he found growing
over mosses and killing them. Stems and leaves of the moss Plagiothecium
sylvaticum were grown through and through by the hyphae of a Pertusaria,
and he observed a leaf of Polytrichum commune pierced by the rhizinae of
a minute Cladonia squamule. The cells had been invaded and the neigh-
bouring tissue was brown and dead.
remains is Lecanora
Perhaps the most voracious consumer of organic
tartarea, more especially the northern form frigida. It is the well-known
cud-bear lichen of West Scotland, and is normally a rock species. It has
an extremely vigorous thickly crustaceous and quick-growing thallus, and
spreads over everything that lies in its path decaying mosses, dead leaves,
other lichens, etc. Kihlman 2 has furnished a graphic description of the way
it covers up the vegetation on the high altitudes of Russian Lapland. More
than any other plant it is able to withstand the effect of the cold winds that
sweep across these inhospitable plains. Other plant groups at certain seasons
or in certain stages of growth are weakened or killed by the extreme cold
of the wind, and, immediately, a growth of the more hardy grey crust of
Lecanora tartarea begins to spread over and take possession of the area
affected very frequently a bank of mosses, of which the tips have been
destroyed, is thus covered up. In the same way the moorland Cladoniae,
C. rangiferina (the reindeer moss) and some allied species, are attacked.
They have no continuous cortex, the outer covering of the long branching
are thus sensitive to cold and
podetia being a loose felt of hyphae; they
liable to be destroyed by a high wind, and their stems, which are blackened
as decay advances, become very soon dotted with the whitish-grey crust of
the more vigorous and resistant Lecanora.
1 Zukal 1879.
2 Kihlman 1890.