PDF - CES (IISc)

23 6
PHYSIOLOGY
material with which they come in contact Others live on dead wood, palings,
etc. where the supply of disintegrated organic substance is even greater ; or
they spread over withered mosses and soil rich in humus.
b. FROM OTHER LICHENS. Bitter 1 has recorded several instances ob-
served by him of lichens growing over other lichens and using up their
substance as food material. Some lichens are naturally more vigorous than
others, and the weaker -or more slow growing succumb when an encounter
takes place. Pertusaria globulifera is one of these marauding species; its
habitat is among mosses on the bark of trees, and, being a quick grower, it
easily overspreads its more sluggish neighbours. It can scarcely be considered
a parasite, as the thallus of the victim is first killed, probably by the action
of an enzyme.
Lecanora subfusca and allied species which have a thin thallus are
frequently overgrown by this Pertusaria and a dark line generally precedes
the invading lichen; the hyphae and the gonidia of the Lecanorae are first
killed and changed to a brown structureless mass which is then split up by
the advancing hyphae of the Pertusaria into small portions. A little way
back from the edge of the predatory thallus the dead particles are no longer
visible, having been dissolved and completely used up. Pertusaria amara
also may overgrow Lecanorae, though, generally, its onward course is
checked and deflected towards a lateral direction; if however it is in a young
and vigorous condition, it attacks the thallus in its path, and ahead of it
appears the rather broad blackish line marking the fatal effect of the enzyme,
the rest of the host thallus being unaffected. Neither Pertusaria seems to
profit much, and does not grow either faster or thicker; the thallus appears
indeed to be hindered rather than helped by the encounter. Biatora (Lecidea)
quernea with a looser, more furfuraceous thallus is also killed and dissolved
by Pertusariae; but if the Biatora is growing near to a withering or dead
lichen it, also, profits by the food material at hand, grows over it and uses it up.
Bitter has also observed lichens overgrown by Haematomma sp. the ; growth
of that lichen is indeed so rapid that few others can withstand its approach.
Another common rock species, Lecanora sordida (L. glaucoma), has a
vigorous thallus that easily ousts its neighbours. Rhizocarpon geographicum,
a slow-growing species, is especially liable to be attacked ; from the thallus
of L. sordida the hyphae in strands push directly into the other lichen in a
horizontal direction and split up the tissues, the algae persist unharmed for
some time, but eventually they succumb and are used up; the apothecia,
though more resistant than the thallus, are also gradually undermined and
hoisted up by the new growth, till finally no trace of the original lichen is
left. Lecanora sordida is however in turn invaded by Lecidea insularis
(L. intumescens} which is found forming
1 Bitter 1899.
small orbicular areas on the