a. From the Substratum. The Ascomycetous fungi, from which so many of the lichens are descended, are mainly saprophytes, obtaining their carbohydrates from dead plant material, and lichen hyphae have in some instances undoubtedly retained their saprophytic capacity. It has been proved that lichen hyphae, which naturally could not exist without the algal symbiont, may be artificially cultivated on nutrient media without the presence of gonidia, though the chief and often the only source of carbon supply is normally through the alga with which the hyphae are associated in symbiotic union.
A large number of crustaceous lichens grow on the bark of trees, and their hyphae burrow among the dead cells of the outer bark using up the 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[860] has recorded several instances observed 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 small orbicular areas on the 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. Pertusaria 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 Pertusaria 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[861] 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 Variolaria, 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 neighbouring tissue was brown and dead.
Perhaps the most voracious consumer of organic remains is Lecanora tartarea, more especially the northern form frigida. It is the well-known cudbear 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[862] 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 podetia being a loose felt of hyphae; they are thus sensitive to cold and 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.