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_. III. ASSIMILATION AND RESPIRATION A. INFLUENCE OF TEMPERATURE