In such districts lichen vegetation is entirely or almost entirely absent.
The fungus seems, on the other hand, to stimulate the algal cells to special development, for those in the lichen are larger than those in the free state, but this is not necessarily adverse to the idea of parasitism, for it is well known that an increase in the size of the cells of the host is often the result of the attacks of parasitic fungi.
Above this grows a species of pine, which becomes dwarfed and disappears at an altitude of about 6000 ft., beyond which is a zone of lichen and moss covered or almost bare rock.
Zukal has considered that the lichen acids protect the lichen from the attacks of animals; the experiments of Zopf, however, have cast doubt on this; certainly lichens containing very bitter acids are eaten by mites though some of the acids appear to be poisonous to frogs.
The lichen flora of temperate regions again is essentially distinguished from the preceding by the frequency of corticolous species belonging to Lecanora, Lecidea and Graphidei.
He investigated the exact relation of fungus and alga and showed that the same alga is able to combine with a number of different fungi to form lichens; thus Chroolepns umbrinus is found as the gonidia of 13 different lichen genera.
The view of the dual nature of lichens had hitherto been based on analysis; the final proof of this view was now supplied by the actual synthesis of a lichen from fungal and algal constituents.
Thus in the case of saxicolous lichens the mineralogical character of the rock has of itself little or no influence upon lichen growth, which is influenced more especially and directly by their physical properties, such as their capacity for retaining heat and moisture.
The relation of the two constituents of the lichen have been briefly stated in the beginning of this article.
The thallus or body of the lichen is of very different form in different genera.