LICHEN COMMUNITIES 371 



As already described Lecanora tartareal spreads freely over the mosses 

 of the tundra. Aigret 2 in a study of Cladoniae notes that CL pyxidata, var. 

 neglecta chooses little cushions of acrocarpous mosses, which are particularly 

 well adapted to retain water. CL digitata, Cl.flabelliformis and some others 

 grow on the mosses which cover old logs or the bases of trees. 



g. ON FUNGI. Some of the fungi, such as Polyporei, are long lived, and 

 of hard texture. On species of Lenzites in Lorraine, Kieffer 3 has recorded 

 15 different forms, but they are such as naturally grow on wood and can 

 scarcely rank as a separate association. 



3. SAXICOLOUS 



Lichens are the dominant plants of this and the following formations, 

 they alone being able to live on bare rock; only when there has been formed 

 a nidus of soil can other plants become established. 



a. CHARACTERS OF MINERAL SUBSTRATA. It has been often observed 

 that lichens are influenced not only by the chemical composition of the 

 rocks on which they grow but -also by the physical structure. Rocks that 

 weather quickly are almost entirely bare of lichens : the breaking up of the 

 surface giving no time for the formation either of thallus or fruit. Close- 

 grained rocks such as quartzite have also a poor lichen flora, the rooting 

 hyphae being unable to penetrate and catch hold. Other factors, such as 

 incidence of light, and proximity of water, are of importance in determining 

 the nature of the flora, even where the rocks are of similar formation. 



b. COLONIZATION ON ROCKS. When a rock surface is laid bare it 

 becomes covered in time with lichens, and quite fresh surfaces are taken 

 possession of preferably to weathered surfaces 4 . The number of species is 

 largest at first and the kind of lichen depends on the flora existing in the 

 near neighbourhood. Link 6 , for instance, has stated that Lichen candelarius 

 was the first lichen to appear on the rocks he observed, and, if trees were 

 growing near, then LicJien parietinus and Lichen tenellus followed soon after. 

 After a time the lichens change, the more slow-growing being crowded out 

 by the more vigorous. Crustaceous species, according to Malinowski 6 , are 

 most subject to this struggle for existence, and certain types from the nature 

 of their thallus are more easily displaced than others. Those with a deeply 

 cracked areolated thallus become disintegrated in the older central areas by 

 repeated swelling and contracting of the areolae as they change from wet 

 to dry conditions. Particles of the thallus are thus easily dislodged, and 

 bare places are left, which in time are colonized again by the same lichen 

 or by some invading species. There may result a bewildering mosaic of 



1 See p. 358. 2 Aigret 1901. 8 Kieffer 1894. 4 Stahlecker 1906. 



9 Link 1795. 6 Malinowski 1911. 



24 2 



