LICHEN COMMUNITIES 389 



C. crispa are examples of this compact growth : they form a soft thick carpet 

 of a yellowish-grey colour. Cladoniae also grow in crowded tufts, but are 

 generally to be found in the more sheltered positions, in valleys between 

 the tundra hills and in the clefts of the rocks, or between great boulders and 

 stones where there is also more moisture. 



The same kinds of lichens occur all over these northern regions. Birger 

 Nilson^ gives as the principal earth-lichens in Swedish Lappland, Alectoria 

 ochroleuca, A. nigricans, Cetraria nivalis, C. cucullata, Cladonia uncialis, 

 Thamnolia {Ceranid) vermiciilaris and Sphaerophorus coralloides. 



Darbishire^ speaks of the extensive beds of various species of Cetraria 

 in Ellesmere Land and King Oscar Land. Alectoria nigricans a.nd A. ockro- 

 leuca were often found in pure communities, but even more frequently in close 

 company with mosses. Though these fruticose lichens are not represented 

 by many species in Arctic regions, they cover a very extensive area and 

 form a very important feature in the vegetation. 



Crustaceous lichens are not wanting : Lecanora tartarea f frigida, L. 

 epibryon and others are to be found in great sheets covering the mosses or 

 the soil, or spreading over the stones and boulders. Cold has no deterrent 

 effect, and their advance is only checked by the presence of perpetual snow. 



e. Desert Lichens. The reduced rainfall of desert countries is un- 

 favourable to general lichen growth and only the more xerophytic species — 

 those with a stout cortex — can flourish in the adverse conditions of excessive 

 light and dryness. Lichens, however, there are, in great numbers as far as 

 individuals are concerned, though the variety is not great. The abundance 

 of the crustaceous Lecanora esculenta in the deserts of Asia has already been 

 noted. Flagey'' found it one of the dominant species at Biskra in the Sahara 

 where it grows on the' rocks. Patouillard* in describing the flora of Tunis 

 speaks of the great patches (societies) of Lecanora crassa f. deserti -which at 

 a distance look like milk spilled on the ground, or if growing on unequal 

 surfaces take the aspect of plaster that has been passed over by some 

 wheeled vehicle. At Biskra species of Heppia grow on the sand. Steiner" 

 also records the frequency oi Heppia and of Endocarpon in the Sahara as well 

 as of Gloeolichens which, as they are associated with gelatinous blue-green 

 algae, can endure extreme and long-continued desiccation. These lichens, 

 however, only form communities in clefts among the rocks where these abut 

 on the desert. In the great plains the sand is too mobile and too often 

 shifted by the sirocco to enable them to settle. 



Bruce Fink" discusses desert lichens and their adaptive characters : 

 crustaceous species with a stout cortex are best able to withstand the long 

 dry periods; conspicuously lobed thalli are lacking, as are lichens with 



^ Nilson 1907. ^ Darbishire 1909. ^ Flagey 1901. ■* Patouillard 1897. 



'^ Steiner 1895. " Bruce Fink 1909. 



