Vegetation of Northern Cape Breton. 295 



{2) areas in which, owing to the Hmiting influence of certain 

 local factors, the succession has culminated in an edaphic climax 

 which is less mesophytic than the regional climax association- 

 type; and (5) areas which have been denuded of their original 

 vegetation, and where secondary succession is taking place. 

 Secondary successions are discussed in a separate section, but 

 they obviously possess many points in common with primary suc- 

 cessions. 



Aside from the views of the regional climax forest (Figs. 

 lo-ii), the primary formation-types of ordinary uplands are 

 pictured only by Figs. 14-16; but see in this connection the 

 figures illustrating secondary formations (Figs. 33-40). 



h. THE ASSOCIATION-COMPLEXES OF ROCK OUTCROPS 



Rock surface association-types. — The first forms of life to 

 grow on a bare rock surface are usually the lichens. Commonly 

 the crustose lichens appear first: species of Bnellia, Lecanora, 

 Lecidia, Rhi::ocarpon, etc. These are closely followed and often 

 accompanied by f oliose lichens : species of Parmelia, Gyrophora, 

 etc. Associated with these may be the fruticose lichen. Stereo- 

 caulon sp., and certain lithophytic mosses, such as Hedwigia 

 ciliata and Grimmia apocarpa. Where the rock slopes steeply, 

 other plants may be entirely absent, owing to their inability to 

 secure a foothold on the bare rock surface, and the succession 

 may become arrested at this early stage. 



But on gentle slopes the conditions are different, for here 

 plants are able to maintain their positions even when entirely 

 unattached to the substratum. Situations of this sort are 

 favorable to the development of the fruticose lichens, notably 

 species of Cladonia (e. g., C. rangifcrina, C. syhatica). These 

 usually establish themselves first in shallow depressions of the 

 rock surface, where moisture conditions are relatively favorable, 

 and from here they may spread laterally in all directions until 

 the surface of the rock becomes completely covered with a loose, 

 essentially unattached mat of vegetation. In company here 

 with the fruticose lichens very commonly grow certain mosses : 

 these may include aiiy of the species mentioned below as charac- 

 teristic of crevices, but particularly Racomitrium canescens and 

 species of Polytrichum. 



