Vegetation of Northern Cape Breton. 303 



down the slope, and thus produces a very unstable rock mass. 

 Between the two extremes are all degrees of intergradation. 



The association-types of boulder tains. — As in the rock out- 

 crop series, two types of habitat are available to plants here, the 

 rock surfaces and the crannies between the fragments. The sur- 

 faces of the boulders are usually overgrown with crustose and 

 foliose lichens. Any of the rock face species previously cited 

 may grow here. These, however, play little or no active part in 

 the talus succession as a whole : the latter is instituted almost 

 entirely by the plants which grow in the crannies. Here, through 

 the further disintegration of the larger rock fragments, and also 

 to some extent from other sources, a soil accumulates. Toward 

 the base of a talus slope soil gathers faster and soil moisture 

 is more abundant than higher up, so that as a rule succession 

 progresses much more rapidly here than elsewhere on the slope. 

 Very commonly the base of a talus slope will be clothed by a 

 mesophytic forest while above there are only scattered trees and 

 shrubs. 



The shade and protection from exposure afforded by the blocks 

 which surround the crannies create here conditions which are 

 congenial to mesophytes as well as to many xerophytes. The 

 pioneer plants may include various species of Cladonia and any 

 of the bryophytes which have been cited as characteristic of 

 crevices in the rock outcrop series. It also commonly includes 

 certain more mesophytic species, such as Ptilidium ciliarc, 

 Hypnuui Schreheri, and Hylocomium splendens. The lichen- 

 bryophyte element may perform an important function in the 

 succession by forming cushions and mats which often spread 

 away from the crannies over the adjoining rock surfaces, creat- 

 ing a substratum favorable for the germination of the spores 

 and seeds of higher plants. The presence in the crannies of a 

 soil, however, permits the growth at the outset, not only of 

 lichens and bryophytes, but of vascular plants as well. Her- 

 baceous plants are sparingly represented by Polypodium vulgarc 

 and a few other species, while the two shrubs, Sambncus racc- 

 mosa and Ruhiis idacns canadensis, usually occupy a prominent 

 position. But both herbs and shrubs are subordinate in impor- 

 tance to trees. These gain a foothold early and may predominate 

 the succession from start to finish. For a long time, at least as 

 long as the intermittent bombardment of the slope continues by 



