Vegetation of Northern Cape Breton. 449 



other possible lines of development will be pointed out. Let it be 

 assumed, as is very commonly the case, that the low spots have 

 become colonized by Sphagnum tenelhtm and 6". Pylaisei, 

 species which lack the cushion-forming habit. Under these 

 circumstances the formation of peat and the building up of 

 the substratum may take place very slowly, being due very largely 

 to the accumulation of sedge remains. But, even at that, it 

 takes place much more rapidly in these lower areas than on the 

 higher ones. As the layer of peat in these lower areas becomes 

 gradually thicker and the ground level is raised higher, the sur- 

 face vegetation spreads out laterally and may override the higher 

 areas ; and in this way there may originate what has been 

 described above as a bog meadow. 



Further advance beyond the bog meadow stage of the succes- 

 sion is dependent primarily, either directly or indirectly, on the 

 activity of various cushion-forming species of Sphagnum. 

 Wherever conditions are congenial to the growth and spread of 

 the mesophytic cushion-forming species (S. papillosum, S. 

 magellanicum, S. pulchrum), bog meadow may gradually give 

 way to wet bog. Indeed these species may have been the 

 important ones from the very outset, so much so that the bog 

 meadow stage in the succession may never have been developed. 

 The factors which condition the presence or absence and the 

 relative abundance when present of these species of Sphagnum 

 doubtless have to do very largely with the amount of water 

 available throughout the season, but it seems likely also, as 

 suggested elsewhere, that the difficulty with which these and 

 other species are able to invade areas already occupied by 5". 

 Pylaisei in particular may be a factor of considerable importance 

 as affecting their establishment on the surface of a bog meadow. 



The transformation in the character of thfe habitat accomplished 

 through the agency of the mesophytic cushion-forming sphag- 

 nums and the manner in which they bring about the elimination 

 of bog meadow or any other type of vegetation which may be 

 present is exceedingly interesting. Heretofore, in the case of 

 bog meadow, what water has not been absorbed by the compact, 

 peaty substratum has been able to run off quite unobstructed 

 over the comparatively smooth, firm surface, with the result that 

 except during wet periods the ground at the surface may have 

 been relatively dry. One of the essential characteristics of the 



