362 George E. Nichols, 



The sphagnunis in relation to the formation of floating mats. — 

 The relatively subordinate role played by the sphagnums in 

 initiating the formation of floating mats was suggested by 

 Ganong ('03, pp. 440-441) and Transeau ('o5-'o6, p. 363), and 

 has been emphasized by Davis ('07), Cooper ('13) and others. 

 From these and the writer's observations it seems certain that 

 in general the appearance of the sphagnum is subsequent rather 

 than antecedent with reference to that of the vascular plants. 

 The inability of the sphagnums of themselves to form a mat 

 may be attributed largely to the lack of coherence and buoyancy 

 in the mass of floating vegetation which they sometimes form. 

 But added to this is the fact that comparatively few species of 

 sphagnum are capable of flourishing with their foliage completely 

 submerged. Of course there are certain sphagnums which are dis- 

 tinctly aquatic in their mode of growth, but among the twenty 

 species which have been recorded from Cape Breton, only 

 two definitely belong in this category, namely, S. Pylaisei and 

 S. cxispidatiim (see in this connection the ecological classifica- 

 tion of bog sphagnums on p. 422). In many mountain ponds these 

 two species grow in great profusion, floating at or just below the 

 surface of the water, and their ecological relations there will be 

 discussed in some detail later (see especially pp. 424, 429), 

 Neither of these two species, however, occurs in any abundance 

 along the coast : in fact, the writer has never seen S. Pylaisei 

 except in the mountains, while S. cuspidatum, though frequently 

 represented in lowland ponds by the var. Torreyi, is seldom of 

 ecological importance here. The important mat pioneers among 

 the sphagnums in the Barrasois bogs, which may be regarded as 

 representative of lowland bogs in general, are 5". papillosum, S. 

 magellanicum, and 5". recurvum. These three species grow best 

 in very wet situations, but they will flourish only where the 

 nature of the substratum is such that, at least throughout most 

 of the growing season, their shoots remain partially raised above 

 water level. The maintenance of this position they are not suffi- 

 ciently buoyant to accomplish themselves, so that the pre- 

 existence of some sort of a support to prevent their sinking 

 below the surface is essential. Hence the importance of shrubs 

 and sedges as pioneers in the development of a "sphagnum mat." 



The marginal ditch and its significance. — The formerly water- 

 filled depressions now occupied by the Barrasois bogs have 



