Vegetation of Northern Cape Breton. 363 



become almost completely filled in through the activity of vege- 

 tation. All that remains in most cases to remind one of the pond 

 stage in the succession is a moat-like marginal diich or fosse, 

 which averages perhaps ten feet in width and up to two or more 

 feet in depth, which may be open and filled with water or 

 occupied by a wet sphagnous swamp, and which, as a rule, more 

 or less completely encircles the area occupied by the bog proper. 

 The significance of this marginal ditch is not wholly clear. Else- 

 where ('15, pp. 207, 208), the writer has been inclined to uphold 

 the explanation first suggested by Davis ('07, pp. 150, 151), which 

 attempts to correlate it with fluctuations in water level. But 

 conditions here in northern Cape Breton are even better explained 

 by Atkinson's theory ('05, pp. 615, 616) that the formation of 

 the ditch is due to the shade produced by the forest along the 

 shore, which hinders or prevents the growth of the mat-forming 

 plants. In several of the forest-encircled Barrasois bogs the 

 ditch is open along the southern shore, i. e., along the shore where 

 the effect of the shade produced by the fringing forest naturally 

 would be most pronounced, while along the northern, least shaded 

 shore it has become completely filled in by vegetation. This 

 condition obviously cannot be explained by the fluctuation theory ; 

 and for that matter, as already mentioned, there is very little 

 seasonal fluctuation in water level in these basins. 



Development of the edaphic climax association-type. — Beyond 

 the wet bog stage, further development is largely dependent on 

 two species of Sphagnum which have not as yet been mentioned : 

 S. fusciim and 5. capillaceum tenellum, particularly the former. 

 Where the hydrophytic (or relatively mesophytic) pioneer sphag- 

 nums are superseded by these relatively xerophytic forms, the 

 surface of the bog may become built up a foot or more above 

 water level. In a mature bog the sphagnums almost everywhere 

 are the predominant plants underfoot. They cover the ground 

 with a continuous, hummocky, mattress-like carpet, which con- 

 sists for the most part of the russet-green 5. fttscum, interspersed 

 with occasional more or less extensive patches of the reddish 5". 

 capillaceum tenelhim. Commonly growing along with the 

 sphagnums, in the older parts of the bog, are two mosses: 

 Polytrichum commune and P. juniperinum, while in some of the 

 higher, drier areas the sphagnums may have become superseded 

 by cladonias or by such bryophytes as Ptiliditim ciliare and 



