428 George E. Nichols, 



stage. The important sphagnums in this connection are 6". 

 pulchrum and the more mesophytic cushion-forming species, but 

 particularly the latter. As a rule these are present in abundance, 

 and so favorable are the conditions for their growth here in the 

 highlands that they commonly give rise to a thick mat which 

 rises steeply from the water's edge to a height of one or two 

 feet. Myrica Gale, Chamaedaphne, and Andromeda are all 

 important as pioneer shrubs, now one, now another playing the 

 leading role. 



Locally certain sedges are more important as pioneers in mat 

 formation than are the shrubs. Carex limosa frequently extends 

 out into the open water from along the shore, growing in length 

 as much as a foot in a single season, and sometimes it is present 

 in sufficient abundance to form the basis of a mat. More com- 

 monly Carex filiformis is the pioneer sedge. The behavior of 

 this species in mat formation has been described by Ganong 

 ('03, pp. 440-441), Transeau ('o5-'o6, p. 363), Davis ('07, 

 135-138), the writer ('15, pp. 198-199), and others. Commonly 

 the sedges are followed by the sphagnums, which build up the 

 surface in the manner already described. On the whole, sedges 

 are much less prominent in the role of mat pioneers than are 

 shrubs. Moreover, the latter, because of the framework 

 afiforded by their strong, woody stems, favor much more the 

 growth of the cushion-forming sphagnums and the consequent 

 thickening and solidifying of the mat. 



As the mat grows outward into the pond, the open water 

 beneath gradually becomes filled in, partly by the sinking of the 

 mat as it becomes thicker and heavier through the continued 

 upward growth at its surface, partly by the dropping down of 

 vegetable debris from the under surface of the mat. Where the 

 outward growth is rapid, the mat may be underlain for some 

 distance shoreward from its outer margin by open water ; where 

 outward growth is slow the filling in beneath may keep pace 

 with it, so that very little of the mat is actually floating. But 

 in either case, wherever the filling in is being accomplished 

 primarily through the intervention of a floating mat, the water 

 in the pond is deep right up to the edge of the mat. and in cases 

 where the mat has become "grounded" clear to its margin, in 

 the manner indicated above, the bank usually sheers precipitously 

 to the bottom. 



