365 



Equisetum arvense. 



Typha latifolia. 



Utricularia minor. 



Lobelia Canbyi. 



Cardamine bnlbosa. 



Seirpus americanus. 



Geum rivale. 



Aspidium thelypteris. 

 The birch-alder association occupies the smallest area of any of the 

 associations, since it forms merely a narrow fringe between the areas of 

 quaking mat and the areas occupied by the arbor vitae association. Some 

 of the same plants are found intermingled with the trees and others on the 

 mat. The tendency is for these bordering shrubs gradually to close in upon 

 the mat areas they enclose until the mat is covered. The shrubs may gain 

 a foothold upon higher points in the mat association from which they spread 

 outward. The principal plants of the birch-alder association are as follows: 



Potentilla fruticosa. 



Aldus incana. 



Betula pumila. 



Hypericum prolincum. 



Salix cordata. 



Physocarpus opulifolius. 



Cephalanthus occidentalis. 



Steironema quadrifolia. 



Silphium terebinthinaceum. 



Ulmaria rubra. 



Phlox glaberrima. 

 By far the largest part of the bog is occupied by the arbor vitae associa- 

 ciation. The association is noticeable from a distance, on account of the 

 presence of these trees of arbor vitae, or white cedar, which gave the bog 

 its name. Trees two feet in diameter are common. A stump, oblong in 

 cross-section, was found to be twenty feet in circumference and five bj r eight 

 feet in diameter. The stump was hollow, so that its age could not be de- 

 termined, but the outer six inches showed about one hundred growth rings, 

 so the tree must have been several hundred years old. Under natural 

 conditions, this association would probably persist for a very long time, 

 as invasion from without goes on very sloAvly. The Thuyas have very com- 



