365 
Equisetum arvense. 
Typha latifolia. 
Utricularia minor. 
Lobelia Canbyi. 
Cardamine bulbosa. 
Scirpus 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 ineana. 
Betula pumila. 
Hypericum prolificum. 
Salix cordata. 
Physoearpus 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 by 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 slowly. The Thuyas have very com- 
