rgtt] DACHNOWSKI—CRANBERRY ISLAND 23 
cessive seasons. On the other hand, the absence of logs and fallen 
timber in the peat of the sphagnum-cranberry zone points clearly 
to a relatively slow encroachment upon the open water by the 
plants. When inundation took place, only the coarsely fibrous 
and incoherent cranberry-sphagnum mat rose with the water 
level, and its vegetation survived. 
As late as 1830 the bog was an extension from the mainland. 
After the formation of the dike, and the consequent rise of the 
water level, most of the mainland became inundated, leaving the 
bog completely an island. With its surface vegetation of mostly 
northern forms, the island is virtually a water culture on a large 
scale. None of the plants are dependent for any important part 
of their food on the mineral soil below the peat. Cranberry 
Island is, therefore, not to be considered merely as a case of the 
conversion of a forest into a marsh under the influence of an 
increased water content in the soil. The analysis of peat samples 
shows that the vegetation now growing upon the peat substratum 
represents quite fully a continuation of the former boreal flora. 
It presents today a somewhat disjointed distribution, but this has 
come about chiefly through recent repeated disturbances in the 
water level of the lake, through a settling and shrinkage of the 
peat soil, through the slow encroachment of the invading southern 
vegetation, and through the formation in places of a better and 
firmer soil. 
The flora 
For convenience three well marked plant zones may be pointed : 
out, each of which is characterized by communities and groups 
of plants easily differentiated from the others. No attempt has 
been made to give full lists of plants, or to correlate the associa- 
tions and successions mentioned with similar conditions elsewhere. 
Essentially the same order of succession and of arrangement of 
plants as has been described for northern bogs is not, of course, to 
be expected. The species are not always the same in the corre- 
sponding formations, but they are systematically related and 
closely similar in ecological structure. 
A fuller floristic treatment is now in preparation, in which 
many of the features are described in detail. 
