1922] WATERMAN—PLANT COMMUNITIES 19 
localities. Betula alba, Populus grandidentata, and P. tremuloides 
are fairly common all through the region. As has been stated, the 
region has been burned probably more than once, although for- 
tunately not in recent years. The extent of area covered by any 
one burning is uncertain. Certain local differences in distribution 
can best be explained on the assumption that the burnings have 
not been very complete; certainly some patches which bear old pines 
and hemlocks must have escaped (fig. 8), but patches of white 
birch and poplars indicate a secondary succession after fires. 
-—Luxuriant growth of Pteris agquilina in mesophytic habitat, second 
Aes hirch and poplar in background. 
The undergrowth varies from the modified pioneer type of the 
ridges nearest the lake to a mesophytic type containing many forms 
belonging to the climax beech-maple-hemlock forest. These meso- 
phytic associations are not distributed in accordance with the geo- 
logic age of the ridges, but are determined rather by edaphic 
conditions. They will be considered in the section on the beech- 
maple-hemlock invasion. In addition to the typical mixed 
ground cover already described, there are two types of undergrowth 
societies unevenly distributed over the ridges, an almost pure 
Pieris aquilina society and a Vaccinium society including V. penn- 
