472 BOTANICAL GAZETTE [DECEMBER 
streams, where the oak timber gradually shades into beech and 
maples.” On the other hand, “evergreen’trees, whether conifer- 
ous or broad-leaved, seem to be just as characteristic of poor soil 
as of any particular kind of climate” (14). Bowman (3) draws 
attention to the limitations of soil composition as a determining 
factor. Cow es has shown that the composition of the rock 
from which any soil may be derived seldom acts in a limiting 
capacity with respect to the species which that soil may support. 
It is only in exceptional cases that a soil, newly weathered, is defi- 
cient in the mineral constituents necessary for plant growth. This 
generalization is particularly applicable in Ontario, where the soil, 
whether it be glacial drift toward the south, or the weathered 
deposits and exposed rocks farther north, is derived from the 
dominantly granitic rock of the Laurentian Plateau. The original 
composition of the soil is seldom a limiting factor, at least in so far 
as the forests of Ontario are concerned. 
Humus factor 
It is scarcely necessary to emphasize the importance of the 
humus content of the soil as an ecological factor; its significance as 
a limiting factor with respect to the forests of Ontario is our chief 
concern. In forest regions the humus content of the soil increases 
the water retaining capacity; increases the porosity, and hence 
the aeration of the soil. Mineral salts are retained by the adsorp- 
tive properties of humus, and incidentally, conditions are made 
more favorable for soil bacteria, which are essential for the growth 
of such species as Fagus. Cows (8) states, “Although bare sand 
supports a.xerophytic flora, the accumulation of a thin humus layer 
is sufficient for forest development, and the Michigan dunes show 
that the most mesophytic of our forests can grow on a sand dune if 
there is present a-humus layer a few centimeters in thickness.” 
In the Algonquin Park region to which reference has been made, 
the Acer or Acer-Fagus forest occupies the ridges, while the Abies- 
Picea forest occupies the lower slopes and lowlands. On the slopes 
where the exposure of the rocks, due to drainage of glacial lakes, 
has been comparatively recent, only a small amount of rock soil 
has accumulated; this is covered by a humus layer, but the two 
are not intimately intermingled by weathering processes. The 
