466 BOTANICAL GAZETTE [DECEMBER 
has been able to observe, the records of the explorers mentioned 
have been even more accurate than has generally been conceded. 
Although the specific limits of forest species have been rather 
definitely outlined, there seems to be no agreement regarding the 
part played in determining these limits by the various factors affect- 
ing forest growth. In this paper an attempt has been made to 
relate the limiting factors to the specific range of tolerance of forest 
trees, and in this way to account for the respective distributions 
of some of the species dominating the forests of Ontario. 
SCHIMPER (21), as a result of his extensive studies in plant geog- 
raphy, concludes that ‘‘the differentiation of the earth’s vegetation 
is thus controlled by 3 factors: heat, atmospheric precipitation 
(including winds), soil. Heat determines the flora, climatic humid- 
ity the vegetation; the soil as a rule merely picks out and blends 
the materials supplied by these two climatic factors, and on its 
own account adds a few details.” 
Investigators have mentioned many factors which affect the 
composition of forests. Drawing his conclusions from the explora- 
tion of Labrador, Low (18) says “the distribution of forest areas 
and the range of the various trees depend upon several factors, 
among which may be mentioned position as regards latitude, height 
above the sea coast, and the character of the soil.” BOwMAN (3 
in the light of his physiographic studies says as follows: 
The distribution of forests is controlled largely by rainfall, although the 
distribution of species within each region is also controlled by insolation, 
temperature, wind velocity, water supply, and geographic relation to post- 
glacial centers of distribution. When more detailed statements are attempted 
many difficulties are encountered in the form of apparent inconsistencies. 
. Some species appear to find their appropriate conditions in different latitudes 
by a change in their habitat; for example, the larch, balsam fir, and white 
birch which in the north grow freely on dry or hilly ground, toward the southern 
limits seek the cold ground in swamps. The white cedar and white pine in 
some places manifest the same tendency. 
FROTHINGHAM (12) in his report on hardwood forests sums UP 
the situation as follows: “How moisture and temperature affect 
the different species in the complexity of forest environmen is still 
so little known that no positive information can be given.” 
