490 BOTANICAL GAZETTE [DECEMBER 
TSUGA CANADENSIS.—This species is among conifers what Acer 
and Fagus are among deciduous trees; it isa climax form. In fact, 
its ranges of tolerance are almost identical with those of the decidu- 
ous forms already mentioned. TJ. canadensis, when contrasted 
with such species as are represented by Pinus Banksiana, serves 
to emphasize the ecological diversity of conifers. BELL (2) states 
that ‘‘this tree maintains a good size to the verge of its range and 
always appears to terminate abruptly.”’ Stands of mature trees 
are to be found as “‘outliers”’ beyond the general area of its dis- 
tribution. This evidence confirms the belief that Tsuga is still 
migrating; that in many instances it has been limited by the time 
factor rather than by environmental factors. 
PoPULUS BALSAMIFERA AND P. TREMULOIDES.—Although P. 
balsamifera generally extends farther north than P. tremuloides, 
having a greater temperature reins of tolerance, its northern limit 
passes south of the latter at 71° W. long.; soil becomes the limiting 
factor in this region. P. balsamifera “appears to confine itself to 
heavy clay soil of the river valleys on the modified drift of the 
Cambrian areas’”’ (Low 18), while ‘“‘P. tremuloides is most plenti- 
ful on the unmodified glacial till of the drift ridges.’’ The seedlings 
of both require a high degree of light intensity, and as such are 
pioneer forms. Southward they occur only where fire and other 
destructive agencies have restored pioneer conditions. The 
abundance of these species of Populus northward, especially south 
and west from Hudson Bay, would indicate that this region is 
biologically young. 
ACER SACCHARUM.—South of the Laurentian Plateau A. sac- 
charum dominates, except in the undrained lowlands. Its range 
of tolerance is limited to a mature soil (that is, well drained, well 
aerated, and containing a relatively large amount of humus inti- 
mately mixed with the rock soil) and low light intensity. The 
humidity of the atmosphere in Ontario is such that it is doubtful 
whether it ever acts as a limiting factor, other conditions being 
favorable. It is evident that the distribution of Acer is chiefly an 
expression of the time factor; the time required to give rise to 4 
deep, well drained humus soil and to shade conditions, and in addi- 
tion the time which is necessary to crowd out those forms which 
