470 BOTANICAL GAZETTE (DECEMBER 
Thuja occidentalis, which extends northward far beyond Acer 
saccharum, does not reach the western limits of the latter, while 
Quercus rubra extends westward beyond the limits of many species 
which are to be found beyond its northern limit. Such phenomena 
doubtless are the result of the fact that different limiting factors 
dominate the several regions forming the boundaries of distribu- 
tion. 
In southeastern Canada it is difficult to find any parallelism 
between the limits of tree species and the boundaries of precipita- 
tion areas. For instance, Juglans cinerea extends throughout areas 
whose respective yearly precipitations are 30-35 in., 35-40 in., 
30-35 in., 40-45 in., and 45-50 in., with no apparent hctiencbiche of 
the lines bounding its growth area. The limits of Fagus americana 
pass through similar areas, with the addition of an area where the 
minimum yearly precipitation is 25 in. This range is of par- 
ticular significance in consideration of the fact that Fagus is gener- 
ally regarded as mesophytic. The absence of a parallelism between 
precipitation and forest limits in the Ontario section is so obvious 
from a consideration of the accompanying map (fig. 2) that further 
emphasis would be superfluous. Other conditions being favorable, 
there is sufficient rainfall for forest growth; or, in other words, 
precipitation does not enter here as a limiting factor. 
WARMING (24) has based his ecological classification of plants 
upon the premise that ‘‘the most potent and decisive factor is the 
amount of water in the soil.” ‘‘ The soil upon which the coniferous 
forest occurs varies widely, yet so far as reliable information is 
available it is always physically or physiologically dry.” “The 
cold winter is a physiologically dry season against which trees can 
protect themselves by defoliation or by xerophytic structure.” 
This may account for the xerophytic structure of the coniferous 
leaf; it is difficult to understand what bearing it has in connection 
with the contention that conifers as a class grow in dry soil condi- 
tions, the winter, even for conifers, being a period of comparative 
dormancy. Deciduous trees are protected against the ‘physio- 
logical dryness” of winter by leaf fall; conifers by having leaves of 
xerophytic structure. Some deciduous trees, for example Quercus, 
are comparatively xerophytic, while others are decidedly meso- 
