1905] WHITFORD—FORESTS OF FLATHEAD VALLEY 281 
A summary of the relation of the prairie to the forest in this region 
is as follows: (1) there is less moisture in the prairie soil than in the 
forest soil; (2) this is due primarily to the smaller amount of rainfall; 
(3) in the prairie formation forests may exist in certain topographic 
situations, as along streams and other bodies of water, and on protected 
hill-sides; (4) in the forest formation prairies exist where the charac- 
ter of the soil is such that it will not easily hold water; by the gradual 
addition of humus such soils may be changed sufficiently in their’ 
water-holding capacity to permit more mesophytic conditions and 
in some instances a climax western larch-Douglas spruce combination. 
Objection may be made to the use of the term mesophyte for plants 
with xerophytic leaves. WARMING classifies all conifers as xero- 
phytes because they grow in dry soils. In the eastern United States 
in contrast with the broad-leaved deciduous trees the conifers are 
undoubtedly xerophytes, comparatively speaking. That is, during 
the summer months the deciduous tree requires more moisture than 
the conifers, therefore the deciduous tree is excluded from the dry 
soils. In the northwestern United States the coniferous forests occupy 
the mesophytic soils almost to the entire exclusion of the deciduous 
clement. As shown in the discussion on the climatic formations, 
this is due to a peculiar climate in which the summers are compara- 
tively cool and dry, and the winters comparatively warm and exces- 
sively wet. In such a climate the deciduous tree is lacking, except 
in edaphic situations, on account of the cool dry summers, because 
with its broad transpiring surface it requires more moisture than it 
is able to get. The narrow-leaved sclerophyllous trees, on the other 
hand, while they do not necessarily thrive during the dry summer 
months, because of their reduced transpiring surface they are per- 
mitted to exist, while the broad-leaved deciduous trees cannot. 
During the winter the reverse is the case, for deciduous trees with 
their bare twigs are better able to endure severe conditions than are 
the conifers. On the other hand, if the climate is moist and warm, 
the conifer is able to do a considerable amount of photosynthetic 
work. Thus the deciduous tree requires more moisture during the 
summer months and is consequently more mesophytic at that season; 
on the other hand, the coniferous tree requires more moisture during 
the winter months and consequently is more mesophytic at that 
