388 BOTANICAL GAZETTE [JUNE 
appearing in the Indian territory. The sand hill region likewise 
narrows toward the north, but much more rapidly, vanishing in 
the Dakotas, and attaining only occasional expression to the 
northward. The submontane region IV skirts the base of the 
Rocky mountains from Athabasca to New Mexico. 
It is unnecessary to speak at length of the physiography of 
the three regions. The most noticeable topographic feature 
that gives character to the floral covering is soil composition. 
This, with altitude and precipitation, comprises the great factors 
which have brought about the differentiation of the prairies into 
three regions. Thus, while the prairies proper differ in the 
character of their gently undulating surface from the ridged and 
broken sand hills, and from the elevated tablelands of the foot- 
hills with their precipitous cafions, the important fact for the 
floral covering is that the soil of the prairie is a close loam 
instead of a loose sand, as in the sand hills, or an argillaceous 
marl, as in the foothills. Secondary to this only, is the 
decrease in precipitation, and the increase in altitude from the 
eastern boundary of the province to the western. Of equal if 
not greater importance for the flora is the question of environ- 
ment, i. ¢., the derivation of the floral elements. The vegeta- 
tive covering of the foothills is derived primarily from the 
mountains, with the exception of the fundament which it has in 
common with the other regions. That of the prairies proper 
has come in part from the wooded region to the east. The flora 
of regions II and IV is a derived one, and the regions may be 
termed open. The sand hills, while they have drawn somewhat 
from II and IV, are at no point in direct contact with the flora 
of another province. The flora is to a large degree endemic, and 
the region is more or less closed. 
The characteristic formations of the prairie pro 
xerophytic, occasionally poophytic. These are meadow, prairie, 
sand hill and foothill formations. Not infrequently, cer®® 
ruderal formations of xerophytic character acquire great — 
nence, as in the Niobrara district of the sand hill region 
Meadow formations are never xerophytic, but always mesophytic 
vince are 
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