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In the present discussion the formations are taken up by 
geologic and physiographic provinces, because they seem not to 
be distinguished so much upon floristic as upon ecologic grounds. 
Temperature conditions will be seen to paly a réle as between a 
province in the extreme south and one in the extreme north, 
but even then chiefly in the floristic content of the annuals and 
lignescent perennials of the prairie formations. Again, the 
breadth of area from east to west, following the lines of 
decrease in rainfall, gives the grass formations of the eastern 
provinces a physiognomy differing from that of the extreme 
western. Finally, as previously mentioned, since edaphic fac- 
tors exert such a marked control upon formations, the consider- 
ation of the grass formations by provinces will give the full force 
of these factors. 
A preliminary word may be said in reference to the grass 
formation of the region asa whole. It was stated above that 
the climate of the region is a grass plains climate, and that the 
grasses form the matrix of the vegetation, this being true even 
where they are not the dominant element. Taking the region 
as a whole, there is a wide range of climatic (hydrometeoric) 
conditions between the east and west boundaries. But even in 
the province of greatest rainfall, climatic conditions, together 
with geologic and physiographic conditions, result in a decidedly 
xerophytic vegetation, The grass vegetation is the chief expo- 
nent of the xerophytic conditions, and certain ecologic types of 
grasses are found through the entire region, as Bulbilis dactyloides, 
the specifically designated ‘buffalo grass.” Throughout the 
entire region, also, the dryness of air and brilliancy of sunshine 
cause adaptations to rapid transitions from active growth to 
dormant conditions, great quantities of nutritive materials being 
stored in the dormant parts, 
THE RIO GRANDE PLAIN. 
PHYSIOGRAPHY AND GEOLOGy.—The area here included in the 
Rio Grande plain is really the continuation of the Atlantic coast 
plain west of the 98th meridian. It is in general a triangle, 
