2038 BOTANICAL GAZETTE | SEPTEMBER 
cases making alkaline flats of varying degrees of saltiness. The 
higher, less alkaline slopes of these bolsons are often covered 
with grama grass formation characteristic of the region. The 
alkaline flats, of course, possess a halophytic vegetation, of 
which salt grasses form a prominent part. 
On the whole, the grass vegetation of the mountain and 
south plateau provinces falls under three types of formation: (1) 
the close grass floor formation; (2) the open bunch grass forma- 
tion; and (3) the salt grass formation. Of these the third will 
be discussed under the special head of ‘halophytic formations.” 
The open bunch grass formation is floristically equivalent to 
the close grass floor formation. The open and bunched charac- 
ter of the formation is due to extreme dryness, and especially to 
the instability of the loose finer igneous débris, or the coarser 
gravelly or stony nature of the soils, 
The close grass floor formation may be designated “‘grama 
grass formation,” from the popular name of the one to several 
prevailing species. In this formation, the great majority of 
species making up the formations of the eastern provinces have 
been sifted out by increasingly arid conditions, and only those 
remain which mark the extreme limit of xerophytic adaptation 
in grasses. A summary of the adaptation features of this type 
of formation shows: (1) rapid transitions from active to dor- 
mant conditions ; (2) great resistance to extreme dryness and 
heat (including fires) while in the dormant stage ; (3) equally 
great recuperative power after extreme treatment, including 
apparent extermination from a given area; (4) large food stor- 
age of fats and sugars in portions which retain vitality during 
dormant periods, thus rendering quick growth possible ; (5) the 
quality of perfect drying zm situ, thus not only covering the soil 
and holding it in place, but also protecting the vital parts. 
PURE FORMATIONS OF PRAIRIE ANNUALS IN THE PRAIRIE PLAINS. 
In the preceding discussion of grass formations, the herba- 
ceous annuals of the prairies have been treated simply as acces- 
sory elements in the formations, but of value in characterizing 
