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1906] SHANTZ—VEGETATION OF THE MESA 37 
Societies. 
Artemisia jrigida society—This is by far the most important 
society of the region. It occurs at the heads of the gullies and on 
the more depressed places of the north Mesa. It is also important 
on alluvial fans and areas where there has been a secondary suc- 
cession. Where this species is abundant, as a rule it shuts out all 
other plants with the exception of the taller species, such as Argemone 
intermedia and Stipa Vaseyi. This society can be distinguished 
for miles because of the silvery appearance of the plant, which is 
so widely distributed over the entire Mesa that it would be considered 
a facies of the formation if it were not for the fact that more extended 
study shows it to be local in its distribution. 
Gutierrezia Sarothrae society—This composite is rather evenly 
distributed over the Mesa region, but it can never be said to replace 
the grasses which are characteristic of the formation. It does dom- 
inate rather large areas, however, particularly in the southern part 
of the Mesa, where it is found associated with many of the character- 
istic plants of the Mesa top. It does not occur so commonly in 
the purer gravel soil as in the clay and gravelly loam. 
Chrysopsis villosa society.—This society is of considerable impor- 
tance on the north portion of the Mesa, where it occurs in the Bou- 
teloua hirsuta consocies. 
The Aristida longiseta society is not extensive, but dominates 
south and east crests and slopes. The Psoralea tenuiflora society 
is extensive, reaching its best development on the hillsides. 
The rainy vernal period favors the development of a number of 
annuals which come into bloom at this time. The most important 
of these is Boebera papposa, which has a very even distribution 
throughout the region studied. It occurs as a ruderal plant, usually 
from 4-10°™ high and bearing very often only one head. In point of 
numbers it probably exceeds all but the facies of the formation. 
However, it succeeds best as a ruderal, and in the formation the small 
plants may be as numerous as 996 per quadrat and still not be espe- 
cially noticeable. Wherever there are open spaces in the formation, 
this society is found. 
Plantago Purshii also occurs as an important annual in the forma- 
