60 A Study of the Vegetation of 
other plants are mat-formers, while a few are more or less 
shrubby in character. 
The bunches of Agropyron are often 1-2 feet apart, perhaps 
only 2-5 occurring in a square meter (Fig. 47). The individual 
bunches or tufts are composed of from 100-350 or more stiff, 
erect stems, reaching a height of 2-3 feet. Several generations 
of dead stems are to be seen in these persistent bunches, the old 
leaves and culms forming a tangle at the bushy base. Conse- 
quently the landscape appears just as characteristic in late sum- 
mer when this dominant is drying out as in the fall when it takes 
on renewed growth, or in early spring when a host of inter- 
stitials for a time cover the ground with a green carpet. Agro- 
pyron is the one grass best adapted for such situations. Its well 
developed roots penetrate the moist crevices in the underlying 
basalt to a depth of 4 or 5 feet. 
Grasses like Festuca ovina ingrata and Koeleria cristata with 
rather short root-systems find this an uncongenial habitat. How- 
ever, a few grasses with very short root-systems and an early 
blooming habit are very successful interstitials. Of. these, Poa 
sandbergu and the low annual, Festuca pacifica, are by far the 
most important. Hundreds of individuals of the latter fre- 
quently occur in a single square meter. Mats of Amntennaria 
dimorpha and dense growths of Plantago purshii are likewise 
common between the scattered bunches of the dominant. These 
interstitials, together with numerous others are quite as common 
in the preceding Poa-Polygonum associes of which this, indeed, 
except for the dominance of Agropyron, might be considered a 
late developmental stage. However, the presence of the rabbit 
brush, Chrysothamnus nauseosus, C. viscidiflorus, Tetradymia 
canescens, Erigeron hispidissimus, and various lupines, espe- 
cially in pockets of deeper soil, indicates the developmental trend 
toward the more typical Agropyron community. 
In the deep soils of the hills bordering the scab-lands the Agro- 
pyron consociation is better developed (Fig. 20). The appear- 
ance of Festuca ovina indicates more favorable life conditions. 
Indeed, the latter sometimes assumes equal importance with 
Agropyron on the more moist slopes. But the larger amount of 
60 
