62 A Study of the Vegetation of 
Its shrubby growth-form, numerous, erect, fascicled, and dense, 
white-woolly branches and white-woolly leaves, combined with a 
greater height-growth than most of its associates, make it a con- 
spicuous plant in the landscape. The non-woolly variety, Chryso- 
thamnus nauseosus graveolens, not less conspicuous, is also very 
abundant. Both plants extend well up the warm valleys and 
canyons but are practically absent from the high prairies eastward. 
Tetradymia canescens, with a growth form similar to the pre- 
ceding half-shrubs, likewise lends a decidedly xerophytic tone to 
.the dry scab-lands. It reaches its eastward extension in this 
region. 
A plant which forms a rather distinct consocies is Elymus 
condensatus. On moist flood-plains this grass reaches a height 
of 3-5 feet. In such situations the stout stems are densely 
tufted and areas of many acres may be dominated by this species. 
In deep soils on the dry hillsides it is not infrequent, but here it 
usually occurs in more or less isolated clumps. 
In the eastward extension of the Agropyron community along 
the rim-rock of the streams into higher altitudes, many of the 
species being confined to the warmer valleys, drop out. In such 
situations Balsamorhiza sagittata is often abundant between the 
bunches of Agropyron. 
In conclusion it may be said that the plants of this consocies 
live typically not only in a climate of lower rainfall but also one in 
which the period of drought, owing to the warmer climate with a 
growing season several weeks earlier, is correspondingly increased 
as compared with that of the well developed prairies of the 
higher altitudes. 
In the preceding pages I have treated the Festuca and Agro- 
pyron communities as climax units of vegetation. Without ques- 
tion, favorable changes of climatic conditions, such as a slight in- 
crease in rainfall, would cause an extension of the Festuca com- 
munity into the area now occupied by the Agropyron consocia- 
tion. The latter in turn would extend its area downward and 
westward into the desert scrub formation. Indeed, such move- 
ments of plant populations are easily traced in the new areas of 
rim-rock and scab-land where they are brought about as a result 
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