10 
BULLETIN 791, U. S. DEPARTMENT OE AGRICULTURE. 
has become well established the percolation of moisture is exceedingly 
slow; and, except after prolonged and heavy rainstorms, a 
surprisingly small proportion of the moisture passes beyond the 
densely matted soil stratum. Accordingly, the difficulty which other 
species encounter in gaining a foothold and their practical failure to 
compete successfully with the grass for the moisture essential to their 
proper development and perpetuation account chiefly for the char- 
acteristically pure stand of small wheat grass where its development 
is undisturbed. 
The roots of the bunched species, slender wheat grass and blue 
bunch wheat grass (fig. 2), extend approximately 3^ times as deep 
into the soil as those of the small 
wheat grass, the average maximum 
depth being about -±0 inches. Hence 
a large proportion of the root- 
absorbing surface of the bunch 
grasses is well below the average 
maximum depth of that of the 
turfed species. There is no appre- 
ciable difference in the root charac- 
teristics of the two bunch grasses 
under consideration. The distance 
between the bunches varies from a 
few inches to several feet, dej)end- 
ing upon the moisture and other 
physical conditions. However, re- 
gardless of the distance between 
the bunches, provided the type is 
fully developed, there is relatively 
little difference in the character, 
density, and luxuriance of the other species which inhabit the inter- 
vening space, the normal stand of which is usually sparse. 
CONDITIONS OF GROWTH AND REPRODUCTION. 
While the wheat grasses thrive under a considerable range of con- 
ditions, their optimum development is reached only where the soil is 
reasonably well decomposed and in a fairly high state of productivity 
and where sufficient moisture is available to supply vigorous plants 
during the first half of the growing season. 
In the area under observation there is usually ample precipita- 
tion early in the spring of the year to saturate the soil (see Table 1 
and fig. 3). Occasionally, however, the rainfall in June and in 
the first half of July is so light that the soil contains insufficient 
moisture for the promotion of vigorous growth. 1 
2.20 
2.00 
/.eo 
<0 
1 PC) 
*J 
■^ 
^0 
**- 
/AO 
• ^ 
/.20 
S 
/.00 
CI 
.60 
V. 
5. 
so 
CI 
?! 
.40 
20 
June 
July 
AuQ. 
Sept. 
Fig. 3. — Average monthly precipitation 
in the wheat-grass formation during 
the growing season, 1914-1917, in- 
clusive. 
P 
1 Growth in the subalpine zone begins about the last week of June. 
