12 BULLETIN 791, U. S. DEPARTMENT OF AGRICULTURE. 
manently established and in vigorous condition (fig. 4). True 
bunch grasses, on the other hand, reproduce entirety from seed, two 
to three years being required to establish fully a seedling plant. 
Thus, while a shoot originating from the rootstock of a turf- forming 
species is largely nourished through the medium of the deep-rooted 
parent plant, the establishment of a bunch-grass seedling is dependent 
upon its own development for moisture and nutriment. Accordingly, 
turfed wheat grasses gain dominion over the soil in the drier situations 
where their rate of occupation may from time to time be more or less 
seriously interrupted through drought, but where the well-established 
plants are seldom killed. In such habitats the bunch grasses are 
usually killed out in the seedling stage, or the established plants, in 
competition for water, are crowded out by the shallow-rooted turfed 
species. On the other hand, habitats which receive considerable pre- 
cipitation and are characterized by soils which permit of ready per- 
colation of water are capable of supporting the deep-rooted plants. 
Such habitats are seldom if ever congenial to the domination, or in- 
deed the conspicuous presence, of the turfed wheat grasses. 
SOIL WATER CONTEXT. 
A comparison of the soil moisture conditions on a typical turfed 
(small wheat grass) area and on a typical bunch grass (blue bunch 
wheat grass) area in close proximity to each other may be made by 
observing the graphs in figure 5. 
Section A of the graph, representing the moisture conditions of the 
soil supporting a typical stand of small wheat grass during the grow- 
ing season of 1915, shows a rather sharp decline in the moisture con- 
tent in the three soil strata studied (0-6, 6-12, and 12-24 inch depths) 
from July 1, which marks the beginning of vigorous spring growth, 
to September 20, the end of the growing period. On July 1 the high- 
est per cent of moisture was recorded in the 0-6 inch layer of soil, 
11.8 per cent of the water content being available for the use of the 
plant. In the 6-12 inch layer of soil for the same period there was 
approximately 1 per cent less moisture than in the upper layer, while 
in the 12-24 inch depth there was 4 per cent less. During the second 
period, however, there was a sharp decline in the moisture of the 
surface layer, and during the third period a rather striking increase. 
In the 6-12 and 12-24 inch depths for the same periods the decline 
was gradual, which is typical of all subsequent periods at the two 
lower depths. 
The most significant facts brought out in section A, however, are 
(1) the rather striking fluctuations in the water content in the 0-6 
inch layer, and (2) the fact that the water content in the 0-6 inch 
depth of soil is reduced to a point at which it becomes unavailable 
