48 
BULLETIN 791, IT. S. DEPARTMENT OF AGRICULTURE. 
as the ruderal-weed species. Blue bunch wheat grass and slender 
wheat grass, on the other hand, absorb nearly all of the water they 
need from soil far below the deepest penetration of roots of the 
annual plants. 
__,Androsace 
... „.._,. .M.onolepis. . _ 
/ tr'c-c ■■€ ,■: 3 .-c "Si. c nuS } 
Tarweed _ 
[ Maa.'a qc rr>era?a j 
Fig. 19. — Plants characteristic of the first-weed stage. 
Table 5. — Comparative depth of roots of typical ruderal-weed species and of 
the chief wheat grasses. 
Plant. 
Depth of roots in inches. 
Maximum. 
Minimum. 
Optimum. 
Ruderal-weed species: 
9 
10 
8 
11 
10 
9 
3 
3.5 
4 
3 
4 
4 
3 
5.5 
6 
Tarweed 
7 
Tolmie's orthocarpus 
5 
Kno tweed 
Douglas knotweed 
7 
Tansy mustard 
6 
Average 
9.1 
3.5 
6.2 
Wheat grasses: 
Small wheat grass 
15 
40 
40 
6 
26 
26 
8 
Blue bunch wheat crass 
32 
Slender wheat grass 
32 
Average 
31.7 
19.3 24 
The rate of percolation of water is much greater on the ruderal- 
weed lands than on the small wheat-grass areas. This is due not 
only to the dense matlike growth of the roots characteristic of the 
