48 



BTJLLETIlSr 791, U. S. DEPARTMEISTT OF AGPJCULTURE. 



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 , 

 ( Androsace difTusa l 



^.„ JVl.onolepis. 



I Mono'/e pis nut tall /an us J 



"Tarweed 

 / l/ladia gi omerata f 



Fig. 19. — Plants characteristic of the first-weed stage. 



Table 5. — Comparative depth of roots of typical ruderal-wccd species and of 



the chief wheat grasses. 



Plant. 



Depth of roots in inches. 



Maximum. Minimum. Optimum. 



Ruderal-wecd species: 



Goosefoot 



Slender-leaved collomia. 



Tarweed 



Tolmie's orthocarpus. . . 



Knotweed 



Douglas knotweed 



Tansy mustard 



Average 



Wheat grasses: 



Small wheat grass 



Blue bimch wheat grass 

 Slender wheat grass 



JLverage 



7 



3 



5.5 



9 



3.5 



6 



10 



4 



7 



8 



3 



5 



11 



4 



7 



10 



4 



7 



9 



3 



6 



9.1 



3.5 



6.2 



15 



6 



8 



40 



26 



32 



40 



26 



32 



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 



