184 Science of Plant Life 



of drought penetrated to greater depths were killed off 

 suffocated when the water stood at higher levels. The 

 distribution of roots in the soil, therefore, is determined prin- 

 cipally by the combined influences of gravity, water, and 

 oxygen. Water and gravity control the direction of growth, 

 and the oxygen supply determines whether or not growth can 

 take place or the roots survive. 



In the plains of eastern Kansas, the roots of plants may 

 penetrate certain soils to depths of from 15 to 20 feet. The 

 absorbing parts of these roots actually reach the water table ; 

 that is, they reach the level at which the soil is saturated, 

 or the level to which water would rise in a well. 



Two or more species of plants are sometimes found associ- 

 ated in dry regions, and locally in dry habitats, because their 

 roots get their water at different levels and hence do not 

 compete with each other. For example, in our Southern 

 deserts the giant cactus commonly grows with the creosote 

 bush. The former plant obtains its water from the super- 

 ficial layers of the soil, while the latter obtains its water at 

 deeper levels. The roots of lawn grass are very superficial, 

 and lawn grass suffers from drought much sooner than do 

 the deeper-rooted dandelion and English plantain that occur 

 with it as weeds. 



In dry regions where plants compete with one another, suc- 

 cess comes mostly to those that secure a sufficient water 

 supply. In moist regions success in competition between 

 plants depends chiefly on ability to reach the light or with- 

 stand shade. 



The pressure of growth. The pressure exerted by roots in 

 penetrating the soil may be very great, amounting to hun- 

 dreds of pounds to the square inch. This is readily appreci- 



