200 General Botany 



In the plains of eastern Kansas, and California, the roots of 

 plants such as alfalfa and sugar beet 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 associated 

 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 Southwestern deserts the giant 

 cactus often grows with the creosote bush. The former plant 

 obtains its water from the superficial layer 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, success 

 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 withstand shade. 



Temperature has a marked effect on root development. Many 

 common annual weeds develop extensive root systems only at 

 temperatures below 68° or 70° F. Poor root development means 

 retarded vegetative growth and small plants. Wheat is an ex- 

 ample of a plant whose root system develops best below 60° F. 

 Its principal absorbing roots are those formed from the first node 

 of the stem. These are formed down near the seed in cool soil. 

 In warm soil the first node may be at, or above, the surface of the 

 soil and the roots are then not advantageously placed in the soil. 



The pressure of growth. In the growth of plant organs hun- 

 dreds, or thousands, of cells are expanding simultaneously through 

 the taking up of water. The pressure which these cells develop 

 is called growth pressure. The pressure exerted by roots in pene- 

 trating the soil may be very great, amounting to hundreds of 



