Roots and Their Environment 183 



gravity, but they tend to grow at right angles to its direc- 

 tion rather than directly toward it, as do primary roots. 

 Secondary roots are transversely geotropic. 



The response of roots to light. Roots tend to grow away 

 from strong Ught. They are negatively phototropic. This 

 tendency may be observed in the aerial roots of ivies and 

 other climbers. No matter from what surface on the stem 

 they arise, they curve around away from the hght. This is 

 advantageous to a climbing plant, for it brings thfe root to 

 the supporting tree or wall. 



The response of roots to water. Particles of water, in dif- 

 fusing from wet places in the soil to places that are drier, also 

 affect the direction of root growth. A root in drier soil will 

 turn toward the direction from which water particles strike 

 it. Continued growth will then bring the root into the 

 moister soil. This response of a root is of great advantage 

 to the plant. It is sometimes stated that roots " seek " the 

 water. In reahty, roots grow toward moist places only when 

 water diffusing from the moist places reaches them and so 

 controls the direction of growth. They do not seek the water, 

 but they are turned to the moist soil by the water itself. 



The distribution of roots in the soil. Another factor that 

 determines the distribution of roots in soil is the oxygen sup- 

 ply. Various plants have different requirements, but all roots 

 doubtless require oxygen for growth. Those who have seen 

 stumps pulled from the land know that the roots go deep in 

 upland sandy soils ; that they do not go so deep in heavy clay 

 soils; and that they are just beneath the surface in swamp 

 and bog land. The principal reason why one finds the roots 

 near the surface in swamps is that these roots were the only 

 ones that continued to Hve and grow. The roots that in times 



