58 SENSITIVENESS OF THE ROOT 



root cap, are sensitive to the stimulus of gravity. If this region 

 of the root is carefully removed with a very sharp razor the 

 root is no longer capable of responding to gravity although it 

 may curve in various directions owing to the irritation pro- 

 duced by cutting. Furthermore, while the tip alone perceives the 

 stimulus of gravity, the curvature occurs two or three mm. back 

 of the tip, i. e., in the region of rapid elongation, so that we have 

 the transmission of the impulse somewhat after the manner of 

 our own nervous system. To be sure there are no specially con- 

 structed cells for conveying this impulse comparable to the nerves, 

 but it has been shown that chemical changes are set up that ex- 

 tend from the tip to the region of curvature where a more ex- 

 tended growth of the cells on the upper side of the root is induced 

 than on the lower side. This results in bending the root down into 

 the soil. These reactions apply to the main or tap root. The 

 lateral branches, or secondary roots, tend to grow more or less at 

 right angles to the stimulus of gravity, while the short tertiary 

 branches radiate out in all directions. Why the stimulus of 

 gravity causes a more extended growth on one side of the tap 

 root than on the other or why the lateral roots are so differ- 

 ently affected can not yet be answered, but the nicety of these 

 reactions in enabling the roots to reach in all directions and 

 thoroughly explore the soil in their quest for food, is very ap- 

 parent. The cells of the root tip are also sensitive to touch or 

 contact. This enables the root to avoid obstacles and grow 

 around rocks and stones. In this case the irritation of the root 

 tip produces a more considerable growth in the zone of curva- 

 ture on the side where the root is irritated with the result that 

 the tip of the root is bent from the object. In this way roots 

 work their way through the soil and avoid obstacles and grow 

 around rocks, for as soon as the irritation of the obstruction is 

 removed by the curvature, gravity will cause the root to grow 

 down again. This sensitiveness to touch is very well shown by 

 fastening with mucilage a bit of sand paper to the tip of the root 

 when the irritation will cause such an excessive growth in the 

 cells of the curving zone as to sometimes cause the tip to move 

 through a complete loop. 



