62 SENSITIVENESS OF THE ROOT 



stop to consider how they gain their sense of direction. Gravity 

 is the stimulus that acts upon the irritable protoplasm of the cells 

 and so brings about a growth reaction that sends the root in the 

 right direction. When roots are placed in a horizontal position 

 and slowly revolved on their longitudinal axes, no curvature re- 

 sults, since all parts of the cells are stimulated alike; but when 

 allowed to rest a curvature results because possibly heavier par- 

 ticles in the cells fall to the lower sides of the cells and so pro- 



end 



Fig. 37. Cross-section of a root of lupine showing tlie origin of the lateral 

 rootlets. Lettering as in Fig. 36. — H. O. Hanson. 



duce an irritation through their unusual position. The cells in 

 the first millimeter and a half of the root tip, or possibly in the 

 root cap, are sensitive principally to the stimulus of gravity — 

 the other cells much less so. If this region of the root is care- 

 fully 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 produced 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 cori- 



