NATURE OF PLANTS 57 



whereas in the spring before the leaves appear they become filled 

 with fluid. 



26. The Sensitiveness of the Root. — We may now ask how 

 does this elaborate root mechanism become so perfectly adjusted 

 to the soil. If the root of a pea or bean seedling is placed hori- 

 zontally in sawdust, after one or two hours it will begin to 

 curve down toward the earth center. No matter in what position 

 it is placed the result is always the same. We are so familiar 



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

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



with the downward growth of roots into the soil that we never 

 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- 

 duce an irritation through their unusual position. Only the cells 

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



