54 SENSITIVENESS OF THE ROOT 



whereas in the spring before the leaves appear they become filled 

 with fluid. 



26. The Sensitiveness of the Root. The question may now be 

 asked how does this elaborate root mechanism come into contact 

 with the soil. If the root of a pea or bean seedling is placed 

 horizontally 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 

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

 stop to consider how it gains its 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, i. e., in the for- 

 mative region, 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 extend 

 from the tip to the region of curvature where a more extended 

 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 



