STRUCTURE AND GROWTH OF ROOTS 133 



When the tendency is to turn toward that which exerts 

 the influence causing the turn, the tropism is said to be 

 positive. When the tendency is to turn away from that 

 which exerts the influence, the tropism is said to be nega- 

 tive. Thus a tropism toward Hght is positive phototro- 

 pism ; a tropism away from light is negative phototropism. 

 Leaves, unless the hght be too intense, are positively pho- 

 totropic, while roots are negatively phototropic. Roots 

 tend to turn away from light. 



Early in this book it was said that " stems grow up and 

 roots grow down." We may now express this by saying 

 that roots are positively geotropic, while stems are nega- 

 tively geotropic. (Geo comes from a word meaning the 

 earth. Recall geography.) 



You remember that plant organs, especially roots, ap- 

 pear to be modified to suit their surroundings. You re- 

 member that we called these modifications responses. 

 Tropisms evidently are one kind of responses. Thus the 

 geotropisms of which we have just been speaking are the 

 responses which the plant shows to gravity, to the pull of 

 the earth's mass. 



That which causes a response is called a stimulus. The 

 tendency of the root to grow down appears to be a response 

 to the stimulus of gravity, for, if we remove the effects 

 of gravity, roots continue to grow in whatever direction 

 they may have started. To observe the result of remov- 

 ing the effect of gravity, sprouting seeds may be pinned 

 to an upright revolving disk, being kept moist by drip- 

 ping water. As the disk rotates, each seedling constantly 

 changes its position with reference to gravity. It occupies 

 no one position long enough for an effect to be prodiiced. 

 Thus the effect of gravity is practically removed, and the 



