I08 BOTANY. 



Exercise 40. 



186. Influence of the earth on stems and leaves.— Place rapidly growing 

 potted plants horizontally. Seedlings in pots, or young plants, or potted 

 hyacinths are good ones to use. In the course of a day observe the positions 

 of the stems and leaves. Sketch some of them. 



187. Gravity acts as a stimulus. — Knight found that the 

 stimulus which influences the root to turn downward is the force 

 of gravity. The reaction of the root in response to this stimulus 

 is geotropism, a turning influenced by the earth. This term is 

 applied to the growth movements of plants influenced by the 

 earth with regard to direction. While the motor zone lies back 

 of the root tip, the latter receives the stimulus, and is the per- 

 ceptive zone. If the root tip is cut off the root is no longer 

 geotropic, and will not turn downward when placed in a hori- 

 zontal position. Growth toward the earth is progeotropism. 

 The lateral growth of secondary roots is diageotropism. 



188. The result with stems. — The stem, on the other hand, 

 which was placed in a horizontal position has become again erect. 



Fig. 83. Fig. 84. 



Germinating pea placed in a hori- In twenty-four hours gravity has caused the 



zontal position. root to turn downward. 



Figures 83, 84. — Progeotropism of the pea root. 



This turning of the stem in the upward direction takes place in the 

 dark as well as in the light, as we can see if we start the experiment 

 at nightfall, or place the plant in the dark. This upward growth 

 of the stem is also influenced by the earth, and therefore is a case 

 of geotropism. The special designation in the case of upright 

 stems is negative geotropism, or apogeotropism, or the stems are 

 said to be apogeotropic. Place a rapidly growing potted plant 

 in a horizontal position by laying the pot on its side. The ends 



