PLANT PHTSIOL007. 



107 



movements, which are all referable to the activities of pro- 

 toplasm, may he grouped under the following heads, viz. : 

 Nutation (or Automatism), Geotropism, Heliotropism and 

 Irritability. 



183. Nutation. — Under this term are gathei-ed those 

 cases in which terminal parts of plants move spontaneously 

 and somewhat regularly in 

 definite directions. It has 

 been observed that the grow- 

 ing ends of climbing plants 

 perform circular nutations; 

 thus in the hop and honey- 

 suckle the free ends of the 

 stems rotate in the direction 

 of the hands of a watch (Fig. 

 56«), while in the yam, bean, 

 and morning-glory the rota- 

 tion is the reverse (Fig. 565). 

 In other cases the nutation 

 is a simple swaying back and forth, as in many leaves and 

 growing shoots. 



184. Mr. Darwin has shown that as soon as a seed ger- 

 minates the little root at once begins a sort of revolving 

 motion, its tip describing more or less elliptical or circular 

 figures. By this circumnutation a root is enabled to 

 find those places in the soil which offer the least resistance 

 to its passage. Moreover, it has been shown that the tip 

 of the root is sensitive to pressure, and when it comes in 

 contact with any object bends from it. In this way the 

 root-tip guides the advancing root through the interstices 

 of the soil, avoiding on every hand the pebbles and harder 

 bits of earth. The root-tip appears also to be sensitive to 



_ Twining stems— o, of 

 hop ; b, of yam. 



