THE MOVEMENTS OF PLANTS. 169 



veloped a change occurs in the rate of growth of the outer 

 leaves ; their inner (ventral) faces now grow more rapidly 

 and the bud expands. Similar spontaneous variation in the 

 growth of different sides of tendrils produces a nodding or 

 waving motion, or even a rotation of the tip, by means of 

 which they are often enabled to reach a support. In most 

 tendrils the region whose growth is hastened travels irregu- 

 larly around the axis, so that their tips rotate in a roughly 

 circular or elliptical orbit from the time the tendril is two- 

 thirds grown until growth ceases. The further changes in 

 the tendril, by which it wraps the tip about the support and 

 coils the remainder into a double spiral, are paratonic move- 

 ments induced by contact. The rotating movements by which 

 twining plants climb are also paratonic and not spontaneous. 

 242. 2. Paratonic movements are also of the highest im- 

 portance for the well-being of the plants concerned. By 

 means of them the different organs are developed in such 

 situations that they can properly perform their work. The 

 stimuli which influence the rate of growth are chiefly light, 

 gravity, heat, mechanical contact, and moisture. The pecul- 

 iar states in which a plant or an organ exists when it can 

 respond to the different stimuli have received different names, 

 and those names indicate the nature of the stimulus. A 

 plant or an organ is heliotropic when it places itself in a cer- 

 tain position with reference to the direction of the rays of 

 light falling upon it ; geotropic, when it reacts thus to the 

 force of gravity ; thermoiropic, when it reacts thus to the 

 presence of a warm body ; hydrotropic, when it reacts thus to 

 the presence of a moist surface, etc. In each case the plants 

 are said to react positively when the movement is toward the 

 source of the stimulus ; negatively, when the movement is 

 away from the stimulus ; transversely, when it is transverse to 

 the direction of the stimulus. These reactions are to a cer- 

 tain extent related to one another, and it will be convenient. 



