THE MOVEMENTS OF PLANTS. 1 93 



is produced by internal causes, called spontaneous move- 

 ments, and (2) those in which the variation in growth results 

 from stimulation by external agents, called paratonic move- 

 ments. 



283. i. Spontaneous movements. Among spontaneous 

 movements are those in which the variation in growth occurs 

 upon different sides of a cylindrical organ, or the two faces of 

 a bilateral one. The opening of all flower and leaf buds illus- 

 trates this movement, which is called nutation. During the 

 development of the interior parts, the outer leaves (often 

 scale-like) which protect them grow more rapidly upon their 

 outer (dorsal) surfaces. They are thus pressed together into 

 a compact bud. When the internal parts are suitably de- 

 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 acceleration of growth travels irregularly 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 re- 

 mainder into a double spiral, are paratonic movements in- 

 duced by contact. The rotating movements by which twin- 

 ing plants climb are also paratonic and not spontaneous. 



284. 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 peculiar states 

 in which a plant or an organ exists when it can respond to 



