36o 



A TEXT-BOOK OF BOTANY, 



in a straight line, move either from one side to the other, or coil 

 or curve about an imaginary axis. This spiral movement is 

 known as circumnutation and is characteristic of twining stems and 

 tendrils, as the hop vine and tendrils of Bryonia ( Fig. i8i ) . Nuta- 

 tion curvatures are due to unequal growth on two sides of the 

 organ and cease when there is a cessation in growth or when 

 the plant has reached maturity. 



The movements of organs due to external stimuli are usually 

 in a direction which shows a relation to the direction of the stim- 

 ulus, as those produced by gravity and light (Fig. 207), and these 



Fig. 207. American senna (Cassia marilandica). The figure at the left shows the pin- 

 nately-compound leaves in the day position when under the influence of light, and the one 

 to the right the drooping position of the leaflets at night. 



movements are of use in bringing the organs into more favorable 

 positions for growth. Stimuli of this kind are spoken of as 

 orienting or tropic. The compound leaves of a number of plants 

 exhibit in addition certain variable and periodic movements, which 

 have their origin in a special mechanism known as the pulvinis. 

 The pulvinis appears as a swelling on the petiole and consists of 

 parenchymatous tissue which is highly turgid, i.e., full of water. 

 Any stimulus, such as mechanical shock, which causes a differ- 

 ence in the degree of turgidity on two sides, will result in a move- 

 ment of the leaves in such plants as Mimosa, Oxalis and locust. 

 The leaves of Mimosa pudica, a common cultivated sensitive plant, 

 show a very rapid response to such stimuli, the leaflets folding 



