DIV. ii PHYSIOLOGY 359 



These chemonastic curvatures of tendrils are evidently of no use to the plants. 

 The same is the case for the nastic movements of tendrils which take place on 

 wounding and on rise of temperature (traumatonasty, thennonasty). On the 

 other hand, chemonastic movements play an important part in some insectivorous 

 plants. 



Very striking chemonastic movements are exhibited by the tentacles 

 of Drosera (Fig. 214). On chemical stimulation these curve so that 

 their upper sides become concave and the glandular heads are thus 

 brought towards the centre of the circular leaf. Such substances as 

 albumen, phosphates, etc., which Drosera can use as food, serve as stimuli 

 (p. 258) ; so also can indifferent and even poisonous substances. Often 

 minimal traces of these substances (e.g. 0'0004 mgr. of ammonium 

 phosphate) suffice to bring about the irritable movement ; when the 

 stimulus is applied to the summit of the tentacle it leads to the 

 curvature at the base of the latter. There is thus in this case as in 

 certain phototropic curvatures, but even more clearly than in these, 

 a separation between the organ of perception which receives the 

 stimulus and the motile organ that effects the movement. The 

 stimulus received by the head of the tentacle must be conducted to 

 the base of the latter. 



An insect that has settled on a marginal tentacle will be brought 

 by this curvature to the centre of the lamina. The short -stalked 

 tentacles borne here send a stimulus to all the marginal ones, causing 

 them to curve inwards. The insect is thus surrounded by many 

 glands and covered with their digestive secretion. 



The curvature resulting from growth is carried out in the same way as in 

 tendrils. After curvature the tentacle has become considerably longer. When 

 growth ceases, the motility of the tentacles is ended so that they can only close 

 over a limited number of times. Further, the tentacles of Drosera in common with 

 tendrils can exhibit thigmonastic, traumatonastic, and thermonastic reactions. 

 Doubtless, however, their chemonastic irritability is the main and most important 

 one. Chemical stimuli are concerned in the movements of other insectivorous 

 plants, e.g. Dionaea and Pinguicula. 



3. SEISMONASTY ( m ) 



In Dionaea the two halves of the leaf -blade (Fig. 217) close 

 together not only as a result of chemical stimuli but also owing to a 

 mechanical stimulus. In contrast to the haptotropic movements of 

 tendrils or of Drosera resulting from contact with solid bodies, in the 

 case under consideration every disturbance resulting from a mechanical 

 shock acts as a stimulus ; the movement can thus be brought about 

 by rain-drops. These movements are termed seismonastic. 



The most familiar example of seismonastic movements is furnished 

 by Mimosa pudica, a tropical leguminous shrubby plant, which owes 

 its name of sensitive plant to its extreme sensitiveness to contact. 



