SECTION 



16.] 



MOVEMENTS. 



153 



473. Leaf-stalks and tendrils are adapted to their uses in climbing by a 

 similar sensitiveness. The coiling of the leaf-stalk is in response to a 

 kind of irritation produced by contact with the supporting body. This 

 may be shown by gentle rubbing or prolonged pressure upon the upper 

 face of the leaf-stalk, which is soon followed by a curvature. Ten- 

 drils are still more sensitive to contact or light friction. This causes the 

 free end of the tendril to coil round the support, and the sensitiveness, 

 propagated downward along the tendril, causes that side of it to become 

 less turgescent or the opposite side more so, thus throwing the tendril into 

 coils. This shortening draws the plant up to the support. Tendrils which 

 have not laid hold will at length commonly coil spontaneously, in a simple 

 coil, from the free apex downward. 



In Sicyos, Echinocystis, and the 

 above mentioned Passion-flowers 

 (471), the tendril is so sensitive, 

 under a high summer temperature, 

 that it will curve and coil prompt- 

 ly after one or two light strokes 

 by the hand. 



474. Among spontaneous move- 

 ments the most singular are those 

 of Desmodium gyrans of India, 

 sometimes called Telegraph-plant, 

 which is cultivated on account of 

 this action. Of its three leaflets, 

 the larger (terminal) one moves 

 only by drooping at nightfall and 

 rising with the dawn. But its two 

 small lateral leaflets, when in a 

 congenial high temperature, by day 

 and by night move upward and 

 downward in a saccession of jerks, 

 stopping occasionally, as if to re- 

 cover from exhaustion. In most 

 plant-movements some obviously 

 useful purpose is subserved : this 

 of Desmodium gyrans is a riddle. 



475. Movements in Flowers are very various. The most remarkable 

 are in some way connected vvitli fertilization (Sect. Xlll.). Some occur 

 under irritation : the stamens of Barberry start forward when touched at 

 tlie base inside : those of many polyandrous flowers (of Sparmannia very 

 strikingly) spread outwardly wlien lightly bruslied : the two lips or lobes 



Fig. 491. Portion of stem and leaves of Telegraph-plant (Desmodium gyrans), 

 almost of natur^ siz^i 



