IV 



Movement and Nervous Action in Plants 63 



The leaf-climbers have clasping petioles as in Clematis and 

 Tropaeolum, or hook themselves up by the tips of their 

 leaves as in Gloriosa ; most of them also revolve like the 

 twiners, and in this way bring their leaves into contact 

 with adjacent branches. When they are young the leaf- 

 stalk or the leaf-tip, or even the whole surface of the leaf 

 (in the climbing fumitory, Corydalis clavicutata), is sen- 

 sitive to contact, bends towards the side on which pressure 

 is exerted, and thus clasps the plant to its support. 



The tendril-bearers, such as the pea and the vine and 

 the passion-flower, are the most evolved climbers, for they 

 have prehensile organs specially adapted for this function. 

 These prehensile organs or tendrils may be modified leaflets 

 as in the pea, or modified leaves as in Lathyrus Aphaca, or 

 flower-stalks as in the vine, or even branches in some rare 

 cases. The shoots of the tendril-bearers revolve as those 

 of the leaf-climbers do, and the tendrils themselves move 

 round and round. Thus the tendrils are brought into 

 contact with surrounding objects, to the touch of which 

 they are often finely sensitive. They curve to what they 

 touch and link themselves around it, after which they 

 usually grow stronger and thicker, and by coiling into a 

 spiral raise the plant nearer to its support. 



Darwin's Observations on Climbing and Twining 

 Plants. — Having now classified the climbing plants as 

 Darwin did, we shall inquire more carefully into what he 

 has told us of their life. 



In regard to twining plants let us quote one of Darwin's 

 observations : " When the shoot of a hop rises from the 

 ground, the two or three first-formed joints or internodes 

 are straight and remain stationary ; but the next formed, 

 whilst very young, may be seen to bend to one side and 

 to travel slowly round towards all points of the compass, 

 moving, like the hands of a watch, with the sun. The 



