THE STEM AND THE LEAF 



75 



The development and mode of operation of tendrils is espe- 

 cially interesting (figs. 58 and 59). The tendril is either a 

 leafless, thread-like branch, as in the grapevine, or a highly 

 modified leaf or part of a leaf, as in the cucumber. When a 

 living and active tendril comes into contact with any suitable 



FIG. 68. Bur cucumber (Sicyos), a tendril climber 



At a, b, c, ft, e, and /successive stages in the history of tendrils are shown. The 



tendril, at first curved, becomes straighter, then curls up at the tip, and finally 



(after becoming attached to some object) coils itself into a close spiral and thus 



draws the plant with considerable force toward the sustaining object 



support for the climber, this contact brings about more active 

 growth on the exterior side of the tendril (that side which 

 does not touch the foreign object). In this way the tendril 

 is made to coil about the support. Soon after the tendril has 

 become attached, the free portions are thrown into coils, and 

 the shortening which results draws the stem of the climber 

 closer to the support. 



