Natural History 621 



Robert Louis Stevenson gives a graphic picture of this form of 

 the struggle for existence in his poem of "The Woodman." The 

 twining movement, like the movement of the root-tip, is regulated 

 by gravity. 



Tendril-bearers respond to another influence, that of contact 

 with a solid body. Like the tip of the hop, the whip-like tendril 

 of the bryony sweeps round in a circle. This movement is not 

 directed by external stimulus, but is purely automatic. But if the 

 tendril touches a twig, a wire, a leaf, or another tendril, it is 

 stimulated by the contact to more rapid growth on the side away 

 from the exciting object, and so curves towards it. As it goes on 

 curving, fresh portions come into contact, and are stimulated, so 

 that the whole tip becomes tightly wrapped round the support. 

 Later the basal part twists into a corkscrew spring once or twice 

 reversed, and then becomes thick and woody. This spring breaks 

 the force of the tug of a passing animal or of a gust of wind, and 

 saves the plant from being torn from its support. In his Climbing 

 Plants Darwin describes the conspicuous success of the bryony 

 in riding the storm which battered many other plants. 



The tendrilled plant is more highly adapted for climbing than 

 the twiner. The latter loses a third of its length in its coils, and 

 while it can use only such supports as are nearly vertical, the ten- 

 dril encircles anything that is solid and not too thick. The tendril 

 represents some part of the plant turned to a new use. In the 

 sweet-pea it takes the place of the apical leaflets, in some vetch- 

 lings it replaces the entire leaf, in some tropical orchids it is a root, 

 in the bryony and in the vines it is a branch. 



If we gently stroke the under side of the bryony tendril we 

 can readily observe the answer-back ; after a few minutes the ten- 

 dril curves towards the side touched. Only a solid body can 

 induce the response ; a stream of water, a violent rain shower, even 

 rubbing with a stick of semi-solid gelatine, has no effect. Here is 

 a fine example of delicate adjustment, for response to rain-drops 

 would be of no use to the plant, would even deprive it of oppor- 



