HAPTOTROPISM 299 



the tendril. Contact is not necessarily, or even usually, 

 transitory ; as the tendril bends round the support new 

 regions make contact, and the movement proceeds till the 

 whole of the tip is used up. Reverse movements also take 

 place, and these, in conjunction with continually renewed 

 stimulus, result in the winding on of a part of the tendril 

 below the point of original contact, and in a closer grip of 

 the whole. When the encircling is complete a fresh reaction 

 sets in, due primarily to the mechanical strain of the plant's 

 mass. The mechanical tissue of the moving, growing tendril 

 is coUenchyma ; after the support is grasped sclerenchyma 

 develops and the resistance to the pull of the plant is much 

 heightened. In many tropical plants the tendril under- 

 goes enormous secondary thickening. Frequently the free 

 portion of the tendril, between the support and the plant, 

 coils into a corkscrew. As it is impossible to make a simple 

 coil between two fixed ends the direction of the twist is 

 reversed at least once (A Fig. 44), The corkscrew acts as a 

 spring ; in gusts of wind it pulls out somewhat, and closes 

 up again, thus diminishing greatly the risk of tearing. Our 

 native Bryonia and the tropical Lagenaria are good examples 

 of this. Fixed by hundreds of tentacles to a fence or 

 hedge, the bryony is extraordinarily secure. We may recall 

 Darwin's description of a bryony hedge in a storm : " It is 

 this elasticity which prevents both branched and simple 

 tendrils being torn away from their supports during stormy 

 weather. I have more than once gone during a gale to 

 watch bryony growing in an exposed hedge with its tendrils 

 attached to the surrounding bushes ; and as the thick and 

 thin branches were tossed to and fro by the wind the tendrils, 

 had they not been excessively elastic, would instantly have 

 been torn off and the plant thrown prostrate. But as it 

 was, the bryony safely rode out the gale, like a ship with two 

 anchors down, and with a long range of cable ahead to serve 

 as a spring as she surges to the storm." 



Nature of Tendrils. — Tendrils are derived from the 

 most various organs. In the simplest case the organ, 

 unaltered except as regards its reactions, acts directly as a 



