Chap. IV. SPIRAL CONTRACTION. 163 



never contract spirally unless they have first seized 

 hold of some object ; if they catch nothing they hang 

 down, remaining straight, until they wither and drop 

 off: this is the case with the tendrils of Bignonia, 

 which consist of modified leaves, and with those of 

 three genera of the Yitaceae, which are modified flower- 

 peduncles. But in the great majority of cases, tendrils 

 which have never come in contact with any object, 

 after a time contract spirally. All these facts taken 

 together, show that the act of clasping a support and 

 the spiral contraction of the whole length of the 

 tendril, are phenomena not necessarily connected. 



The spiral contraction which ensues after a tendril 

 has caught a support is of high service to the plant ; 

 hence its almost universal occurrence with species 

 belonging to widely different orders. When a shoot 

 is inclined and its tendril has caught an object above, 

 the spiral contraction drags up the shoot. When the 

 shoot is upright, the growth of the stem, after the 

 tendrils have seized some object above, would leave it 

 slack, were it not for the spiral contraction which 

 draws up the stem as it increases in length. Thus 

 there is no waste of growth, and the stretched stem 

 ascends by the shortest course. When a terminal 

 branchlet of the tendril of Cobaea catches a stick, we 

 have seen how well the spiral contraction successively 

 brings the other branchlets, one after the other, into 

 contact with the stick, until the whole tendril grasps 

 it in an inextricable knot. When a tendril has caught 

 a yielding object, this is sometimes enveloped and 



