HOME PLANTS AND THEIR WATS. 21 



drils, on the other hand, have no hook at the end, and here 

 the tendrils are sensitive to the touch on any side. 



14. There is a remarkable movement which occurs in 

 tendrils after they have caught an object, and which ren- 

 ders a tendril a better climbing organ than any sensitive 

 leaf. This movement is called spiral contraction. When 

 a tendril first seizes an object it is quite straight, with the 

 exception of the extreme tip, which is firmly curled round 

 the object seized. But in a day or two the tendril begins 

 to contract, and ultimately assumes the corkscrew-like 

 form represented in the figures. It is clear that in spirally 

 contracting the tendril has become considerably shorter ; 



Tendril Coil. 



and, since the end of the tendril is fixed to a branch, it is 

 obvious that the stem must be dragged nearer to the object 

 which its tendril has caught. Thus, if a shoot of bryony 

 seizes a support above it, the contraction of the tendril will 

 pull up the shoot in the right direction. So that in this 

 respect the power of spiral contraction gives a tendril- 

 climber an advantage over leaf-climbers which have no con- 

 tracting power, and, therefore, no means of hauling them- 

 selves up to supporting objects. 



15. But the spiral contraction of tendrils has another 

 use, and this is probably the most important one. This 

 use depends on the fact that a contracted tendril acts like 

 a spiral spring, and is thus converted into a yielding instead 



