Chap. IV. SPIRAL CONTRACTION. 107 



their own axes, at the proper rate and in the proper 

 direction, might avoid becoming twisted ; but I hare 

 seen no such case. 



In the above illustration, the parallel strings were 

 wound round a stick ; but this is by no means neces- 

 sary, for if wound into a hollow coil (as can be done 

 with a narrow slip of elastic paper) there is the same 

 inevitable twisting of the axis. When, therefore, a free 

 tendril coils itseK into a spire, it must either become 

 twisted along its whole length (and this never occurs), 

 or the free extremity must turn round as many times 

 as there are spires formed. It was hardly necessary 

 to observe this fact; but I did so by affixing little 

 paper vanes to the extreme points of the tendrils of 

 Echinocydis and Passijlora quadrangularis ; and as 

 the tendril contracted itself into successive spires, the 

 vane slowly revolved. 



We can now understand the meaning of the spires 

 being invariably turned in opposite directions, in 

 tendrils which from having caught some object are 

 fixed at both ends. Let us suppose a caught tendril 

 to make thirty spiral turns all in the same direction ; 

 the inevitable result would be that it would become 

 twisted thirty times on its own axis. This twisting 

 would not only require considerable force, but, as I 

 know by trial, would burst the tendril before the thirty 

 turns were completed. Such cases never really occur ; 

 for, as already stated, when a tendril has caught a 

 support and is spirally contracted, there are always 

 as many turns in one direction as in the other ; so that 



