Cuapr. IV. SPIRAL CONTRACTION. 167 
their own axes, at the proper rate and in the proper 
direction, might avoid becoming twisted; but I have 
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 itself 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 
Echinocystis and Passiflora 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 
