TENDRIL-BEARERS. 97 



spirally turned round it, they will inevitably become twisted. 

 Hence a straight coloured line, painted along the internodes of a 

 twining plant before it has wound round a support, becomes 

 twisted or spiral after it has so wound round. I painted a red 

 line on the straight internodes of a Stimulus, Mikania, Ceropegia, 

 Convolvulus, and Phaseolus, and saw it become twisted as the 

 plant wound round a stick. It is possible that the stems of some 

 plants by spontaneously turning on 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 necessary, 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. Hence when a 

 tendril, which is free at its end, coils itself into a spire, it must 

 either become twisted along its whole length (and this is a case 

 which I have never seen), or the free extremity must turn round 

 as many times as there are spires formed. It was hardly neces- 

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

 vanes to the extreme points of the tendrils of the Echinocystis and 

 Passiflora quadrangular is ; and as the tendril contracted itself into 

 successive spires, the vane slowly revolved. 



We can now understand the meaning of the spires being in- 

 variably turned in opposite directions in those tendrils which, 

 having caught some object, are thus fixed at both ends. Let us 

 suppose a caught tendril to make thirty spiral turns in one direc- 

 tion ; the inevitable result will be that it will become thirty times 

 twisted on its own axis. This twisting not only would require 

 considerable force, but, as I know by trial, would burst the ten- 

 dril before the thirty turns were completed. Such a ease never 

 really occurs ; for, as already stated, when a tendril has caught a 

 support ami has spirally contracted, there arc always as many 

 turns in one direction as in the other ; so that the twisting of the 

 axis in the one direction is exactly compensated by that in the 

 other. We can further see how the tendency is given to make coils 

 in an opposite direction to those, whether turned to the right or to 

 the left, which are first made. Take a piece of string, and let it 

 hang down with the lower end fixed to the floor; then wind the 

 upper end (holding the string quite loosely) spirally round a per- 

 pendicular pencil, and this will twist the lower part of the string ; 

 after it has been sufficiently twisted,* will be seen to curve itself 

 into an open spire, with the curves running in an opposite direc- 



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