Page:Darwin - On the movements and habits of climbing plants.djvu/98

 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 Humulus, 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 necessary 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 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 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 direction; 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 tendril before the thirty turns were completed. Such a case never really occurs; for, as already stated, when a tendril has caught a support and has spirally contracted, there are 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 perpendicular pencil, and this will twist the lower part of the string; after it has been sufficiently twisted, it will be seen to curve itself into an open spire, with the curves running in an opposite