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. 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.
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