Leon has seen
seven or eight such alternations. Whether the spires turn once or
more than once in opposite directions, there are as many turns in the
one direction as in the other. For instance, I gathered ten attached
tendrils of the Bryony, the longest with 33, and the shortest with
only 8 spiral turns; and the number of turns in the one direction was
in every case the same (within one) as in the opposite direction.
The explanation of this curious little fact is not difficult. I will
not attempt any geometrical reasoning, but will give only a practical
illustration. In doing this, I shall first have to allude to a point
which was almost passed over when treating of Twining-plants. If we
hold in our left hand a bundle of parallel strings, we can with our
right hand turn these round and round, thus imitating the revolving
movement of a twining plant, and the strings do not become twisted.
But if we hold at the same time a stick in our left hand, in such a
position that the strings become 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 wound round.
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