Moreover
the whole end of the petiole or tendril, if it seizes nothing, bends
after a time abruptly downwards and inwards, showing that its outer
surface has gone on growing after the inner surface has ceased to
grow. That growth is the chief cause of the spiral contraction of
tendrils may be safely admitted, as shown by the recent researches of
H. de Vries. I will, however, add one little fact in support of this
conclusion.
If the short, nearly straight portion of an attached tendril of
Passiflora gracilis, (and, as I believe, of other tendrils,) between
the opposed spires, be examined, it will be found to be transversely
wrinkled in a conspicuous manner on the outside; and this would
naturally follow if the outer side had grown more than the inner
side, this part being at the same time forcibly prevented from
becoming curved. So again the whole outer surface of a spirally
wound tendril becomes wrinkled if it be pulled straight.
Nevertheless, as the contraction travels from the extremity of a
tendril, after it has been stimulated by contact with a support, down
to the base, I cannot avoid doubting, from reasons presently to be
given, whether the whole effect ought to be attributed to growth.
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