The spiral contraction of tendrils is quite independent of their
power of spontaneously revolving, for it occurs in tendrils, such as
those of Lathyrus grandiflorus and Ampelopsis hederacea, which do not
revolve. It is not necessarily related to the curling of the tips
round a support, as we see with the Ampelopsis and Bignonia
capreolata, in which the development of adherent discs suffices to
cause spiral contraction. Yet in some cases this contraction seems
connected with the curling or clasping movement, due to contact with
a support; for not only does it soon follow this act, but the
contraction generally begins close to the curled extremity, and
travels downwards to the base. If, however, a tendril be very slack,
the whole length almost simultaneously becomes at first flexuous and
then spiral. Again, the tendrils of some few plants never contract
spirally unless they have first seized hold of some object; if they
catch nothing they hang down, remaining straight, until they wither
and drop off: this is the case with the tendrils of Bignonia, which
consist of modified leaves, and with those of three genera of the
Vitaceae, which are modified flower-peduncles.
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