It is a singular
circumstance that immature tendrils revolve at their full velocity
before they become sensitive, but in a useless manner, as in this
state they can catch nothing. This want of perfect co-adaptation,
though only for a short time, between the structure and the functions
of a climbing-plant is a rare event. A tendril, as soon as it is
ready to act, stands, together with the supporting petiole,
vertically upwards. The leaflets borne by the petiole are at this
time quite small, and the extremity of the growing stem is bent to
one side so as to be out of the way of the revolving tendril, which
sweeps large circles directly over head. The tendrils thus revolve
in a position well adapted for catching objects standing above; and
by this means the ascent of the plant is favoured. If no object is
caught, the leaf with its tendril bends downwards and ultimately
assumes a horizontal position. An open space is thus left for the
next succeeding and younger tendril to stand vertically upwards and
to revolve freely. As soon as an old tendril bends downwards, it
loses all power of movement, and contracts spirally into an entangled
mass. Although the tendrils revolve with unusual rapidity, the
movement lasts for only a short time.
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