Secondly, those endowed with irritable organs, which when
they touch any object clasp it; such organs consisting of modified
leaves, branches, or flower-peduncles. But these two classes
sometimes graduate to a certain extent into one another. Plants of
the third class ascend merely by the aid of hooks; and those of the
fourth by rootlets; but as in neither class do the plants exhibit any
special movements, they present little interest, and generally when I
speak of climbing plants I refer to the two first great classes.
TWINING PLANTS.
This is the largest subdivision, and is apparently the primordial and
simplest condition of the class. My observations will be best given
by taking a few special cases. When the shoot of a Hop (Humulus
lupulus) rises from the ground, the two or three first-formed joints
or internodes are straight and remain stationary; but the next-
formed, whilst very young, may be seen to bend to one side and to
travel slowly round towards all points of the compass, moving, like
the hands of a watch, with the sun. The movement very soon acquires
its full ordinary velocity. From seven observations made during
August on shoots proceeding from a plant which had been cut down, and
on another plant during April, the average rate during hot weather
and during the day is 2 hrs.
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