Movements and Habits of Climbing Plants | Page 7

goes on revolving,
but much more slowly; for the internodes, until they have grown to
some little length, always move slowly. If we look to the one, two, or
several internodes of a revolving shoot, they will be all seen to be more
or less bowed, either during the whole or during a large part of each
revolution. Now if a coloured streak be painted (this was done with a
large number of twining plants) along, we will say, the convex surface,
the streak will after a time (depending on the rate of revolution) be
found to be running laterally along one side of the bow, then along the
concave side, then laterally on the opposite side, and, lastly, again on
the originally convex surface. This clearly proves that during the
revolving movement the internodes become bowed in every direction.
The movement is, in fact, a continuous self-bowing of the whole shoot,
successively directed to all points of the compass; and has been well
designated by Sachs as a revolving nutation.
As this movement is rather difficult to understand, it will be well to
give an illustration. Take a sapling and bend it to the south, and paint a
black line on the convex surface; let the sapling spring up and bend it to
the east, and the black line will be seen to run along the lateral face
fronting the north; bend it to the north, the black line will be on the
concave surface; bend it to the west, the line will again be on the lateral
face; and when again bent to the south, the line will be on the original
convex surface. Now, instead of bending the sapling, let us suppose
that the cells along its northern surface from the base to the tip were to

grow much more rapidly than on the three other sides, the whole shoot
would then necessarily be bowed to the south; and let the longitudinal
growing surface creep round the shoot, deserting by slow degrees the
northern side and encroaching on the western side, and so round by the
south, by the east, again to the north. In this case the shoot would
remain always bowed with the painted line appearing on the several
above specified surfaces, and with the point of the shoot successively
directed to each point of the compass. In fact, we should have the exact
kind of movement performed by the revolving shoots of twining plants.
{9}
It must not be supposed that the revolving movement is as regular as
that given in the above illustration; in very many cases the tip describes
an ellipse, even a very narrow ellipse. To recur once again to our
illustration, if we suppose only the northern and southern surfaces of
the sapling alternately to grow rapidly, the summit would describe a
simple arc; if the growth first travelled a very little to the western face,
and during the return a very little to the eastern face, a narrow ellipse
would be described; and the sapling would be straight as it passed to
and fro through the intermediate space; and a complete straightening of
the shoot may often be observed in revolving plants. The movement is
frequently such that three of the sides of the shoot seem to be growing
in due order more rapidly than the remaining side; so that a semi-circle
instead of a circle is described, the shoot becoming straight and upright
during half of its course.
When a revolving shoot consists of several internodes, the lower ones
bend together at the same rate, but one or two of the terminal ones bend
at a slower rate; hence, though at times all the internodes are in the
same direction, at other times the shoot is rendered slightly serpentine.
The rate of revolution of the whole shoot, if judged by the movement of
the extreme tip, is thus at times accelerated or retarded. One other point
must be noticed. Authors have observed that the end of the shoot in
many twining plants is completely hooked; this is very general, for
instance, with the Asclepiadaceae. The hooked tip, in all the cases
observed by me, viz, in Ceropegia, Sphaerostemma, Clerodendron,
Wistaria, Stephania, Akebia, and Siphomeris, has exactly the same kind
of movement as the other internodes; for a line painted on the convex
surface first becomes lateral and then concave; but, owing to the youth

of these terminal internodes, the reversal of the hook is a slower
process than that of the revolving movement. {10} This strongly
marked tendency in the young, terminal and flexible internodes, to
bend in a greater degree or more abruptly than the other internodes, is
of service to the plant; for not only does the hook thus formed
sometimes serve to catch a support, but (and