Movements and Habits of Climbing Plants | Page 4

Introductory remarks--Description of the twining of the Hop--Torsion
of the stems--Nature of the revolving movement, and manner of ascent-
-Stems not irritable--Rate of revolution in various plants--Thickness of
the support round which plants can twine--Species which revolve in an
anomalous manner.
I was led to this subject by an interesting, but short paper by Professor
Asa Gray on the movements of the tendrils of some Cucurbitaceous
plants. {2} My observations were more than half completed before I
learnt that the surprising phenomenon of the spontaneous revolutions of
the stems and tendrils of climbing plants had been long ago observed
by Palm and by Hugo von Mohl, {3} and had subsequently been the

subject of two memoirs by Dutrochet. {4} Nevertheless, I believe that
my observations, founded on the examination of above a hundred
widely distinct living species, contain sufficient novelty to justify me in
publishing them.
Climbing plants may be divided into four classes. First, those which
twine spirally round a support, and are not aided by any other
movement. 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. 8 m. for each revolution; and none of the revolutions varied
much from this rate. The revolving movement continues as long as the
plant continues to grow; but each separate internode, as it becomes old,
ceases to move.
To ascertain more precisely what amount of movement each internode
underwent, I kept a potted plant, during the night and day, in a
well-warmed room to which I was confined by illness. A long shoot
projected beyond the upper end of the supporting stick, and was
steadily revolving. I then took a longer stick and tied up the shoot, so
that only a very young internode, 1.75 of an inch in length, was left free.
This was so nearly upright that its revolution could not be easily

observed; but it certainly moved, and the side of the internode which
was at one time convex became concave, which, as we shall hereafter
see, is a sure sign of the revolving movement. I will assume that it
made at least one revolution during the first twenty-four hours. Early
the next morning its position was marked, and it made a second
revolution in 9 hrs.; during the latter part of this revolution it moved
much quicker, and the third circle was performed in the evening in a
little over 3 hrs. As on the succeeding morning I found that the shoot
revolved in 2 hrs. 45 m., it must have made during the night four
revolutions, each at the average rate of a little over 3 hrs. I should add
that the temperature of the room varied only a little. The shoot had now
grown 3.5 inches in length, and carried at its extremity a young
internode 1 inch in length, which showed slight changes in its curvature.
The next or ninth revolution was effected in 2 hrs. 30 m. From this time
forward, the revolutions were easily observed. The thirty-sixth
revolution was performed at the usual rate; so was the last or
thirty-seventh, but it was not completed; for the internode suddenly
became upright, and after moving to the centre, remained motionless. I
tied a weight to its upper end, so as to bow it slightly and thus detect
any movement; but there was none. Some time before the last
revolution was half performed, the lower part of the internode ceased to
move.
A few more remarks will complete all that need be said about this
internode. It moved during five days; but