第2章 TWINING PLANTS(1)

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. 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, and had subsequently been the subject of two memoirs by Dutrochet.

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 Ispeak 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 9hrs.; 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.Ishould 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 the more rapid movements, after the performance of the third revolution, lasted during three days and twenty hours.The regular revolutions, from the ninth to thirty-sixth inclusive, were effected at the average rate of 2 hrs.

31 m.; but the weather was cold, and this affected the temperature of the room, especially during the night, and consequently retarded the rate of movement a little.There was only one irregular movement, which consisted in the stem rapidly making, after an unusually slow revolution, only the segment of a circle.After the seventeenth revolution the internode had grown from 1.75 to 6 inches in length, and carried an internode 1.875 inch long, which was just perceptibly moving; and this carried a very minute ultimate internode.After the twenty-first revolution, the penultimate internode was 2.5 inches long, and probably revolved in a period of about three hours.At the twenty-seventh revolution the lower and still moving internode was 8.375, the penultimate 3.5, and the ultimate 2.5 inches in length;and the inclination of the whole shoot was such, that a circle 19inches in diameter was swept by it.When the movement ceased, the lower internode was 9 inches, and the penultimate 6 inches in length;so that, from the twenty-seventh to thirty-seventh revolutions inclusive, three internodes were at the same time revolving.