The lower and older internodes, which continued to revolve, were incapable, on repeated trials, of twining round a thin stick; showing that, although the power of movement was retained, this was not sufficient to enable the plant to twine.I then moved the stick to a greater distance, so that it was struck by a point 2.5 inches from the extremity of the penultimate internode; and it was then neatly encircled by this part of the penultimate and by the ultimate internode.After leaving the spirally wound shoot for eleven hours, I quietly withdrew the stick, and in the course of the day the curled portion straightened itself and recommenced revolving; but the lower and not curled portion of the penultimate internode did not move, a sort of hinge separating the moving and the motionless part of the same internode.After a few days, however, I found that this lower part had likewise recovered its revolving power.These several facts show that the power of movement is not immediately lost in the arrested portion of a revolving shoot; and that after being temporarily lost it can be recovered.When a shoot has remained for a considerable time round a support, it permanently retains its spiral form even when the support is removed.
When a tall stick was placed so as to arrest the lower and rigid internodes of the Ceropegia, at the distance at first of 15 and then of 21 inches from the centre of revolution, the straight shoot slowly and gradually slid up the stick, so as to become more and more highly inclined, but did not pass over the summit.Then, after an interval sufficient to have allowed of a semi-revolution, the shoot suddenly bounded from the stick and fell over to the opposite side or point of the compass, and reassumed its previous slight inclination.It now recommenced revolving in its usual course, so that after a semi-revolution it again came into contact with the stick, again slid up it, and again bounded from it and fell over to the opposite side.
This movement of the shoot had a very odd appearance, as if it were disgusted with its failure but was resolved to try again.We shall, I think, understand this movement by considering the former illustration of the sapling, in which the growing surface was supposed to creep round from the northern by the western to the southern face; and thence back again by the eastern to the northern face, successively bowing the sapling in all directions.Now with the Ceropegia, the stick being placed to the south of the shoot and in contact with it, as soon as the circulatory growth reached the western surface, no effect would be produced, except that the shoot would be pressed firmly against the stick.But as soon as growth on the southern surface began, the shoot would be slowly dragged with a sliding movement up the stick; and then, as soon as the eastern growth commenced, the shoot would be drawn from the stick, and its weight coinciding with the effects of the changed surface of growth, would cause it suddenly to fall to the opposite side, reassuming its previous slight inclination; and the ordinary revolving movement would then go on as before.I have described this curious case with some care, because it first led me to understand the order in which, as I then thought, the surfaces contracted; but in which, as we now know from Sachs and II.de Vries, they grow for a time rapidly, thus causing the shoot to bow towards the opposite side.
The view just given further explains, as I believe, a fact observed by Mohl (p.135), namely, that a revolving shoot, though it will twine round an object as thin as a thread, cannot do so round a thick support.I placed some long revolving shoots of a Wistaria close to a post between 5 and 6 inches in diameter, but, though aided by me in many ways, they could not wind round it.This apparently was due to the flexure of the shoot, whilst winding round an object so gently curved as this post, not being sufficient to hold the shoot to its place when the growing surface crept round to the opposite surface of the shoot; so that it was withdrawn at each revolution from its support.
When a free shoot has grown far beyond its support, it sinks downwards from its weight, as already explained in the case of the Hop, with the revolving extremity turned upwards.If the support be not lofty, the shoot falls to the ground, and resting there, the extremity rises up.Sometimes several shoots, when flexible, twine together into a cable, and thus support one another.Single thin depending shoots, such as those of the Sollya Drummondii, will turn abruptly backwards and wind up on themselves.The greater number of the depending shoots, however, of one twining plant, the Hibbertia dentata, showed but little tendency to turn upwards.In other cases, as with the Cryptostegia grandiflora, several internodes which were at first flexible and revolved, if they did not succeed in twining round a support, become quite rigid, and supporting themselves upright, carried on their summits the younger revolving internodes.
Here will be a convenient place to give a Table showing the direction and rate of movement of several twining plants, with a few appended remarks.These plants are arranged according to Lindley's 'Vegetable Kingdom' of 1853; and they have been selected from all parts of the series so as to show that all kinds behave in a nearly uniform manner.
The Rate of Revolution of various Twining Plants.
(ACOTYLEDONS.)
Lygodium scandens (Polypodiaceae) moves against the sun.
H.M.
June 18, 1st circle was made in6018, 2nd 6 15 (late in evening)19, 3rd 5 32 (very hot day)19, 4th 50 (very hot day)20, 5th 60
Lygodium articulatum moves against the sun.
H.M.
July 19, 1st circle was made in 16 30 (shoot very young)20, 2nd 150
21, 3rd 80
22, 4th 10 30
(MONOCOTYLEDONS.)
Ruscus androgynus (Liliaceae), placed in the hot-house, moves against the sun.
H.M.
May 24, 1st circle was made in614 (shoot very young)25, 2nd221
25, 3rd337
25, 4th322
26, 5th250
27, 6th352
27, 7th411
