We have referred to the wide sweep of the rotating extremity of the hop, but a still more remarkable instance has been recorded in another plant. This was Ceropegia Gardneri, a tropical Asclepiad. The top was allowed to grow out almost horizontally to the length of 31 inches. This shoot had three long internodes terminated by three shorter ones. The whole shoot revolved in a direction opposite to the course of the sun, and therefore in a contrary direction to the hop, occupying from five hours and a quarter to six hours and three quarters in each revolution. On account of the great length of this shoot the circle through which it moved was about 5 feet in diameter, or 16 feet in circumference, and the apex travelled through this circle at the rate of 32 or 33 inches per hour. "The weather being hot," Mr. Darwin writes, "the plant was allowed to stand on my study-table; and it was an interesting spectacle to watch the long shoot sweeping this grand circle, night and day, in search of some object round which to twine."
The greater number of twining plants climb in a direction opposed to the course of the sun. The hop and the bryony pursue the same course as the sun. The bean (Phaseolus vulgaris), purple convolvulus, and great white convolvulus, follow a course opposed to the sun. The bitter-sweet (Solanum dulcamara), which is a poor climber, turns in both directions. Of the Chili nettle (Loasa aurantiaca), a common greenhouse climber, out of eighteen plants eight revolved in opposition to the sun, five followed the course of the sun, and four turned first in one direction and then in the other.
<!-- image -->Fig. 23.—Bitter-Sweet (Solanum dulcamara).
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During the experiments on twining plants several interesting facts were evolved which may be mentioned incidentally. For instance, when a revolving shoot is arrested by a stick, and before it has had time to make its first circle round it, the stick is removed, the shoot springs forward, not perhaps to catch the retreating stick, but showing that it must have been pressing against it with some force. After a shoot has wound itself round a stick, if the support be withdrawn, the spiral will remain for a little time, and then the shoot will straighten itself again, and again commence revolving in search of a new support. Although our indigenous twiners are able to ascend by twining round a support as thin as a thread, they cannot twine round an object five or six inches in diameter: the honeysuckle being the only twiner that will encircle trees. Exotic twiners in tropical forests we know will, on the contrary, ascend large forest trees. In all the examples experimented upon the rotation appeared to proceed during the night precisely at the same rate as during the day, showing that light appears to have but little influence on climbing plants. Indeed, one physiologist, Mohl, has affirmed that twining plants are but little sensitive to light.
The conditions under which plants rotate and twine most favourably, are the usual ones under which they would naturally perform the other functions of life, namely, good health and a moderate amount of warmth. The twining Polygonum (Polygonum convolvulus), however, twines only during the middle of the summer; in the autumn they will grow vigorously, but without any inclination to climb.
Fig. 24.—The Twining Polygonum (Polygonum convolvulus).
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Most of the garden beans, of the scarlet runner kind, are excellent twiners, whilst some of the varieties exhibit no tendency to twine.
The only other point to which we shall allude in twining plants is the different rate of revolution in different plants. In the hop, the shortest period recorded for a revolution was two hours; in the bryony, two hours and a half; in the kidney bean, five minutes less than two hours; the white convolvulus, one hour and forty minutes; in the trumpet flower (Tecoma), six hours and a half; in Ceropegia, five hours and a quarter; in a climbing fern (Lygodium), five hours for one species and eight hours for another; in Lapageria rosea, eight hours and three-quarters in a hot-house, and eleven hours in a greenhouse; in a species of honeysuckle it was eight hours; and in an exotic (Sphaerostema) it was eighteen hours and a half.
Although these twiners are described as "those which twine spirally round a support, unaided by any other movement," it has been seen that they possess very remarkable movements of their own, which are intimately related to, and are indeed sufficient to account for, their spiral twining. The same kind of movement, that of rotation, or circumnutation, which we have seen in operation in the young radicle of germinating seeds, in cotyledons, leaves, &c., here reaches a higher development, and achieves a more palpable result.
A remarkable genus of twining plants, belonging to the Amaryllis family, has not yet received the attention they deserve. Passing through one of the "stoves" in Kew Gardens we remarked three or four species of these climbing Amaryllids, and were struck by the peculiar twisting of the petiole of the leaves, which led to their closer examination. In one species (Bomarea Carderi) the leaves are lanceolate, on short, flattened footstalks. Soon after they are expanded, and before they fall back into their places, the leaves twist over and expose the under surface to the light, so that the true under surface becomes, practically, the upper surface (fig. 25). The most strange circumstance connected with this reversal of the leaves, is the fact that the under surface of the leaf, as though prepared for the twisting, is smooth, and presents the usual characteristic epidermal cells of an upper surface, whereas the true upper surface, which by twisting becomes practically the under surface, is furnished with short obtuse hairs, such as might be expected to occur on the under surface of a leaf. In order that there might be no mistake in our interpretation of these facts, we requested Mr. W. S. Gilburt, who has devoted himself successfully to the study of the minute anatomy of plants, to examine and favour us with his opinion. Undoubtedly, he says, the entire structure of the leaf is reversed in order to fulfil the conditions of its reversed position. This seems to us quite an unique illustration of accommodation to circumstances. It still remains to us a puzzle why the leaves should thus reverse themselves. The plants were growing so that the leaves were constantly in contact with small objects, and if the twisting of the petiole was occasioned by effort at clasping, it must have exhibited some evidence, but not a single petiole had embraced anything, and all the leaves had turned over, topside under.
Fig. 25.—Leaf of Bomarea Carderi, the petiole twisted in the reversal of the leaf.
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The second class of climbing plants perform this act by means of the ordinary foliaceous organs, or by supplementary ones, which are often modifications of leaves. Those which climb by means of their leaves may do this by embracing the support with the footstalk, or by elongations of the midrib. The most familiar of leaf climbers is the traveller's joy or clematis, which belongs to a genus including many climbers, such as the splendid large-flowered kinds such great favourites in recent times. Some of the species of clematis retain the power of twining to a limited extent, sometimes in the direction of the sun, and with others in opposition to it. Not uncommonly the same twig will twine two or three times in one direction, then grow erect for a while, and afterwards twine again in the opposite direction. They must therefore be regarded as very inferior twiners. It would be expected, a priori, that with this twining power, the terminal joints also rotate, and this is the fact. In one species the quickest revolution was made in five hours and a half, in another in four hours and twenty minutes, in another in three hours and threequarters, and in another in one hour and fifty minutes.
The petioles, or leafstalks, are so far sensitive to the touch that after being rubbed, or otherwise irritated, they bend towards the point of irritation, and if a stick or twig presents itself in that direction, the leafstalk bends round, and embraces it. If no object is encountered by the bending petiole it soon straightens itself again. The petioles are most sensitive when young; in some species the older petioles lose their power of responding to irritation altogether. In one instance a fragment of thin cotton thread, weighing only one-sixteenth of a grain, caused a petiole to bend perceptibly.
"A thin stick placed so as to press lightly against a petiole (of Clematis flammula) having a leaflet a quarter of an inch in length, caused the petiole to bend in three hours and a quarter. In another case a petiole curled completely round a stick in twelve hours. These petioles were left curled for twenty-four hours, and the sticks were then removed, but they never straightened themselves. I took a twig thinner than the petiole itself, and with it lightly rubbed several petioles four times, up and down; these in an hour and three-quarters became slightly curled; the curvature increased during some hours, and then began to decrease, but after twenty-five hours from the time of rubbing, a vestige of the curvature remained. Some other petioles similarly rubbed twice, that is, once up and once down, became perceptibly curved in about two hours and a half. They became straight again in about twelve hours."
Fig. 26.—Traveller’s Joy (Clematis vitalba).
<!-- image -->When the petiole embraces a twig it swells perceptibly for two or three days, and ultimately becomes twice as thick as one which has embraced nothing. The same happens also in the case of other leaf-climbers. A section of such a swollen petiole, when examined under the microscope, exhibited an entire change of structure, whereby it had become more rigid and woody, simulating the structure of the stem. It would seem, therefore, that this change in the structure of the clasping petiole is one likely to be serviceable to the plant, by giving greater strength to the curved portion, and thus enabling it to hold more firmly to its support, and withstand greater shocks; in addition to which the greater thickness of the petiole would lessen its chance of being forcibly unwound again from the twig it had embraced. Several species of Tropaeolum presented somewhat similar phenomena in many respects. They climb also by means of the curvature of the petioles of the leaves. The petioles are in some species more sensitive than those of clematis. The slightest rub caused them to bend in about three minutes in one case, and in another species the petiole, after a slight rub, became curved in six, eight, ten, and in twenty minutes. It is not unusual to see the green fruit capsules of the common nasturtium in gardens bent over abruptly upon the stem, and even occasionally making a complete turn, or loop. This habit has been noticed also in other species.
Fig. 27.—Swollen petiole of Clematis vitalba.
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Two of the commonly cultivated climbing annuals are leaf-climbers. These are Maurandia Barclayana and Lophospermum scandens. No special feature necessary to be noted here was developed in the experiments on these plants, but they are mentioned chiefly on account of the facility with which they may be cultivated by those who may desire to repeat these observations for themselves, and trace all the phenomena of leaf-climbing.