this plant, however; is a most feeble twiner.Loasa aurantiaca (Leon, p.351) offers a much more curious case: I raised seventeen plants: of these eight revolved in opposition to the sun and ascended from left to right; five followed the sun and ascended from right to left; and four revolved and twined first in one direction, and then reversed their course, the petioles of the opposite leaves affording a point d'appui for the reversal of the spire.One of these four plants made seven spiral turns from right to left, and five turns from left to right.Another plant in the same family, the Scyphanthus elegans, habitually twines in this same manner.I raised many plants of it, and the stems of all took one turn, or occasionally two or even three turns in one direction, and then, ascending for a short space straight, reversed their course and took one or two turns in an opposite direction.The reversal of the curvature occurred at any point in the stem, even in the middle of an internode.Had I not seen this case, I should have thought its occurrence most improbable.It would be hardly possible with any plant which ascended above a few feet in height, or which lived in an exposed situation; for the stem could be pulled away easily from its support, with but little unwinding; nor could it have adhered at all, had not the internodes soon become moderately rigid.With leaf-climbers, as we shall soon see, analogous cases frequently occur; but these present no difficulty, as the stem is secured by the clasping petioles.
In the many other revolving and twining plants observed by me, Inever but twice saw the movement reversed; once, and only for a short space, in Ipomoea jucunda; but frequently with Hibbertia dentata.
This plant at first perplexed me much, for I continually observed its long and flexible shoots, evidently well fitted for twining, make a whole, or half, or quarter circle in one direction and then in an opposite direction; consequently, when I placed the shoots near thin or thick sticks, or perpendicularly stretched string, they seemed as if constantly trying to ascend, but always failed.I then surrounded the plant with a mass of branched twigs; the shoots ascended, and passed through them, but several came out laterally, and their depending extremities seldom turned upwards as is usual with twining plants.Finally, I surrounded a second plant with many thin upright sticks, and placed it near the first one with twigs; and now both had got what they liked, for they twined up the parallel sticks, sometimes winding round one and sometimes round several; and the shoots travelled laterally from one to the other pot; but as the plants grew older, some of the shoots twined regularly up thin upright sticks.Though the revolving movement was sometimes in one direction and sometimes in the other, the twining was invariably from left to right; so that the more potent or persistent movement of revolution must have been in opposition to the course of the sun.It would appear that this Hibbertia is adapted both to ascend by twining, and to ramble laterally through the thick Australian scrub.
I have described the above case in some detail, because, as far as Ihave seen, it is rare to find any special adaptations with twining plants, in which respect they differ much from the more highly organized tendril-bearers.The Solanum dulcamara, as we shall presently see, can twine only round stems which are both thin and flexible.Most twining plants are adapted to ascend supports of moderate though of different thicknesses.Our English twiners, as far as I have seen, never twine round trees, excepting the honeysuckle (Lonicera periclymenum), which I have observed twining up a young beech-tree nearly 4.5 inches in diameter.Mohl (p.134)found that the Phaseolus multiflorus and Ipomoea purpurea could not, when placed in a room with the light entering on one side, twine round sticks between 3 and 4 inches in diameter; for this interfered, in a manner presently to be explained, with the revolving movement.
In the open air, however, the Phaseolus twined round a support of the above thickness, but failed in twining round one 9 inches in diameter.Nevertheless, some twiners of the warmer temperate regions can manage this latter degree of thickness; for I hear from Dr.
Hooker that at Kew the Ruscus androgynus has ascended a column 9inches in diameter; and although a Wistaria grown by me in a small pot tried in vain for weeks to get round a post between 5 and 6inches in thickness, yet at Kew a plant ascended a trunk above 6inches in diameter.The tropical twiners, on the other hand, can ascend thicker trees; I hear from Drs.Thomson and Hooker that this is the case with the Butea parviflora, one of the Menispermaceae, and with some Dalbergias and other Leguminosae. This power would be necessary for any species which had to ascend by twining the large trees of a tropical forest; otherwise they would hardly ever be able to reach the light.In our temperate countries it would be injurious to the twining plants which die down every year if they were enabled to twine round trunks of trees, for they could not grow tall enough in a single season to reach the summit and gain the light.