Field and Woodland Plants. William S. Furneaux
round which it can twine.
The rate at which the top of the stem revolves varies in different plants, and also in the same plant according to the temperature and other conditions affecting the growth. In some species the upper portion describes a complete circle in less than two hours during warm weather, while in others a single revolution may occupy one or two days.
It will be seen, from the nature of these movements, that the revolving stem is far more likely to come in contact with erect, rather than with horizontal supports, and observations made on twining stems will show that they seldom fix themselves round supports which are placed horizontally or only on a slight incline. In fact, some of these stems seem quite unable to twist themselves spirally except round an axis that is either erect or forms a very large angle with the horizontal plane.
Should the twining stem succeed in reaching a favourable prop, it immediately commences to bend itself round and round, forming a more or less compact spiral; and it is probable that the slight pressure, caused by the contact, acts as a stimulus which incites the peculiar mode of growth.
The direction which the spiral takes is not always the same. In the Hop, Honeysuckle, and the Climbing Buckwheat or Black Bindweed, the direction is always the same as that of the hands of a clock; while in the Bindweeds the spiral is invariably contra-clockwise. Further, it is not possible to compel any species to turn in a direction opposite to that which it naturally follows. Its stem may be forcibly twined in the wrong direction any number of times, but the free end will always follow its natural course as soon as it is left undisturbed.
Stem of the Bindweed, Twining to the Left.
Should the stem of a young twining plant fail to reach a suitable support, it bends over, not being sufficiently rigid to support itself, and at last the apex reaches the ground. Then, starting afresh from this second position of rest, it begins to ascend, and its upper end again commences to revolve as before. The chances are that it will, from this second position, find something round which it can twine; but failing this its summit may again and again bend to the ground, thus renewing its attempts from various positions more or less distant from one another, and in each effort so made the revolving upper end of the stem gradually lengthens, and describes a larger and larger circle in search for a favourable prop.
A twining stem sometimes has the advantage of additional support afforded by the stiff nature of the base of the stem, which is often rendered still more rigid by a twist or torsion resembling that of the strands of a rope. Such advantage is often still further increased by the presence of longitudinal ridges of the stem, frequently bearing rows of hooked prickles or hairs that hold on to any object touched. Again, the base of the stem, even though it reaches nothing round which it can twine, sometimes takes the form of a spiral, thus forming a good foundation for the upper portion as it seeks out a convenient prop. Yet another contrivance to secure the same end may be observed in the Greater Bindweed and some other plants. The stems, failing to secure a favourable hold, twine round one another, thus producing a kind of rigid cable for the support of the upper extremities as they revolve in order to find stems round which to form their spirals.
Should all the methods and contrivances of the twining plant fail it in its attempts to secure an uppermost place among the surrounding herbage or shrubs, it is compelled to trail along the ground. But such a position is most disadvantageous and unnatural to it, and usually results in a stunted and sickly plant that may produce no flowers.
Most of the twining plants of our country are of short duration. Many, like the Climbing Buckwheat, are annuals; while others, as the Hop and the Bindweeds, though they have perennial roots, produce fresh stems each season. The Honeysuckle and the Bittersweet, however, have perennial, woody stems which increase in thickness year by year, though the latter does not twine very much, and seems to take an intermediate place between the typical twiners and the plants which support themselves by merely interlacing their stems with the neighbouring plants or shrubs.
Stem of the Hop, Twining to the Right.
Some twining stems are unable to form their spirals round thick supports, and after making some attempt to do so grow off at a tangent to seek some less bulky prop. It has been observed, for instance, that the Hop cannot grasp a pole that is more than four inches in diameter.
In many cases, too, the spirals of the twining stem increase in diameter after they are first formed, and can thus adapt themselves to the increasing size of a living stem round which they have grown. The spirals of the Honeysuckle, however, do not increase in this way; and consequently, when they surround the trunk or branch of a young tree, the latter is constricted, often to such an extent that it is strangled and becomes stunted in its growth.
Another class of climbing plants cling to their surroundings by means of tendrils, which are modifications of leaves or shoots that grow spirally like the stems we have been considering.
Whatever be the origin of a tendril, it generally grows straight until it has reached some favourable support, and in order to obtain such support it performs circular movements similar to those of the tips of twining stems. Like these stems, too, the tendril is always sensitive, and forms a close spiral round the object it touches.
Some tendrils will grow spirally without ever touching a support, but these often become stunted and wither, while those which reach and embrace a stem or other structure are apparently incited to a luxuriant growth by the stimulating effect of the pressure produced.
When the tip of a tendril is successful in gripping a stem firmly, the portion behind it often takes part in the spiral movement, thus becoming shorter, and pulling the support towards its own plant in such a manner as to bring it within the reach of additional tendrils.
Of course the tendrilled plants have a much better chance of securing a suitable support than the twiners, for the latter have to depend on the searching and clinging powers of but one structure, while the tendrils are usually very numerous on the same plant, and throw themselves out in all directions in search of the required aid. The production of tendrils as a means of support is also much more economical than the method of clinging by a twining stem, for the former are usually very slender, while the latter must necessarily be sufficiently thick to convey the nutritive requirements of the whole plant; and thus the process of clinging by tendrils is more in accordance with the usual economy of Nature.
We have observed that twining stems can, as a rule, twine round only those supports which are erect or nearly so. This is not the case with tendrils, which are better adapted for twisting round horizontal stems and leafstalks. Often, too, they pass from one branch or leaf to another, and so secure the plant to which they belong by fastenings both above and below. Further, while the clasping part of a tendril often becomes hard and rigid, the portion between this and the plant may remain green and flexible. This latter portion also frequently forms a new spiral in the opposite direction, thus rendering the connexion between the plant and its support so supple and elastic that no damage is likely to accrue from the motions caused by the wind.
The tendrils which form long spirals are generally modified stems or leaves, or they may be elongated leaflets of a compound leaf. Those which are modified stems may be distinguished by their growth from the axils of the leaves, denoting that they had their origin in axillary buds after the manner of branches generally; and also, sometimes, by the fact that they bear imperfect leaves in the form of little scales. The tendrils of the Common or White Bryony (p. 96) are of this nature; while those of the Grape Vine are either modified floral stems or altered flower-stalks.
In some cases the entire leaf may be changed into a tendril, in which instance its true nature is revealed by the presence of a bud in its axil, as in many ordinary foliage leaves. More frequently, however, the 'leaf-tendril' is an altered leaflet of a compound leaf, such as we see in the Peas and Vetches; and it is interesting to note in such cases that the loss entailed by the conversion of leaflets into tendrils is often compensated for by the formation of leaf-like stipules which are capable of performing the function