Читать книгу Field and Woodland Plants - William S. Furneaux - Страница 16

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Many plants have stems which grow to a considerable length, and which are at the same time too weak to support the plants in the erect position. A considerable number of these show no tendency to assume an upward direction, but simply trail along the surface of the ground, often producing root fibres at their nodes to give them a firmer hold on the soil and to absorb additional supplies of water and mineral food. Some, however, grow in the midst of the shrubs and tall herbage of thickets and hedgerows, or in some other position in which it becomes necessary to strive for a due proportion of light, and such plants would stand but a small chance in the struggle for existence if they did not develop some means of securing a favourable position among their competitors.

These latter are collectively spoken of as climbing plants; but it is interesting to note that in their seedling stage they are all erect, and it is only after they reach a certain height that they commence to assume some definite habit by which they obtain the necessary support, or to develop special organs by which they can cling to objects near them.

Some climbers produce no special organs for the purpose of fastening themselves to surrounding objects, but trust entirely to the wandering and more or less zig-zag nature of their feeble stems, and thus reach the open light merely by a process of interweaving, as in the case of the Hedge Bedstraw (Galium mollugo). Others adopt this same method of interweaving, but at the same time develop some kind of appendages to give them additional support. Thus, the Rough Water Bedstraw (G. uliginosum), which sometimes reaches a height of four or five feet, has recurved bristles all along its slender stem, and these serve as so many little hooks, holding the plant securely on to the neighbouring rank herbage of the marsh or swamp in which it grows, while the rigid leaves further assist by catching in the angles of surrounding stems.

Another good example is to be seen in the common Goose-grass or Cleavers (G. aparine) of our hedgerows, which also reaches a height of four or five feet, and clings very effectually by means of the hooked bristles of its stems and leaves.

The Marsh Speedwell (Veronica scutellata), though it grows to a height of only one foot, is too weak to stand erect without support, and it has quite a novel method of securing the aid of the plants among which it grows. Its two topmost leaves at first stand erect over the terminal bud, so that they are easily pushed through the spaces in the surrounding herbage as the stem lengthens. They then diverge, and even turn slightly downwards, thus forming two supporting arms, the holding power of which is further increased by the down-turned teeth of their margins. This process is repeated by the new pairs of leaves formed at the growing summit of the stem, with the result that the plant easily retains the erect position.

Prickles of the Wild Rose.

The Wild Roses and Brambles growing in the hedgerows support themselves among the other shrubby growths by the interlacing of their stems, but are also greatly aided by the abundance of prickles with which these stems are armed. The prickles, even if erect, would afford considerable assistance in this respect; but it may be observed that they are generally directed downwards, and often very distinctly curved in this direction, and so serve to suspend the weak stems at numerous points.

We often find the Bramble growing in abundance on heaths and downs, in situations where suitable props do not exist. In this case the younger shrubs simply trail along the ground, or form low arches as the weight of the stems and their appendages cause the apex to bend to the ground. Yet if we turn to the older shrubs of several years' growth we find that they have succeeded in reaching a height of some feet. The first stems of these shrubs formed low arches as we have just described, and then they gave rise to branches which were first erect, but were afterwards bent downwards in the same manner, forming arches rising higher than their predecessors. This continued, year after year, till at last a long series of stems, forming arch above arch, reached the present height, the older stems, at the bottom, now dead, serving to support the whole mass above.

Ivy, Showing the Rootlets or Suckers.

Some climbing stems produce little roots by means of which they can cling firmly to available supports. Such are very common among tropical plants, but our Ivy affords a splendid example. The roots so formed may appear in clusters at special points of the stem, or in long lines running longitudinally on it, and they are produced on trailers as well as on climbers. In fact, we can draw no fine distinction between the former and the latter in this respect, and even the Ivy will sometimes trail along the ground after the manner of the Periwinkle, which roots itself at several points as it proceeds.

The rootlets of the Ivy and other climbers of the same habit always avoid the light; and if they are not originally formed on the side of the stem facing the supporting surface, they soon turn towards the latter, and give rise to little clinging suckers that firmly adhere. If they come in contact with a bare rock, or with a surface from which no nutriment can be derived, they serve the one purpose of clinging only; but if they reach even a small amount of nutritive soil, they produce absorbent fibres that are capable of extracting food.

The ivy usually clings to the bark of trees or to old walls, the crevices of which often contain some small amount of transported soil, or more or less organic soil formed by the growth and decay of low forms of vegetable life; and thus the tree is enabled to obtain a little food from the objects that give it the necessary mechanical support.

The well-known Virginian Creeper (Ampelopsis) produces rootlets by means of which it can cling to very smooth surfaces. Its light-avoiding 'tendrils' always turn to the wall or other supporting body; and, on coming in contact with it, give off little branches which diverge like the toes of the tree-frog, and produce little adhesive discs which hold on firmly by the aid of a sticky secretion.

Perhaps the most interesting of all climbing plants are those which twine their stems around the props afforded by the neighbouring growths. As before stated, the stems of these plants are erect when very young; but after they have reached a certain height the top of the stem bends to one side, and then, as the growth proceeds, it turns slowly round and round, describing a circle in the horizontal plane, thus seeking some support 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 of leaves. In fact, we often find that the size of the stipules is proportional to the number of tendrils produced; and that when the leaflets are considerably reduced in number by their conversion into tendrils, not only are the stipules large and leafy, but the stem itself may be extended laterally into broad wing-like expansions which do the work of foliage leaves.

Interesting illustrations of this are to be found in the Yellow Vetch—a rather rare plant sometimes seen in sandy fields—in which all the leaves are converted entirely into tendrils, and their function performed by very large leafy stipules; also in the Narrow-leaved Everlasting Pea of bushy places, in which the leaflets of the compound leaves are all converted into tendrils with the exception of two, the work of which is aided by the stipules and by the 'wings' of the stem and petioles. In the Rough-podded Vetch, too, the stems and petioles are winged to serve the same end; and other British members of this genus have either large stipules or winged stems, or both, to compensate for the loss of leaflets that have been modified into tendrils.

In other climbers the blade of the leaf is not reduced in size, even though the leaf serves the purpose of a tendril, the function of clinging being assigned exclusively to the petiole or leaf-stalk. This may be observed in the Wild Clematis and the Bryony, in both of which the petiole forms a ring round any branch or stem with which it comes in contact. These petioles are apparently equally sensitive on all sides, and are therefore ready to cling to any available support, whether above or below. In the Clematis the leaves are at first at right angles to the stem of the plant, but they afterwards turn downwards, and thus transform themselves into so many anchors which give additional aid in supporting the climber among the other hedgerow plants and shrubs.