Field and Woodland Plants. William S. Furneaux
them; and their stamens are afterwards raised by the lengthening of the corolla until they touch the stigma. Thus the flowers attempt to secure cross-pollination; but, failing this, pollinate themselves.
In the Common Arum or Cuckoo Pint, described on p. 106, we have an example of a flower of peculiar construction, surrounded by a very large bract in which insects are imprisoned and fed until the anthers are mature, and then set free in order that they might carry the pollen to another flower of which the stigmas are ripe.
Sometimes the flowers of the same species assume two or three different forms as far as the lengths of the stamens and pistils are concerned, the anthers of one being of just the same height as the stigma of another, so that the pollen from the former will dust that portion of the body of the insect which rubs against the latter Examples are to be found among the Primulas, and in the Purple Loosestrife, both of which are described in their place.
In some flowers the stamens are irritable, rising in such a manner as to strike the insects that visit them; and in these cases the anthers almost invariably deposit pollen on that portion of the insect's body which is most likely to come in contact with the stigma of the next flower visited. Again, in Sages, the anthers are so arranged that they are made to swing, as on a see-saw, to exactly the same end.
These few examples will suffice to show that the structure and conformation of flowers are subservient to the one great purpose of securing the most suitable means of the distribution of pollen, and the student who recognises and studies the various forms of flowers in this connexion will find his work in the field doubly interesting.
III
CLIMBING PLANTS
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