Part 24

Should the reader desire to watch the growth of the pollen tubes, he can easily do so by shaking some pollen cells (preferably large ones, such as those of some lilies) on to a solution of sugar, and watching them at intervals with the aid of a lens. In the course of a few hours the pollen tubes will be seen to protrude, and these eventually grow to a considerable length.

In order that the ovules of a flower may develop into seeds, it is necessary that they become impregnated by pollen from the anthers of the same species, and this is brought about in the following manner: The pollen cells having been transferred by some means to the mature stigma, they adhere to the surface of the latter, and, deriving their nourishment from the secretion of the stigmatic cells, as above described, proceed to throw out their tubes. These tubes force their way between the cells of the stigma and style, and enter the ovary. Each tube then finds its way to one of the ovules, which it enters by means of a minute opening in its double coat called the micropyle, penetrates the embryo-sac, and reaches the ovum or egg-cell. The ovule is now impregnated or fertilised, and the result is that the ovum divides and subdivides into more and more cells till at last an embryo plant is built up. The ovule has thus become a seed, and its further development into a mature plant depends on its being transferred to a suitable soil, with proper conditions as to heat and moisture.

If the flower concerned is a perfect one, and the ovules are impregnated by pollen from its own anthers, it is said to be self-fertilised; but if the pollen cells that fertilise the ovules have been transferred from a distinct flower, it is said to be cross-fertilised.

Now, it has been observed that although self-fertilisation will give rise to satisfactory results in some instances, producing seeds which develop into strong offspring, cross-fertilisation will, as a rule, produce better seeds. In fact, self-fertilisation is not at all common among flowers, and the pollen has frequently no effect unless it has been transferred from another flower. In a few cases it has been found that the pollen even acts as a poison when it is deposited on the stigma of the same flower, causing it to shrivel up and die. In many instances the structure and growth of the flower is such that self-pollination is absolutely impossible; and where it is possible the seedlings resulting from the process are often very weak.

It has already been hinted that the wonderful variety of form and colour exhibited by flowers has some connexion with this important matter of the transfer of pollen, and the reader who is really interested in the investigation of the significance of this great diversity will find it a most charming study to search into the advantages (to the flower) of the different peculiarities presented, especially if he endeavours to confirm his conclusions by direct observations of the methods by which the pollen cells are distributed to the stigmas.

Pollen cells are usually distributed either by the agency of the wind or by insects; and it is generally easy to determine, by the nature of the flower itself, which is the method peculiar to its species.

A wind-pollinated flower is generally very inconspicuous. It produces no nectar, which forms the food of such a large number of insects, and has no gaudy perianth, nor does it emit any odour such as would be likely to attract these winged creatures. Its anthers generally shed an abundance of pollen, to compensate for the enormous loss naturally entailed in the wasteful process of wind-distribution, and the pollen is so loosely attached that it is carried away by the lightest breeze. Further, the anthers are never protected from the wind, but protrude well out of the flower; and the stigma or stigmas, which are also exposed, have a comparatively large area of sticky surface, and are often hairy or plumed in such a manner that they form effectual traps for the capture of the floating pollen cells.

An insect-pollinated flower, on the other hand, has glands (nectaries) for the production of nectar, and its perianth is usually of such a conspicuous nature that it serves as a signal to attract the insects to the feast. (In some instances the individual flowers are very small, but these are generally produced in such clusters that they become conspicuous through their number.) Often it emits a scent which assists in guiding the insects to their food. Its stamens are generally so well protected by the perianth that the pollen is not likely to be removed except by the insects that enter the flower; and the supply of pollen is usually not so abundant as in the wind-pollinated species, for the insects, travelling direct from flower to flower, convey the cells with greater economy. The stigmas, too, are generally smaller, and are situated in such a position that, when mature, they are rubbed by that portion of the insect's body which is already dusted with pollen.

As we watch the nectar-feeding insects at work, we not only observe that the flowers they visit possess the general characters given above as common to the insect-pollinated species, but also that, in many instances, the structure of the flower is such that the transfer of pollen from anthers to stigma could only be accomplished by the particular kind of insect which it feeds. Various contrivances are also adopted by many flowers to attract the insects which are most useful to them, and to exclude those species which would deprive them of nectar and pollen without aiding in the work of pollination. Thus, some flowers are best pollinated by the aid of certain nocturnal insects, which they attract at night by the expansion of their pale-coloured corollas and by the emission of fragrant perfumes. These close their petals by day in order to economise their stores and protect their parts from injury while their helpers are at rest. Others require the help of day-flying insects: these are expanded while their fertilisers are on the wing, and sleep throughout the night.

We do not propose to give detailed accounts of the various stratagems by which flowers secure the aid of insects in this short chapter. Several examples are given in connexion with the descriptions of flowers in subsequent pages, but a few typical instances, briefly outlined here, will give the reader some idea of features which should be observed as flowers are being examined.

In many flowers the anthers and the stigma are not mature at the same time, and consequently self-pollination is quite impossible. With these it often happens that the anthers and stigma alternately occupy the same position, so that the same part of the body of an insect which becomes dusted with pollen in one flower rubs against the stigma of another.

Other flowers, such as the Forget-me-not, in which both stamens and stigma are ripe together, project their stigmas above the stamens at first, in order that an insect from another flower might touch the stigma before it reaches the stamens, and thus cross-pollinate 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.

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.

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.