A very remarkable instance of a different kind is supplied by the relations existing between the American species of Yucca and the small white-winged moths of the genus Pronuba. The following succinct account is given by Professor G. H. Carpenter:[7] “The female of these moths has not only the palps of the first maxillæ developed, but the region of the maxillæ (palpiger) whence they spring produced into a pair of long, flexible, hairy processes. By means of these she collects from the anthers pollen, which she deliberately carries to the stigma to ensure fertilization. With her piercing ovipositor—a most abnormal development among moths—she bores through the tissue of the pistil, and by means of the flexible egg-tube, protrusible beyond the ovipositor, lays her eggs close to the ovules of the Yucca. The caterpillar when hatched feeds on the growing seed of the plant, which would never develop were it not for the action of the Pronuba moth. This action is most wonderful, in that the moth herself gets no benefit from it. Her food canal is degenerate, and her jaws, useless for sucking, are devoted altogether to the gathering of the pollen; she does not feed in the perfect state. Doubtless her ancestors did so, and were first attracted to the Yucca in search of honey, though the act of pollination is now performed only for the sake of the offspring.”
Among certain lower animals and plants symbiotic connection is often most intimate. For instance, in the body-wall of certain Sea Anemones and Holothurians there are small green cells which were long believed to be part of the animal, and which puzzled naturalists because they contained chlorophyll, that remarkable green substance characteristic of plants, which gives to them the power of forming food out of its raw inorganic materials. These cells are now known to be minute seaweeds (Algæ), which spend their lives in the animal tissues to the benefit of both organisms. The plant, by virtue of its chlorophyll, absorbs carbon dioxide, decomposes it, and gives out oxygen, which is eagerly seized on by the animal. The animal in its turn liberates carbon dioxide, which is required by the plant. Similar relations exist between Algæ and some of the lowly Radiolarians and Foraminifera; in these cases, the animals being very minute, the plant partner plays a more conspicuous rôle. It is noteworthy that these Algæ are quite capable of living and multiplying separately, free from the body of the animals, and the animals also are capable of pursuing an independent existence.
Let us turn now to the relations existing between flowers and insects, which form one of the most picturesque and romantic features of field life, and of which the materials for study and observation are ever at our own doors. What is a flower? A flower is a group of modified leaves set apart for the business of sexual reproduction. The essential parts or sporophylls are of two kinds, which may be borne on the same flower or on separate flowers on one plant, or on separate plants. These are the stamens, bearing pollen grains (or microspores), from which male cells arise; and carpels, which contain ovules, each enclosing an embryo sac or megaspore, in which is an ovum or female cell.
Each stamen consists usually of a slender stalk, the filament, bearing an oblong head, the anther, which contains four chambers, or pollen sacs, filled with pollen grains; these, when mature, escape into the air by the rupturing of the walls of the chambers.
Each carpel contains in its lower part an ovary, while its upper part presents to the air a surface charged with nutrient substance, the stigma, which is often raised on a slender stalk, the style.
To secure the production of seed, the first necessary step is pollination, or the transfer of pollen from the stamen to the stigma. When this is effected—the means will be considered immediately—and a pollen grain alights on the surface of the stigma, which is usually sticky or hairy to aid its retention there, the pollen grain commences growth, and sends out a slender tube (the pollen tube), which pursues its way through the substance of the stigma, down the style, into the ovary, and from its tip a male cell passes out and fuses with the ovum. In most flowers the pollen tube is not called on to make any great effort of growth, the distance between stigma and ovary being very small; but occasionally, as in Crocus and Lily, this may amount to half a foot. The result of this act of fertilization is that the ovum and ovule grow, the former forming eventually the embryo, or young plant, the latter the seed in which the embryo is enclosed. In order that fertile seed may be produced it is often necessary, and usually desirable, that the pollen which reaches the stigma should not belong to the same flower, but to a different flower of the same species; cross-pollination being the rule among seed plants, self-pollination the exception. To secure the former, and to avoid the latter, many highly interesting devices are found, materially affecting the structure and development of flowers.
The essential parts of a flower, then, consist of stamens and carpels. Flowers consisting of no other parts but either or both of these are not common, but we may compare, for example, the rarely produced flowers of the Duckweeds (Lemna), in which a tiny group of two stamens and a carpel represents one flower, or, according to some views, a group of three flowers. More commonly the flower is much more composite, consisting mostly of four sets of organs, arranged in whorls or rings, or more rarely in close spirals. In the centre is a group of carpels; outside them—in other words, slightly lower on the stem—a ring, or two rings, of stamens, few or many; then a ring of petals, forming the corolla, usually coloured, leaf-like, and conspicuous; and outside of them a ring of sepals, forming the calyx, generally green and leaf-like. The main function of the calyx is protective; it encloses the essential organs and guards them till they are mature, when the flower opens and stamen and stigma play their parts. The calyx is usually tough, and often covered with hairs, or with a sticky substance, to keep the flower safe and ward off the attacks of insects or other small devourers. If we turn to the corolla we find a singular variety of size, form, and colour. To account for this, it is necessary to consider the means by which pollen is distributed. There are two chief ways in which pollen is conveyed from flower to flower—by means of the wind, and by means of flying insects. If we examine wind-pollinated flowers, such as Hazel (Corylus), Scotch Fir (Pinus), or Reed-mace (Typha), we note the small size of the flowers and the great abundance of pollen. Compare these with insect-fertilized flowers, such as Buttercup (Ranunculus), Flax (Linum), Snapdragon (Antirrhinum), or one of the Orchids. In these the flowers are much larger owing to the increased size of the petals, which are of brilliant colour and of various shape. Pollen is mostly much reduced in quantity, since insects flying direct from flower to flower afford a far more economical mode of distribution than is offered by the wind. The pollen grains, moreover, are sticky and covered with tiny spines or knobs, to render them more liable to adhere to the body or head of an insect; the pollen grains of wind-fertilized flowers being, on the other hand, smooth, dry, and dust-like. Again, these insect-pollinated flowers usually possess little glands which secrete nectar, the sugary syrup which by digestion in a bee’s body becomes honey. Here, then, is the inter-relation established: the insect helps the plant by carrying its pollen from flower to flower, and in its turn is helped by the provision of delicious food. And what about the showy petals, and the fragrance that so often marks these entomophilous flowers? They are advertisements, designed to catch the attention of the necessary insects as they fly about. Not only does the corolla by its bright colour attract insects, but markings of various shapes and tints upon the petals are generally held to be honey-guides—sign-posts directing the insects to the nectar and to the pollen. These are especially conspicuous in many of the irregular flowers to which reference will be made shortly, in which the insects are encouraged to approach the flowers in a particular way. An example
Fig. 14.—Flower of Erodium petræum. 2/1.