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