colouring matter dissolved in the cell-sap (pinks and blues) or to small coloured solid bodies (_chromoplasts_) contained in the cells (reds and yellows). Chlorophyll being absent, the coloured petals do not assist assimilation: they are purely advertisements, though incidentally they often fulfil a useful protective rôle for the important organs which they surround. In this latter connection their sensitiveness to changes of light and temperature, which causes them to close in dark or cold weather, is a very familiar phenomenon; as is also the excellent protection which they provide in flowers such as those of the _Labiatæ_, where, fused together into a tube, they form a kind of cave in which the stamens and pistil nestle securely. [Illustration: FIG. 24.--FRUIT OF CORIARIA JAPONICA. 1/1.] An exceptional use of petals, where indeed they are used for the purposes of advertisement, but to secure the dispersal not of the pollen, but of the seeds, is illustrated in Fig. 24. In the genus _Coriaria_ the staminate and pistillate organs are borne on separate flowers. The flowers of both kinds are small and inconspicuous. But in the “female” flowers the petals persist after flowering, and, becoming fleshy and comparatively large, enclose the seed in a pulpy berry-like envelope, which no doubt serves the same purpose as a true berry in securing seed-dispersal by being devoured by birds. In _C. terminalis_, which comes from the Himalayas, the “ripe” corolla is bright orange; in _C. japonica_, from Japan, it is at first coral-red, and when mature velvet-black. The _stamens_, which form the next ring (sometimes a double ring or a close spiral), are much less leaf-like than the sepals or petals, yet there can be no doubt that they are descended from leaf-shaped organs; this is especially clear from the study of certain primitive fossil types, in which the corresponding organs which bear the pollen are actually leaf-like. In most of the present-day Seed Plants the stamens conform to a uniform type--a slender stalk (_filament_) bearing a head (_anther_) containing four chambers, in which are produced _pollen grains_, which escape when the flower is mature by the splitting of the enclosing walls. The ways in which the pollen is then conveyed to the pistil of other flowers have been referred to briefly on a previous page (p. 82). The stamens in many flowers are few, and their number usually bears a relation to the number of the other floral parts; in other flowers, for instance Rose and St. John’s wort (_Hypericum_), they are of large and indefinite number. The peculiar arrangement of the pollen in Orchids has been already noted (p. 94).