14 FLIES AND COLOURS. [CHAP. honey on glass slips placed over paper of other colours, yellow, orange, red, green, black, and white. Then I continually transposed the coloured paper, leaving the honey on the same spots; but the bee always flew to the blue paper, wherever it might be. Bees appear fortunately to prefer the same colours as we do. On the contrary, flowers of a livid yellow, or fleshy colour are most attractive to flies; and moreover while bees are attracted by odours which are also agreeable to us, flies, as might naturally be FIG. 7.-Malva sylvestris. FIG. 8.-Malva rotundifolia. expected from the habits of their larvæ, prefer some which to us seem anything but pleasant. Among other obvious evidences that the beauty of flowers is useful in consequence of its attracting insects, we may adduce those cases in which the transference of the pollen is effected in different manners in nearly allied plants, sometimes even in the same genus. Thus, as Dr. H. Müller has pointed out, Malva 1.] MALVA. EPILOBIUM. 15 sylvestris (Fig. 7) and Malva rotundifolia (Fig. 8) which grow in the same localities, and therefore must come into competition, are nevertheless nearly equally common. In Malva sylvestris, however (Fig. 9), where the branches of the stigma are so arranged that the plant cannot fertilise itself, the petals are large and conspicuous, so that the plant is visited by numerous insects; while in Malva rotundifolia, the flowers of which are comparatively small and rarely visited by insects, the st St FIG. 9.-Stamens and stigmas of FIG. 10. Ditto of Malva rotundifolia. branches of the stigma are elongated, and twine themselves (Fig. 10) among the stamens, so that the flower readily fertilises itself. Another interesting case is afforded by the genus Epilobium. Epilobium angustifolium has large purplish flowers in conspicuous heads (Fig. 11), and is much frequented by insects; while E. parviflorum (Fig. 12) has small solitary flowers and is seldom visited by insects. Now in the former species their visits are necessary, because the stamens ripen and 16 GERANIUM. [CHAP shed their pollen before the pistil, so that the flower is consequently incapable of fertilising itself. In E. parviflorum, on the contrary, the stamens and pistil come to maturity at the same time. Let us take another case—that of certain Geraniums. In G. pratense (Figs. 5 and 6, p. 8) all the stamens open, shed their pollen, and wither away before the pistil comes to maturity. The flower cannot therefore fertilise itself, and depends entirely on the visits of FIG. 11.-Epilobium angustifolium. FIG. 12.-Epilobium parviflorum. insects for the transference of the pollen. In G. pyrenaicum, where the flower is not quite so large, all the stamens ripen before the stigma, but the interval is shorter, and the stigma is mature before all the anthers have shed their pollen. It is therefore not absolutely dependent on insects. In G. molle, which has a still smaller flower, five of the stamens come to maturity before the stigma, but the last five ripen simultaneously with it. Lastly, in G. pusillum, which is least of all, the stigma ripens even before the stamens. Thus, then, we have a series more I.] SCROPHULARIA. 17 or less dependent on insects, from G. pratense to which they are necessary, to G. pusillum, which is quite independent of them; while the size of the corolla increases with the dependence on insects. In those species in which self-fertilisation is prevented by the circumstance that the stamens and pistil do not come to maturity at the same time, the stamens generally ripen first. The advantage of this is probably connected with the visits of bees. In those flowers which grow in bunches the lower ones naturally open first. Consequently in any given spike the flowers are at first all male; subsequently the lower ones, being the older, have arrived at the female stage, while the upper ones are still male. Now it is the habit of bees to begin with the lower flowers of a spike and work upwards. A bee, therefore, which has already dusted herself with pollen from another flower, first comes in contact with the female flowers, and dusts them with pollen, after which she receives a fresh supply from the upper male flowers, with which she flies to another plant. There are, however, some few species in which the pistil ripens before the stamens. One is our common Scrophularia nodosa. Now why is this? Mr. Wilson has given us the answer. S. nodosa is one of our few flowers specially visited by wasps; the honey being not pleasing to bees. Wasps, however, unlike bees, generally begin with the upper flowers and pass downwards, and consequently in wasp flowers it is an advantage that the pistil should ripen before the stamens. But though the stamens generally ripen before the pistil, the reverse sometimes occurs. Of this a very C 18 ARISTOLOCHIA-ARUM. [CHAP. interesting case is that of the genus Aristolochia. The flower is a long tube, with a narrow opening closed by stiff hairs which point backwards, so that it much resembles an ordinary eel-trap. Small flies enter the tube in search of honey, but from the direction of the hairs it is impossible for them to return. Thus they are imprisoned in the flower, until the stamens have ripened and shed their pollen, by which the flies get FIG. 13.-Diagrammatic section of Arum. , hairs; a, anthers; st, stigmas. thoroughly dusted. Then the hairs of the tube shrivel up, thus releasing the prisoners, which carry the pollen to another flower. Again, in our common Arums-the Lords and Ladies of village children—the well-known green leaf incloses a central pillar, near the base of which are arranged a number of stigmas (st, Fig. 13), and above |