cross-fertilisation is found may fitly conclude the more exact consideration of the present topic, and preface the abstract philosophy which directs attention to the bearing of the facts of fertilisation on the constitution and regulation of the world of life at large. A very interesting mechanism for effecting cross-fertilisation is seen in the case of flowers which, like the peas, beans, and their leguminous neighbours, present a very characteristic form of blossom. These flowers possess ten stamens (Fig. 25), nine united to form a bundle, and one remaining single. The flower is peculiar in that St it consists of one very large petal (Fig. 25, B, a), behind or above, two at the sides or "wings" (bb), and two united to form the boatshape "keel" (c) below. When an insect alights on the side petals or "wings" (Fig. 25, C, al), the keel (c) is thereby x pressed downwards, and the pollen of the stamens (st) and the tip of the pistil are al made to project, so as respectively to dust the insect's breast with pollen, and to receive therefrom the foreign pollen gathered previously from another flower. Sprengel himself noticed that the union. of the stamens favoured this conjoined action. When the weight of the insect's body is removed, the stamens and pistil resume their normal position. If, as Sir John Lubbock remarks, "the two ends of the wings (in a sweet-pea) be taken and thumb, and pressed down so as to imitate the effect VOL. CCLI. NO. 1812. 3 A ου FIG. 25 (c). ca between the finger produced by the pressure of an insect, the keel is depressed with the wings, while the pistil and stamens are thus partly uncovered." In the bean, when the wings are similarly pressed down, the stigma of the pistil, and then the "beard" of the style, laden with pollen, project from the keel, which is of coiled conformation. When a bee exercises the necessary pressure, the pistil of the bean will first strike its body and become fertilised by fresh pollen, whilst the pollen-laden style of the flower will, secondly, leave fertilising matter on the bee's body for application to the pistil of another flower. A dead-nettle (Fig. 21), with its irregular flower, presents a favourable and readily understood example of the manner in which a special form of flower is adapted for the special insect which cross-fertilises it. A bank of dead-nettles is to humble-bees what a country-fair is to juveniles, in that it presents the insects with a store of sweets specially intended for their delectation. In shape, the sage (Fig. 25, A) or dead-nettle flower, as everyone knows, exhibits a wide mouth, bounded by a very much arched upper lip, whilst a divided lower lip is also conspicuous enough. The green cup-like calyx has its sepals united, whilst the very irregularly shaped corolla is composed of united petals. There are four stamens-two long and two short-the fifth stamen of botanical expectation being abortive. The stamens are peculiar in position, inasmuch as they lie along the arch of the petals, instead of surrounding the pistil. The style is very long, and forked at its tip (sg), and it moreover depends below the anthers as in fuchsia (Fig. 24). The honey for which the bees visit the dead-nettle is situated far down within the flower, and if we make a vertical section of the corolla, we shall find a circle of hairs (Fig. 21, h) placed inside the petals at their lower portion. Now, in what special fashion is the mechanism thus described, brought into play in the fertilisation of the dead-nettle tribe? The reply may be found in a simple study of a dead-nettle on a warm summer's day, when insect-life and the blossoming of flowers together seem to attain the acme of activity and development. The bee approaches the flower, and finds in the lower lip of the blossom a convenient doormat on which to alight. Here the insect gains a point d'appui for the movement of the proboscis, which probes the depths of the corolla so as to reach the nectar, and easily thrusts aside the circlet of stiff hairs presenting an impassable barrier to a less robust as well as uninvited insect guest. The acts of the insect, in so far as the work of honeygetting is concerned, end thus. Meanwhile, however, it has likewise been performing its unconscious part in the fertilisation of the flower. The position of the stamens under the hooded petal has been noted. Such a position assures two results-firstly, that the stamens shall be brought in contact with the bee's body; and secondly, that the pistil shall likewise touch the insect in order that foreign pollen, obtained from a previously visited dead-nettle, shall be deposited on the stigma. The stigma, as we have seen, depends below the anthers. Hence it must be the first object with which the bee comes in contact. Fertilisation by the foreign pollen is thus secured before the stamens have dusted the insect with the flowers' own pollen. As Dr. Ogle has pointed out, the position of the stamens doubtless facilitates in a marked degree the proper placing of pollen on the insect's body. If the anthers had lain side by side, the bee's head might have been dusted on parts which do not touch the stigma as the insect enters the flower; whilst even the eyes of the bees might have become disadvantageously covered with pollen. There is, in short, the closest possible correlation between the structure of the flower and the form and size of the insect which fertilises it. Such correlation is exhibited in, if anything, an increased degree in the genus Salvia, belonging to the dead-nettle order (Labiata), also including the sage (Fig. 25). Salvia (Figs. 22, 23) attracted the notice of Sprengel-Rector at Spandau--who, in his "Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen" ("The Secrets of Nature in the Structure and Fertilisation of Flowers"), published in 1793, was one of the first to direct attention to the fact that nature's law was "cross" and not "self-fertilisation "—or, as he himself expressed it, "nature does not desire that any complete flower should be fertilised by its own pollen." It is interesting to note that a species of Salvia (S. splendens) occurring in the New World appears to be cross-fertilised through the agency of humming-birds; these fairy-like birds thus discharging in this case the functions of the insects to which some species approach so nearly in size. The trumpet creepers (Tecoma radicans) and trumpet honeysuckle (Lonicera sempervivens) are probably fertilised by moths and by humming-birds as well. In Salvia officinalis (Fig. 22, A), the general form of which closely resembles that of the dead-nettle, the stamens ripen before the pistil; and as, moreover, the stigma (p) is placed above the anthers (a), self-fertilisation is an impossibility. When, however, the stamens have shed their pollen, they shrivel up, and the pistil as it ripens developes the stigma, so that it elongates, curves downwards, and thus assumes a position (Fig. 23, B, st) in which it cannot escape contact with the back of the bee entering the flower (Fig. 23, A). The insect's back, it may be noted, is st a exactly that region which the ripe stamens in a younger and necessarily different flower will have dusted with foreign pollen. But the economy of Salvia includes yet other appliances for more effectually securing fertilisation by the insect. There are but two well-developed stamens (Fig. 22, B) in the flower. These organs have widely separated anther-cells; and when in an undisturbed condition, each stamen is seen to consist of a stalk (the filament) (f), to which another and movable stalk (the connective) (m), bearing an anther-cell at each end, is attached. Only one of these anther-cells (a') is fully developed in each stamen. The connective, like a swing-bar, can (m) be pushed backwards on its axis so as to bring the fully developed or upper anther-cell (a) to a horizontal position (Fig. 22, C). Such a result is actually brought about by the bee. Thrusting its head into the flower in the search for nectar, the insect pushes before it the FIG. 26. ORCHID FLOWER. lower end of the swing-bar, and thus brings the upper end of the bar with its ripe anther (a') in contact with its back (Fig. 23, A). This latter region is thus dusted with pollen, and when the insect flies to another Salvia flower in which the pistil is ripe, the stigma (Fig. 23, st), as we have seen, will in due course receive the pollen through contact with the back of the bee. A single paragraph only is permissible regarding the curious details connected with the fertilisation of the Orchids, which possess 1 1 flowers (Fig. 26) of markedly irregular shape. The lip (7) in such a flower as Orchis mascula, a common British species, is very broad; whilst the nectary to which bees desire admittance is extremely long (Fig. 28, n). The pollen forms two club-shaped masses (Figs. 27, 28 a), each adherent to a disc (Fig, 12, d), which in turn lies within the rostellum or cup (r). When touched, the rostellum breaks across, and thus allows the two glutinous discs (12, d) to become exposed. When a bee visits this peculiar flower, it pushes its proboscis into the nectary (n) for the sake of the honey contained therein. At the same time, the insect comes in contact with the discs of the pollen-masses (Fig. 27), these masses becoming adherent to the insect's head. A pencil pushed into an orchis detaches the pollen-masses after the fashion of the insect's unconscious act. At first, the pollen-masses remain erect like two abnormal horns on the insect's head; but gradually they assume a horizontal position, so that the insect cannot fail to charge the next orchid-pistil it enters with the pollen-masses. The stigma, or top of the pistil (Fig. 26, st, st), is so placed in these flowers that pollen-masses borne on a bee's head are certain to strike this surface, and thus fertilise the contents of the ovary. It is probable that as each pollen-mass consists of several packets of pollen-grains, one mass may contain material enough to fertilise several flowers; each stigma, through its viscid surface, detaching sufficient pollen from the mass for its fertilisation. The admirable adaptation of flower to insect and insect to flower, thus witnessed, is in no detail better exemplified than in the fact that the pollen-mass at first retains a vertical and then assumes a horizontal position in the insect's head. So long as the pollen-mass is vertical, fertilisation is impossible; and hence the vertical position persists so long as the bee is engaged in visiting the flowers of the plant from which it has derived pollenThus self-fertilisation is prevented; so that, as Sir Joseph Hooker puts it, by the time the horizontal position of the pollenmass is assumed, "the bee has visited all the flowers of the plant from which it took the pollen, and has gone to another plant." masses. To enter into further illustration of the contrivances through which the fertilisation of flowers is secured would be to encroach on the province of the technical and practical botanist. Such details are "writ large" in the pages of every botanical text-book. In the works of Mr. Darwin-and especially in the "Fertilisation of Orchids"-the reader anxious for further details may find a perfect encyclopædia of facts constituting a veritable romance of botanical science. It, however, remains to us in the present instance |