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Ch. VIII.] FLOWERS FERTILIZED BY BIRDS.
hanging downwards, like an inverted candelabrum. From the centre of the circle of flowers is suspended a number of pitcher-like vessels, which, when the flowers expand, in February and March, are filled with a sweetish liquid. This liquid attracts insects, and the insects numerous
insectivorous birds, including the species I have mentioned and many kinds of humming-birds. The flowers are so disposed, with the stamens hanging downwards, that the birds, to get at the pitchers, must brush against them, and thus convey the pollen from one plant to another. A second species of Marcgravia that I found in the woods around Santo Domingo has the pitchers placed close to the pedicels of the flowers, so that the birds must approach them from above; and in this species the flowers are turned upwards, and the pollen is brushed off by the breasts of the birds. In temperate
latitudes we find many flowers fertilised by insects attracted by honey-bearing nectaries; and in tropical America not only bees, moths, and other large insects carry the pollen from one flower to another, but many flowers, like the Marcgravia, are specially adapted to secure the aid of small birds, particularly humming-birds, for this purpose. Amongst these, the “palosabre,” a species of Erythrina, a small tree, bearing red flowers,
that grew in this valley, near the brook, often drew my
FOXGLOVE AND HUMBLE-BEE.
the tube, where they are caught as if in a trap, their only way of exit being closed by the bill of the bird. Whilst the bird is probing the flower, the pollen of the stamens is rubbed in to the lower part of its head, and thus carried from one flower to fecundate another.. The bottom of the flower is covered externally with a thick, fleshy calyx—an effectual guard against the attempts of bees or wasps to break through to get at the honey. Humming-birds feed on minute insects, and the honey would only be wasted if larger ones could gain access to it, but in the flower of the palosabre this contingency is simply and completely guarded against.
Many flowers have contrivances for preventing useless insects from obtaining access to the nectaries. Amongst our English flowers there are scores of interesting examples, and I shall describe the fertilisation of one, the common foxglove, on account of the exceeding simplicity with which this object is effected, and to draw the attention of all lovers of nature to this other branch of subject on which the labours of Darwin and other naturalists have of late years thrown a flood of light. The pollen of the foxglove (Digitalis purpurea) is carried from one flower to another by the humble bee, who, far more than the hive bee, that “improves each shining hour," deserves to be considered the type of steady, persevering industry. It improves not only the hours of sunshine, but those of cloud, and even rain; ard, long before the honey-bee has ventured from its door, is at work bustling from flower to flower, its steady hum changing to an important squeak as it rifles the blossoms of their sweets. The racemes of purple bells held up by the foxglove are methodically visited by it, commencing at the bottom
flower, and ascending step by step to the highest. The four stamens and the pistil of the foxglove are laid closely against the upper side of the flower. First a stamen on one side opens its anthers and exposes its pollen. The humble-bee, as it bustles in, brushes this off. Then another stamen exposes its pollen on the other side, then another and another; but not till all the pollen has been brushed off does the cleft-end of the pistil open, and expose its viscid stigma. The humblebee brushes off the pollen into its hairy coat from the upper flowers of one raceme and carries it direct to the lowest flowers of another, where the viscid stigmas are open and ready to receive it. If the humble-bee went first to the upper flowers of the spike and proceeded downwards, the whole economy of this plant to procure cross fertilisation would be upset.* The open flower of the foxglove hangs downwards. The lower part, or dilated opening of the tube, is turned outwards, and has scattered stiff hairs distributed over its inner surface ; above these the inside of the flower hangs almost perpendicularly, and is smooth and pearly. The large humblebee bustles in with the greatest ease, and uses these hairs as footholds whilst he is sucking the honey; but the smaller bees are impeded by them, and when, having at last struggled through them, they reach the pearly, slippery precipice above, they are completely baffled. I passed the autumn
* Darwin mentions having seen humble bees visiting the flowering spikes of the Spiranthes autumnalis (ladies' tresses), and notices that they always commenced with the bottom flowers, and crawling spirally up sucked one flower after the other, and shows how this proceeding ensures the cross fertilisation of different plants.—“ Fertilisation of Orchids," 127.
SPIDERS THE PREY OF WASPS.
of 1857 in North Wales, where the foxglove was very abundant, and watched the flowers throughout the season, but only once saw a small bee reach the nectary, though many were seen trying in vain to do so.
Great attention has of late years been paid by naturalists to the wonderful contrivances amongst flowers to secure cross fertilisation, but the structure of many cannot, I believe, be understood, unless we take into consideration not only the beautiful adaptations for securing the services of the proper insect or bird, but also the contrivances for preventing insects that would not be useful from obtaining access to the nectar. Thus the immense length of the Angræcum sesquipedale of Madagascar might, perhaps, have been more easily explained by Mr. Wallace, if this important purpose had been taken into account.
The tramway in some parts was on raised ground, in others excavated in the bank side. In the cuttings the nearly perpendicular clay slopes were frequented by many kinds of wasps that excavated round holes of the diameter of their own bodies, and stored them with stingparalysed spiders, grasshoppers, or horse-flies. Amongst these they lay their eggs, and the white grubs that issue therefrom feed on the poor prisoners. I one day saw a small black and yellow banded wasp (Pompilus polistoides) hunting for spiders; it approached a web where a spider was stationed in the centre, made a dart towards it-apparently a feint to frighten the spider clear of its web; at any rate it had that effect, for it fell to the ground, and was immediately seized by the wasp, who stung it, then ran quickly backwards, dragging the spider after it, up a