Fig. 44. Alpine Butterwort
(Pinguicula alpina).
A, section through the flower.
K, calyx. bh, bristly prominences.
sp, spur. st, stamen.
n, stigma. B, stigma and
stamen more magnified.
After H. Müller.
Our birthwort (Aristolochia clematitis) and the Cuckoo-pint (Arum maculatum) are pit-fall flowers, whose long corolla-tubes have an enlargement at the base, in which both pistil and stamens are contained. In the birthwort (Fig. 43) the narrow entrance-tube is thickly beset with stiff hairs (A, b), whose points are all directed towards the base. Little flies can creep down quite comfortably into the basal expansion, but once there they are kept imprisoned until the flower, in consequence of the pollination of the stigma, begins to wither, the first parts to go being these very bristles (B, b´), whose points, like a fish-weir, prevented the flies from creeping out. Other 'fly-flowers,' as for instance the Alpine butterwort (Pinguicula alpina) (Fig. 44), securely imprison the plump fly as soon as it has succeeded in forcing itself in far enough to reach, with its short proboscis, the nectar contained in the spur (sp) of the corolla. The backward-directed bristles hold it fast for some time, and it is only by hard pressing with the back against the anthers (st) lying above it, and against the stigma (n), that it ultimately succeeds in getting free, but it never does so without having either loaded itself with pollen, or rubbed off on the stigma the pollen it brought with it from another similar flower. The Alpine butterwort is protogynous, that is to say, the pistil ripens first, the pollen later, so that the possibility of self-fertilization is altogether excluded.
It would be impossible to give even an approximate idea of the diversity of the contrivances for securing fertilization in flowers without spending many hours over them, for they are different in almost every flower, often widely so, and even in species of the same genus they are by no means always alike; for not infrequently one species is adapted to one circle of visitors, and its near relative to another. Thus the flower of the common Daphne (Daphne mezereum) (Fig. 45, A and C) is adapted to the visits of butterflies, bees, and hover-flies, while its nearest relative (Daphne striata) (Fig. 45, B and D) has a somewhat narrower and longer corolla-tube, so that only butterflies can feast upon it. This example shows that there are exclusively 'butterfly flowers,' but specialization goes further, for there are flowers adapted to diurnal and others to nocturnal Lepidoptera. The former have usually bright, often red colours, and a pleasant aromatic fragrance, and in all of them the nectar lies at the bottom of a very narrow corolla-tube. To this class belong, for instance, the species of pink, many orchids, such as Orchis ustulata, and Nigritella angustifolia of the Alps, which smells strongly of vanilla; also the beautiful campion (Lychnis diurna) and the Alpine primrose (Primula farinosa). The flowers adapted to nocturnal Lepidoptera are characterized by pale, often white colour, and a strong and pleasant smell, which only begins to stream out after sunset, and indeed many of these flowers are quite closed by day. This is the case with the large, white, scentless bindweed (Convolvulus sepium), which is chiefly visited and fertilized by the largest of our hawk-moths (Sphinx convolvuli). The pale soapwort (Saponaria officinalis) exhales a delicate fragrance which attracts the Sphingidæ from afar, and the sweet smell of the honeysuckle (Lonicera periclymenum) is well known, and has the same effect; an arbour of honeysuckle often attracts whole companies of our most beautiful Sphingidæ and Noctuidæ on warm June nights, to the great delight of the moth-collecting youth.
Fig. 45. Daphne mezereum (A and C) and Daphne striata (B and D). The former visited by butterflies, bees, and flies, the latter by butterflies only. A and B, vertical sections of the flowers. St, stamens. Gr, style. n, nectary. C and D, flowers seen from above. After H. Müller.
I cannot conclude this account of flower-adaptations without considering the orchids somewhat more in detail, for it is among them that we find the most far-reaching adaptations to the visits of insects. Among them, too, great diversity prevails, as is evident from the fact that Darwin devoted a whole book to the arrangements for fertilization in orchids, but the main features are very much the same in the majority. Figure 46 gives a representation of one of our commonest species (Orchis mascula), A shows the flower in side view, B as it appears from in front. The flower seems as it were to float on the end of the stalk (st), stretching out horizontally the spur (sp) which contains the nectar. Between the large, broad under lip (U), marked with a honey-guide (sm), and offering a convenient alighting surface, and the broad, cushion-like stigma (n) lies the entrance to the spur. Fertilization occurs in the following way:—The fly or bee, when it is in the act of pushing its proboscis into the nectar-containing spur, knocks with its head against the so-called rostellum (r), a little beak-like process at the base of the stamens (p). The pollen masses are of very peculiar construction, not falling to dust, but forming little stalked clubs, with the pollen grains glued together, and so arranged that they spring off when the rostellum is touched and attach themselves to the head of the insect, as at D on the pencil (Fig. 46). When the bee has sucked up the nectar out of the spur, and then proceeds to penetrate into another flower of the same species, the pollinia have bent downwards on its forehead (E), and must unfailingly come in contact with the stigma of the second flower, to which they now remain attached, and effect its fertilization. What a long chain of purposeful arrangements in a single flower, and no interpretation of them is available except through natural selection!
Fig. 46. Common Orchis (Orchis mascula). A, flower in side view. st, stalk. sp, spur with the nectary (n). ei, entrance to the spur. U, lower lip. B, flower from in front. p, pollinia. Sm, honey-guide. ei, entrance to the nectar. na, stigma. r, rostellum. U, lower lip. C, vertical section through the rostellum (r), pollinium (p). ei, entrance. D, the pollinia removed and standing erect on the tip of a lead-pencil. E, the same, somewhat later, curved downwards.
And how diversely are these again modified in the different genera and species of orchids, of which one is adapted to the visits of butterflies exclusively, as Orchis ustulata, another to those of bees, as Orchis morio, and a third to those of flies, as Ophrys muscifera. These flowers are adapted to insect visits in the minutest details of the form of the petals, which are smooth, as if polished with wax, where insects are not intended to creep, but velvety or hairy where the path leads to the nectar, and at the same time to the pollen and the stigma. And then there is the diversity in the form and colour of the 'honey-guides' on the 'alighting surface,' that is, the under lip of the flower, upon which the insect sits and holds fast, while it pushes its head as far as possible into the spur, so that its proboscis may reach the nectar lying deep within it! Even though we cannot pretend to guess at the significance of every curve and colour-spot in one of the great tropical orchids, such as Stanhopea tigrina, yet we may believe, with Sprengel, that all this has its significance, or has had it for the ancestors of the plant in question, and in fact that the flower is made up of nothing but adaptations, either actual or inherited from its ancestors, although sometimes perhaps no longer of functional importance.