This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
The flowers belonging to the genus Ophrys are formed somewhat on the same plan as those of Orchis, but they have no spur, and the rostellum is double. The Bee orchis (O. apiferei), Fig. 124, however, differs widely from the other allied British species. The two pouch-formed rostellums, the viscid disk, and the position of the stigma, are nearly the same, but the stalks of the pollen masses are long, thin, flexible, and too weak to stand upright. The distance of the pollen masses from one another, and the shape of the pollen grains is moreover variable. The anther cells open soon after the flower expands, and the pear-shaped pollen masses drop out, so as to hang directly over the stigma, with which a breath of air is sufficient to bring them in contact. While therefore in most species of Orchis and Ophrys, self-fertilisation appears to be impossible, in the Bee Ophrys, as R. Brown long ago pointed out (Trans. Linn. Soc., v. xvi.) it is carefully provided for. Darwin has examined hundreds of flowers, and has never seen reason in a single instance to believe that pollen had been brought from one flower to another; and he has met with very few cases in which the pollen mass failed to reach its own stigma. He has never seen an insect visit the flowers of this species, and R. Brown suggested that the resemblance of the flower to bees was to deter insects from visiting them. Darwin does not think this probable. He believes also that, though this species habitually fertilises itself, the curious arrangements which it possesses in common with other allied species, are of use in securing an occasional cross, even if only at very long intervals.
Fig. 124. - Ophrys apifera.
Ophrys arachnites is by some botanists (for instance by Bentham) regarded as a mere variety of O. apifera, but the stalks of the pollen masses are not much more than half as long, without any diminution of thickness. In proportion, therefore, and in their stiffness, they more nearly resemble the other section of the group. Mr. Moggridge, however, has found at Mentone intermediate forms, not only between O. arachnites and O. apifera, but also between these, O. aranifera and O. Scolopax. O. aracJinites and O. apifera do not in England appear liable to pass into one another.
In the Musk orchis (Herminium monorchis), the stalks of the pollen masses are short, and the disks large. This species does not produce honey, but has a strong odour, especially at night.
Habenaria chlorantlia (the Large Butterfly orchis) has both a sweet scent and honey. It is much frequented by insects. The anther cells are widely separated; the pollinia slope backwards, and are much elongated; the viscid disk is circular, prolonged on its imbedded side into a short, drum-like pedicel.
When exposed to the air this drum contracts on one side, and alters the direction of the pollen mass, thus bringing it (as in Orchis mascula) into such a position that it comes in contact with the stigmatic surface of the flower to which it is carried.
Habenaria bifolia (the Lesser Butterfly Orchis) is by Bentham and other high authorities, considered as a mere variety. Yet, as Darwin points out, it differs in many important particulars. The viscid disks are oval; the viscid matter itself is of somewhat different character; the drum-like pedicel is rudimentary; the stalk of the pollen mass is much shorter; the packets of pollen shorter and whiter; and the stigmatic surface more distinctly tripartite.
The genus Cephalanthera (Fig. 125, Cephalanthsra grandiflora) differs from those hitherto described in not possessing a rostellum, and in having the pollen grains single. The flower stands upright, and the labellum is formed of two portions; a base, and a small triangular flap, which at first closes the tube; then turns back, thus forming a small landing place in front of a triangular door, situated half way up the tube: and lastly rises up again and closes the entrance. The pollen mass is situated just above the stigma; and while the flower is in bud, or at any rate before it becomes quite open, the pollen grains which rest on the sharp edge of the stigma, emit a number of tubes which deeply penetrate the stigmatic tissue. These serve partially, but, as Darwin has shown, only partially, to fertilise the flower; he suggests that the principal use of this closing of the flower and emission of the pollen tubes is probably to retain the pollen, which would otherwise fall out of the flower. In this curious manner, however, they are retained in a proper position until the flower is visited by insects, to which they readily adhere; and which are necessary to ensure the perfect fertility of the plant.
Fig. 125. - Cephalanthera grandiflora
Fig. 126. - Listera ovata.
Listera ovata (the Twayblade, Fig. 126) has been carefully described by Sprengel, by whom the structure and action of the rostellum was, however, misunderstood, and by Dr. Hooker (Philosophical Transactions, 1854), who described the flower with accuracy and minuteness; but the relations of the flower to insects, and consequently the true functions of the various parts, were first perceived by Waechter. The pollen masses lie immediately above the rostellum; the pollen is friable and would not of itself adhere to insects, but this is effected by a very remarkable contrivance (see Hildebrand, p. 53). The moment the summit of the rostellum is touched it expels a large drop of viscid fluid, which glues the pollen to the insect or other body. A very slight touch, even for instance with a human hair, is sufficient to produce this remarkable phenomenon. This species is exclusively visited by ichneumons.
Neottia nidus avis (the Bird's Nest Orchis) agrees in the essential points with Listera, though in the position of the honey, etc, it offers minor differences.
Cypripedium (the Ladies' Slipper, Figs. 127 and 128, C. longifolium), the lower lip has the form of a slipper, whence the name. This genus has two fertile anthers, which are rudimentary in other Orchids, while the one which is present in them is represented by a singular shield-like body. The opening into the slipper is small, and partly closed by the stigma and this shield-like body, which lies between the other two anthers. The result is that the opening into the slipper has a horseshoe-like form, and that bees or other insects which have once entered the slipper (Figs. 127-8) have some difficulty in getting out again. While endeavouring to do so they can hardly fail to come in contact with the stigma, which lies under the shield-like representative of the middle anther. As the margins of the lip are inflected (Figs. 127 - 8q), the easiest exit is at the two ends of the horseshoe, and by one or other of these (Fig. 127 e) the insect generally escapes, in doing which, however, it almost inevitably comes in contact with, and carries off some of the pollen, from the corresponding anther. The pollen of this genus is immersed in a viscid fluid, by means of which it adheres firstly to the insect, and secondly to the stigma, while in most Orchids it is the stigma which is viscid. In a Trinidad species, Coryanthes macrantha, according to Dr. Cruger, the basal part of the lip forms a bucket, which secretes a copious fluid which wets the wings of the bees, and by rendering them temporarily incapable of flight, compels them to creep out through the small passages close to the anther and stigma; thus securing, though by different means, the object which in Cypri-pedium is effected by the inflected margins of the labellum. (Jour. Linn. Soc, 1864.)
Fig. 127. - Flower of Cypripediiun lon-gifolium. Front view.
Fig. 128. - Ditto. Seen from the side.
Such are a few of the remarkable contrivances existing among British Orchids. I must refer those who wish for more detailed information, to Mr. Darwin's charming work.
Although I have thought it well to confine myself for the most part to illustrations taken from our common wild flowers, I cannot resist mentioning the case of Catasetum, one of the Vandeas, which as Mr. Darwin says, are "the most remarkable of all Orchids." In Catasetum, the pollinia and the stig-matic surfaces are in different flowers, hence it is certain that the former must be carried to the latter by the agency of insects. The pollinia moreover are furnished with a viscid disk, as in Orchis, but the insect has no inducement to approach, and in fact does not touch, the viscid disk. The flower, however, is endowed with a peculiar sensitiveness, and actually throws the pollinium at the insect. Mr. Darwin has been so good as to irritate one of these flowers in my presence: the pollinium was thrown nearly three feet, when it struck and adhered to the pane of a window.
Fig. 129 - Side view of Catasetum saccatum, with all the sepals and petals removed except the labellum.
This irritability, however, is confined to certain parts of the flower. Fig. 129 represents a male flower of Catasetum saccatum, which is also shown in section in Fig. 130. In this figure it will be seen that the pollinium (ped) is curved and in a state of considerable tension, but retained in that position by a delicate membrane. Now insects alight as usual on the lip of the flower (l), and it will be seen that in front of it are two long processes called antennae (a n). In some species of Catasetum both these antennae are highly irritable; in the present species the right-hand one is apparently functionless; but the moment the insect touches the left-hand one, the excitement is conveyed along it, the membrane retaining the polli-nium is ruptured, and the latter is immediately jerked out of the flower, by its own elasticity, with considerable force, with the viscid disk (d) foremost, and in such a direction as to come in contact with the head of the insect which had touched the antenna. On subsequently visiting a female flower the insect brings the pollen into contact with the stigma.
Fig. 130. - Section of ditto, with all the parts a little expanded.