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Orchid Fertilisation

The great source of information on this, as on so many other important and interesting questions in Natural Science, is to be found in the writings of Charles Darwin, who has written, upon this subject, the well-known work "On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects." This book must always be referred to by those who wish to understand the very curious structures that adapt many Orchids, in a very peculiar degree, to benefit by the visits of insects, while a smaller number are adapted for self-fertilisation alone. Not a little information is contained also in the works of Dr. Hermann Muller (of the chief of which a translation has been published, entitled "The Fertilisation of Flowers"), of Professor Asa Gray, of Delpino, and of many others, whom it is impossible here even to enumerate.

An endeavour will be made here to bring together some of the more interesting results arrived at by the various writers, selecting a few of the more characteristic examples from the species native in Britain, and thereafter giving a short account of some exotic species that exhibit peculiarities not met with in British Orchids. The flowers of Orchids in general are very curious in form and arrangements of their parts. This is well seen in the many figures of Orchids that have appeared in this dictionary (see Aceras, Aerides, Calanthe, Cattleya, Chysis, Coelogyne, Cymbidium, Cypripedium, Dendrobium, Epidendrum, Masdevallia, Maxillaria, and Miltonia).

But, despite this diversity, it is never difficult to recognise, that each flower is supported on a stalk, twisted half round, and that, when cut across, there are very many ovules visible in the stalk, and that this part really encloses the ovary, which is, therefore, inferior, and consists of three carpels, joined usually along their edges only. The flowers show six parts externally, viz., three sepals, much alike, and three petals, of which the lower one, in the mature flower, is almost always larger, broader, and more handsomely coloured than the others, and is called the labellum. It is often peculiar in form also, and very frequently it ends in a spur, which projects backwards, as the nectary. This latter, frequently, does not exude nectar into the tube, but contains it in the thin cells lining the latter; the walls of these cells are so thin as to allow insects (even moths) to pierce them readily, so as to obtain the juice. The tube is often nearly absent, but it is often of great length, and permits the honey to be secured only by insects with long probosces (such as some moths and butterflies possess), or by boring holes from without, as humble bees often do. This latter method, of course, does not benefit the flower at all. The essential, or reproductive, organs in the flower, consist of the stigmas and the fertile stamen, or, in Cypripedieae, the two fertile stamens, and the remains of two stamens, or of one stamen reduced to the state of a staminode, i.e., of a stamen no longer of use in forming pollen, but modified to perform some other function for the welfare of the plant. The relation of the larger parts of the flower to one another, will be more easily understood from the accompanying woodcut of the flower of Laelia albida. The sepals are the three narrower parts, situated behind, and overlapped by, the three petals, of which the lower in the figure is the labellum or lip. The dark, central spot is the opening of the nectary; and the upper part of the spot, is the situation of the stigmas. In Orchids, only two of the three are developed as stigmas, and often--in fact, usually--they unite. Just above the dark spot is seen the column, united to the labellum by two ridges. This column is regarded as formed by the fusion into one body of the upper part of the carpels, the fertile stamens, whether one or (in Cypripedium) two, and probably three or (in Cypripedium) two undeveloped stamens; but of the latter, as a rule, no clear trace remains. At the tip of the column is a body--the rostellum--usually regarded as the third stigma become useless as a stigma, but of great importance in securing the conveyance of pollen from one flower to the stigma of another. The labellum is also believed to be of compound nature, and to be formed of two metamorphosed stamens, and the median petal, closely united in growth. The structure of the fertile stamen, and the process by which pollen is conveyed from it to the stigma of another flower, by insect aid, in a common British Orchid will next be described, selecting, as a widespread and easily-obtained plant, the Spotted Orchis (Orchis maculata). In this species, the perfect stamen is situated on the front of the column; and in the anther there are two pouches, in which the pollen grains are inclosed. The grains are joined together into small masses, and these, again, are joined to form a single mass (pollinium) in each chamber. Each pollinium is formed like a club, supported on a slender stalk (caudicle), which ends below in a round, sticky ball. The two balls lie almost side by side, and are covered by an elastic membrane formed by the rostellum. This membrane prevents the disks from drying, which would render them useless. It is at first continuous over the disks, but it soon tears across, and, on being touched, it is easily dragged down, leaving them bare.

When a suitable insect (e.g., a moth) visits the flowers, and conveys the pollen of one flower to the stigmas of the next one visited, it pushes the proboscis down the tube of the labellum, and, in doing so, its proboscis, or its head, rubs against the membrane, and, depressing it, lays bare the sticky disks of the pollinia. These, as soon as they are touched, adhere to the part with which they are in contact, and, on the insect withdrawing its head, one or both of the pollinia are also withdrawn. They are erect when first withdrawn, but, in about half a minute, as the disks dry, the pollinia bend forward, and thus come to lie nearly alongside the proboscis, or other body, in the very best position to bring them into contact with the stigmas of flowers visited later. Owing to the little interval that elapses before the pollinium is in the suitable position, the insect has usually visited all the flowers on a stem before any of the pollen could be left in flowers of the plant from which it was taken; hence, cross-fertilisation between different plants is favoured. When a pollinium touches a stigma, some of the small masses adhere, and are dragged off, and thus one pollinium suffices for several stigmas. Insects have frequently been taken bearing pollinia, and often several pollinia are found adhering to the proboscis and head of an insect.

O. maculata, like most of this family, remains barren if insects are prevented from visiting the flowers, and aid in transferring the pollen is not given. There is no means in this plant of conveying pollen to the stigma, without extraneous help, and the adaptations for cross-fertilisation are complete.

It may, therefore, be regarded as the type of a large section of Orchids, in so far as concerns its entire dependence on extraneous aid, though, in minor points, there is a considerable diversity in the methods of favouring cross-fertilisation; e.g., in O. pyramidalis, the sticky disks are united into a concave band, which adheres to the proboscis of the insect visitors. The pollinia pass through the same change of position as in O. maculata.

Listera ovata, the "Twayblade," which is an inconspicuous, greenish Orchid, growing in grassy places, has the pollen masses lying immediately above the rostellum. The pollen masses are dry, and, therefore, cannot adhere of themselves to insects; but when the tip of the rostellum is touched, even very gently, by the insect, a drop of sticky fluid oozes out, and fixes the pollen grains in the suitable position to reach the stigmas of the next plant visited. The long, narrow, spurless labellum, secretes honey into a groove down the middle of the upper surface. The insect follows up this guide as it climbs up the labellum, and, on reaching the upper end of the groove, it is almost certain to touch the rostellum, and to have the pollen-masses affixed to its head. For a little the rostellum sinks, and closes the way to the stigmas; but soon it rises, leaving free access to the stigmas for later visitors to the flower. These visitors are attracted by a new supply of nectar in the groove. They are almost all small insects, with the proboscis short or absent, the nectar lying open. The genus Cypripedium--represented by C. Calceolus, the "Lady's Slipper", a scarce species in England, and by many tropical species, e.g., C. spectabile--belongs to a group that possess two fertile stamens, situated at each side, on the lower surface, of a broad, shield-like plate (the representative of the single fertile stamen in other Orchids). This plate overhangs the stigmatic surface, which lies on a prominent overhanging rostellum immediately below the plate. The labellum is much like the front half of a slipper. It forms a chamber, open above, and large enough to allow small bees to enter. This they do readily, to gnaw the hairs that line the middle of the floor. The lips on each side are incurved, so that, when an insect wishes to escape by the opening, it cannot crawl out as it entered; nor is there room for it to fly out. But, at the stalk of the labellum, a small opening is left on each side, where, if strong enough, by squeezing between the labellum and the stigma, and then between the labellum and either stamen, the prisoner can escape, carrying away one of the pollinia. As the stigma is touched before the anther, the flower cannot be fertilised with its own pollen, but only with that from one previously visited. Few flowers are formed on each plant.

Among exotic Orchids, the adaptations for fertilisation by pollen from other flowers are far more curious and striking even than the above, and the peculiarities in form and structure of the rostellum, the pollinia, and other parts of the flower, are often very strange, and cause them to resemble insects (e.g., in the Fly Orchis), or a minute dove, or other objects too numerous to describe, or even to mention.

Mr. Darwin has described fully some of the more remarkable, among which is the extremely curious genus Catasetum. In this genus of Orchids, the flowers are of different sexes, and they are so unlike that on C. tridentatum three supposed genera were based; the characters being drawn from the form of the flowers. These were Catasetum, based on the male; Myanthus, on the hermaphrodite; and Monachanthus, on the female flowers. The occasional presence of these flowers on the same plants has enabled botanists to understand their mutual relations. Cross-fertilisation must be effected before seeds can be formed; and this requires insect agency. In Catasetum (the male flower), the labellum rises up at the back of the flower (the ovary not making the usual half-twist), and is shaped like a monk's cowl. It contains no exuded nectar, but bees and other insects gnaw the inner surface greedily. The central column projects forward below the labellum, and bears the anther on its upper surface, the pollinia and disk being embedded in the tissues till set free by the mechanism now to be described. From each side of the rostellum grows a long, slender, curved horn, or "antenna"; and both lie within the hollow of the labellum. The right antenna occupies the bottom of the space, with its tip just projecting beyond the left margin; the left one curves upwards along the back of the labellum.

The slightest touch on either antenna immediately transmits some stimulus to the membrane over the disk of the pollinia, causing it to be rent; the disk springs out, by sudden removal of tension from the bent caudicle, and the whole mass is thrown forward, with the sticky disk in front, against the insect, if the antenna has been touched by an insect, and at once adheres in the position most likely to secure conveyance to the stigma of the female flower. The pollinia are not sensitive to contact of insects with any part of the flower except the antennae; but the position of these organs renders this contact almost certain to occur when the labellum is being gnawed. The female flowers of this species (formerly called Monachanthus viridis), resemble the male flowers in the position and general form of the labellum, which differs only in minor details. The column also agrees in position, but is smaller; the pollinia are rudimentary; the disk does not cohere with the pollinia, but soon falls away; and the antennae are absent. On the other hand, the stigmas and the ovaries are developed, and the seeds ripen abundantly.

The form called Myanthus barbatus is very different in appearance from the other two, and Darwin points out that it comes near, in its structure, to the flowers of Catasetum callosum and of C. saccatum, but that it is hermaphrodite, though not known to seed. He suggests that it may be a reversion to the ancestral form. Owing to a bend in the ovary, the column and the long fringed labellum have reversed the positions they occupy in the other forms In the large group of Epidendreae.of which so many are cultivated in hot-houses, the adaptations for cross-fertilisation, though less complex than in Catasetum, well deserve a short notice. Cattleya is selected as an easily-obtained type; but Laelia, Chysis, and other genera, only differ from Cattleya in minor details. The positions of the labellum and of the column will be readily made out. The column is prominent, and arches forward, having the stigmas on the front or lower surface, near the tip. Just beyond them is the rostellum, the lower surface of which is thickly covered with sticky liquid. Beyond this, again, on the tip, is the anther. The pollen grains are joined to form four chief masses, which end in caudicles, to which many separate small groups, each of four grains, are stuck. The caudicles have no disks; they lie so that their tips are on the dry, membranous, upper surface of the rostellum. The labellum usually has the sides folded up so as to nearly surround the column. The nectary is a narrow tube between the labellum and the base of the column in front. When an insect of suitable size comes to suck the nectar, it must push its way between the labellum and the front, or lower, surface of the column; and must get well in below the latter before reaching the nectar. In doing this, its back presses against the stigmas, and leaves on them pollen from any previously visited flower. In retreating, the insect presses its back against the lower surface of the rostellum, and becomes smeared with the fluid, some of which is also forced on to the caudicles, and causes them to stick to the insect; and the pollinia are thus drawn out and fixed to it, to be transferred to the next flower visited. It will be observed that the method of affixing the pollinia in these plants reminds us a little of that previously described as occurring in Listera ovata; but the larger size renders it more easy to study the process in Cattleya and its allies, and the differences in details are sufficiently obvious. Some Orchids are now so specialised to adapt themselves for cross-fertilisation alone, that the stigma dies (as if poisoned) when pollen from the same flower is placed on it; e.g., species of Burlingtonia and of Oncidium. Between this extreme and habitual self-fertilisation in cleistogamic flowers, every stage is to be encountered. In many cases, where mechanical obstacles prevent the pollen of a flower being transferred to its stigma, the ovules are well fertilised (e.g., in Orchis maculata) if pollen is placed on the stigma of the same flower.

An interesting example of the way in which an Orchid, possessed of structures intelligible in their origin only as suited to cross-fertilisation, is converted by a slight alteration, so as to enable its ovules to be fertilised by pollen of the same flower, is well seen in the Bee Orchis (Ophrys apifera). In this plant, the parts are, in many respects, similar to those of Orchis maculata; but the pollinia have long, slender, flexible stalks, and the mass of grains in each slips out of the pouch of membrane, and hangs in front of the stigma, retained by the disk, and the slightest swaying movement causes it to strike against the stigma, and to leave pollen on it. The flower is not much, if at all, visited by insects, and it possesses no attractions for them.

A striking anomaly is that observed by Dr. F. Muller, in South Brazil, in which an Epidendrum possessed two lateral stamens that secured self-fertilisation, and a median one, from which the pollen could be removed only by insect or other extraneous aid. The rarity with which this is effected, is indicated by the absence of attractions for insects in the flower. A considerable number of self-fertilised Orchids have been put on record. One of the latest papers on this subject is entitled, "On the Contrivances for Insuring Self-Fertilisation in some Tropical Orchids," by Henry O. Forbes, in the "Journal of the Linnean Society," vol. xxi., pp. 538-49, tt. 16 & 17.