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.
This order is the subject of Mr. Darwin's admirable work, "On the Various Contrivances by which British and Foreign Orchids are fertilised by Insects," from which the following facts are taken. The order contains sixteen British genera, several of them extremely curious and pretty. The species with long nectaries are fertilised by Lepidoptera, those with shorter ones, as a general rule, by bees and flies; Epipactis latifolia, it is said, exclusively by wasps, so that, according to Darwin, "if wasps were to become extinct in any district, so would the Epipactis latifolia" Other species on the contrary such as Epipactis viridifolia, and Ophrys apifera (the Bee Orchis) habitually fertilise themselves. It is remarkable that in some Orchids the ovules are not developed until several weeks, or even months, after the pollen tubes have penetrated the stigma. (Hildebrand, Bot. Zeit., 1863 and 1865. Fritz Muller, Bot. Zeit., 1868.)
The flower in this order is very abnormal. There is, except in Cypripedium, only one anther, which is confluent with the style, forming the so-called "column." The anther is divided into two cells, which are often so distinct as to appear like two separate anthers. The pollen in most Orchids coheres in masses, which are supported by a stalk or "caudicle;" the pollen masses with their stalks are called "pollinia." The styles are theoretically three in number; but the stigma of the upper one is modified into a remarkable organ called the "rostellum," and those of the two lower ones are often confluent, so that they appear like one.
Orchis mascula (Fig. 120) is perhaps our commonest species.
Fig. 121 represents the side view of a flower from which all the petals and sepals have been removed, except the lip (/) half of which has been cut away, as well as the upper portion of the near side of the nectary (n). The pollen forms two masses (Figs. 121, 122a, and 123), each attached to a tapering stalk, which gives the whole an elongated pearlike form, and is attached to a round sticky disk (Fig. 123d), which lies loosely in a cup-shaped envelope or rostellum (r). This envelope is at first continuous, but the slightest touch causes it to rupture transversely, and thus to expose the two viscid balls (dd). Now suppose an insect visiting this flower: it alights on the lip (/), and pushing its proboscis down the nectary to the honey, it can hardly fail to bring the base of the proboscis into contact with the two viscid disks, which at once adhere to it, so that when the insect draws back its proboscis, it carries away the two pollen masses. It is easy to imitate this with a piece of grass, and to carry away on it the two pollen masses and their stalks. If, however, the pollinium retained this erect position when the insect came to the next flower, it would simply be pushed into or against its old position. Instead, however, of remaining upright, the pollinia, by the contraction of the minute disk of membrane to which they are attached, gradually turn downwards and forwards, and thus when the insect sucks the next flower, the thick end of the club exactly strikes the stigmatic surfaces (st st). The pollinium or pollen-mass consists of packets of pollen grains, fastened together by elastic threads. The stigma, however, is so viscid, that it pulls off some of these packets, and ruptures the threads, without removing the whole pollinium, so that one pollinium can fertilise several flowers.
Fig. 120 - Orchis mascula.
Fig. 121. - bide view of flower, with all the petals and sepals cut off except the lip, of which the near half is cut away, as well as the upper portion of the near side of the nectary .
Fig. 122. - Front view of flower, with all sepals and petals removed except the lip.
Fig. 123. - The two pollinia.
This description applies in essentials not only to Orchis mascula, but also to O. Morio, O. fusca, O. macidata, and O. latifolia, as well as to Aceras anthropophora (the Man orchis), in all of which the pollinia undergo, after removal from the anther cells, the curious movement of depression, which is necessary in order to place them in the right position to strike the stigmatic surface.
0. pyramidalis differs from the above group in several important points. The two stigmatic surfaces are quite distinct, and the rostellum is brought down, so as to overhang and partly close the entrance to the nectary. The viscid disks which support the pollen masses, are united into a single saddle-shaped body. The lower lip is furnished with two prominent ridges, which serve to guide the proboscis of the insect into the orifice of the nectary. It is of course important that the proboscis should not enter obliquely, for in that case the pollen masses would not occupy exactly the right position.
Following Darwin and other botanists, I have applied to the spur of Orchis the term "nectary." As a matter of fact, however, the flowers of this genus produce no honey; whence Sprengel applied to them the term "Scheinsaftblumen" or "Sham-honey-flowers." Darwin does not, however, think that moths (by which the flowers of this group are principally fertilised) could be so deceived for generation after generation; and as he has observed that the membrane of the interior of the spur is very delicate, and the cellular tissue extremely juicy, he suspected that insects possibly pierce the membrane, and suck the juicy sap lying beneath. His suggestion has been confirmed by H. Muller, and he himself in a subsequent memoir ("Ann. and Mag. of Nat. His.," 1869, p. 143) speaks confidently on the point. Del-pino, on the contrary, is confident that the species examined by him (O. sambucina, O. morio, O. mascula, and O. metadata) do not secrete honey either on or under the epidermis.