The great tribe of British Neotteæ is characterized by a free anther standing like a hood behind the stigma; the pollen grains are tied together with threads, and attached to a viscous cap lying on the top of the rostellum. The Epipactis palustris is a type of this group of orchids. Its spike is short, and the pink blossoms stand out horizontally from the stem on long ribbed footstalks, which contain the ovaries. [Fig. 83] A is a side view of the flower in its natural position, with the lower sepals alone removed. The labellum, or lowest petal, is interrupted in the middle by a kind of flexible hinge: the basal part is a cup-shaped trough, at times abounding in nectar; the extreme part is a wavy leaf ([fig. 83] B). The entrance to the nectary cup is nearly closed by the hood and the large anther; but, on account of the elasticity of the hinge, the weight of an insect is sufficient to give access to the nectar; but no sooner is the labellum relieved of the weight, than it springs up into its natural position, and the insect creeps backwards and comes out at the top of the flower with the viscous cap clasped round its proboscis, and the pollen grains attached to it ready to fertilize another blossom. In [fig. 84], C represents a section of the Epipactis, and D is a front view of the column.
Fig. 85. Listera ovata:—Side view of flower, with sepals and petals cut away: a, anther; col, summit of column; p, pollen; r, rostellum; s, stigma; l, labellum; n, nectar-secreting furrow.
Of all the British Orchids, the Listera ovata, or Twayblade ([fig. 85]), has the most curious structure. It grows in woods and pastures, has a creeping root, oval leaves, a downy stem, and yellowish green flowers. [Fig. 85] represents a lateral view of a blossom, with all the sepals and petals removed, except the labellum. In this plant the rostellum (r) is large, thin, convex in front, concave behind, and arches over (s) the stigmatic surface. When the flower is full blown, the anther cells (a) are already open, and the naked and friable pollen grains united by a few threads, which form the pointed tips of the pollinia, rest upon the concave back of the rostellum. The labellum, which is contracted at the base, is exceedingly long, hanging down like a narrow ribbon. It is divided half-way up, and furrowed along the middle, from the bifurcation close up to the base of the stigmatic surface (s). The borders of the furrow are globular, and secrete much nectar. The rostellum is internally divided into a series of longitudinal cells, or chambers, which contain and expel viscid matter with violence on the slightest touch, and the viscid matter sets hard in two or three seconds, and soon assumes a purplish brown tint. So exquisitely sensitive is the rostellum, that a touch from the thinnest human hair suffices to cause the explosion. As the pointed tops of the loose pollinia lie on the crest of the rostellum, they are always caught by the exploded drop. This never fails. So rapid is the explosion, and so viscid the fluid, that it is difficult to touch the rostellum with a needle quick enough not to catch the pollinia already attached to the partially hardened drop, and consequently the slightest touch of any small insect which enters the flower, suffices to explode the rostellum, and the pollinia which attach themselves to its proboscis are carried by it to the next flower to adhere to the viscid stigma and fertilize it. Mr. Darwin has seen two Hymenopterous insects retreat from one of these plants with bright yellow pollinia on their heads, and Mr. C. K. Sprengel saw an insect of that kind leave pollen upon a stigma. The action and structure of Neottia Nidus-avis is almost identically the same as that of Listera ovata.
The Malaxis paludosa, or Bog Malaxis, the smallest of British Orchids, is a rare plant, and differs from all of them in having its labellum turned upwards instead of downwards. Its lower margin clasps the column, making the entrance into the flower tubular. In this orchis the upper sepal and two upper petals are reflexed, to allow insects freely to visit the flower. In many orchids the labellum is properly directed upwards, but assumes its usual position as the lower lip by the twisting of the ovarium, or pedicel, of the flower. In the Malaxis paludosa, however, the twisting has been carried to such an excess, that the flower occupies the same position it would have held if the ovarium had not been twisted at all, and which the stalk ultimately assumes when ripe by the process of gradual untwisting. This little plant belongs to a genus distinguished by having a movable and deciduous anther, and is one of the British orchids that have allied forms among the exotic genera. All Dr. Lindley’s vast tribes of Epidendreæ, and the still more numerous and splendid Vandeæ, have not a single British representative.
The structure of the exotic orchids is often very complicated, and they possess many properties unknown in the British genera. Their labellum, or lower lip, is so varied, and sometimes so singular, that it baffles description; besides, it often possesses peculiar motions, sometimes from structure, at other times from irritation. In the Bolbophyllum Rhizophoræ, the labellum is attached to the column by a very narrow thin strap, elastic as india rubber, which oscillates in a singular manner; while that of the B. barbigerum has a beard of fine hairs in almost constant agitation. The irritability of the labellum in many of the allied forms of the orchids is one of their remarkable properties; the slightest touch sets them into motion. The Australian genus Caleana possesses it in the highest degree; for when an insect settles on its labellum, it suddenly shuts up against the column, and encloses it, as it were, in a box.
Some of the exotic orchids have a pseudo-diœcious character. Thus some species of Catasetum have two long horns, or antennæ, attached to the rostellum, which stand over the labellum, and the pedicels of the anthers are fastened down in a curved position; so, when an insect alights upon the labellum and touches the antennæ, the excitement is conveyed by them to the rostellum, the attached edge of the disc of the anthers is ruptured, and they straighten themselves with such force, that not only do they drag the balls of pollen and anther cells from their places of attachment, but the whole pollinium is jerked forward over and beyond the tips of the horns, to the distance of two or three feet. The insect, disturbed by so sharp a blow, or after having eaten its fill, flies with the pollen adhering to it to fertilize the female plant, which differs from the male in having no antennæ. Thus the agency of insects is as requisite to fertilize these semi-diœcious as hermaphrodite orchids.
The Vanilla, which is cultivated for its aromatic pods in Tahiti, Bourbon, and the East Indies, does not bear fruit without artificial aid, which shows that the American insect, which fertilizes it in its own native home, is not indigenous in the places mentioned. It appears that many exotic orchids require a less elevated temperature than has hitherto been supposed.
The form and position of the nectary are exceedingly varied. In certain species, both of the native and tropical orchids, they are always dry; but Mr. Darwin has discovered that, in these cases, the walls of the nectaries are thick and formed of two coats, and that a liquid is contained between them, to which the insect penetrates by piercing the inner wall. The exotic orchids are, for the most part, larger, and require larger insects to fertilize them than our small ones, whose organs are generally microscopic. A curious instance, both of this and of the extraordinary form of the nectary, is found in the Angræcum sesquipedale, a Madagascar orchid, with large six-rayed flowers, like stars formed of snow-white wax. It has a green whip-like nectary, sometimes as much as a foot long, and, from the structure of the plant, it appears that the pollinia never could be withdrawn, until a large moth, with a wonderfully long proboscis, attempts to drain the last drop of nectar from the bottom of the nectary.
Notwithstanding the vast diversity in the form of the orchids, they are homologous in their general structure. Mr. Robert Brown was the first to observe, that an orchid flower consists of fifteen organs, arranged alternately, three within three, in five whorls. This accords perfectly with a system of spiral vessels developed at an early age in all orchids. Mr. Brown and three of the greatest living botanists[[78]] have each traced the spirals from six bundles surrounding the ovary in the footstalk to the different organs of the flower, and have found that they consist of fifteen bundles corresponding to the fifteen organs of the flower; namely, three sepals, three petals, six anthers in two whorls (three of which are rudimentary), and three pistils, with their stigmas: these are arranged in alternate whorls, and undergo many modifications. The pistils and anthers are confluent, and form the column; the uppermost stigma becomes the rostellum; the three inner anthers are rudimentary, one forming the front of the column, and the other two forming the membranous sides of the hood which protects the pollen; and, lastly, the two lower anthers are united to the sides of the lowest petal, and form the labellum, which accounts for its great size, frequent tripartite form, and peculiar manner of attachment.