Orchids

ORCHIDS, the name given to members of the orchid family (Orchidaceae), one of the most numerous and interesting groups of flowering plants, usually with beautiful and often with exceed ingly handsome and highly fragrant flowers. Orchids are found in moist climates very widely throughout the world, except in the polar regions, but they occur in by far their greatest diversity and abundance in humid tropical forests. The orchids are all perennial herbs and are comprised in two groups : terrestrial orchids, which grow in the ground, and (2) epiphytic orchids, epiphytes, which grow perched upon trees, found in the tropics where they form an important feature of the vegetation. Most orchids of the temperate zone are terrestrial.

Floral Structure.

The flowers of orchids, though extremely diverse within certain limits, are all formed upon one common plan, which is only a modification of that observable in such flowers as those of the narcissus. Such flowers consist essentially in the presence of a six-parted perianth, the three outer segments of which correspond to a calyx, the three inner ones to a corolla.

These segments spring from the top of the ovary which is in ferior instead of superior as in the lily. Within the perianth, and springing from its sides, or apparently from the top of the ovary, are six stamens whose anthers contain powdery pollen-grains. These stamens encircle a style which is the upward continuation of the ovary, and which shows at its free end traces of the three originally separate but now blended carpels of which the ovary consists.

An orchid flower has an inferior ovary, but with the ovules on the walls of the cavity (not in its axis or centre), a six-parted perianth, a stamen or stamens and stigmas. The main distinguish ing features consist in the fact that one of the inner pieces of the perianth becomes in course of its growth much larger than the rest, and usually different in colour, texture and form. So dif ferent is it that it receives a distinct name, that of the "lip" or "labellum." In place of the six stamens we commonly find but one (two in Cypripedium), and that one is raised together with the stigmatic surfaces on an elongation of the floral axis known as the "column." Moreover, the pollen, instead of consisting of sep arate cells or grains, consists of cells aggregated into "pollen masses," the number varying in different genera, but very gen erally two, four or eight. In Cypripedium all three stigmas are functional, but in most orchids only the lateral pair form recep tive surfaces, the third being sterile and forming the rostellum which plays an important part in the process of pollination, often forming a peculiar pouch-like process in which the viscid disk of the pollen-masses is concealed.

It would appear, then, that the orchid flower differs from the more general monocotyledonous type in the irregularity of the perianth, in the suppression of five out of six stamens, and in the union of the one stamen and the stigmas. In addition to these modifications, which are common to nearly all orchids, there are others generally but not so universally met with ; among them is the displacement of the flower arising from the twisting of the inferior ovary, in consequence of which the flower is so completely turned round that the "lip," which originates in that part of the flower, conventionally called posterior, or that nearest to the supporting stem, becomes in course of growth turned to the anterior part of the flower nearest to the bract. Other common modifications arise from the union of certain parts of the perianth to each other, and from the varied and often very remarkable outgrowths from the lip. These modifications are associated with the structure and habits of insects and their visits to the flowers.

Cross-Fertilization by Insects.

In the common orchids of British meadows, Orchis maculata, 0. inascula (Shakespeare's long purples), etc., the general structure of the flower is as de scribed. In addition there is in this particular genus, as indeed in many others, a long tubular spur or horn projecting downwards from the back of the lip, whose office it is to secrete and store a honeyed juice; the forepart of the lip forms an expanded plate, usually larger and more brightly coloured than the other parts of the flower, and with hairs or ridges and spots of various kinds according to the species. The remaining parts of the perianth are much smaller, and commonly are so arranged as to form a hood overarching the "column." This column stands up from the base of the flower, almost at right angles to the lip, and it bears at the top an anther, in the two hollow lobes of which are concealed the two pollen-masses, each with its caudicle terminating below in a roundish gland, concealed at first in the pouch-like rostellum at the front of the column. Below the anther the surface of the column in front is hollowed out into a greenish depression covered with viscid fluid—this is the two united stigmas.