Fig. 318.—A Diagram of an Orchid-flower. B, Cephalanthera. Stylar-column: a anther; s stigma; at the foot are seen scars indicating the position of the parts which have been removed.
Order 2. Orchidaceæ. The epigynous, petaloid perianth is strongly zygomorphic in having the posterior leaf of the interior whorl, the lip (labellum), differing from all the other leaves in form, size, and colour (except Apostasieæ); the position of the labellum is very frequently reversed, being turned forwards and downwards by the twisting of the ovary (Fig. [318] A). Only 1 of the stamens—the anterior of the external whorl—is developed and bears an anther (by the twisting of the ovary it is turned posteriorly and upwards); the others are entirely wanting (indicated by * in Fig. [318] A) or present as staminodes (Fig. [318] A, δ δ) (except Apostasieæ, Cypripedileæ); the filaments are united with the style to form a column (Fig. [318] B), the stylar-column[30] (gynostemium), and the anther (a) is thus placed on its apex and exactly behind or over the stigma (s). The anther is 4-locular; the pollen-grains do not separate (except Apostasieæ, Cypripedileæ) but remain united either in tetrads or in masses, which correspond to a pollen-mother-cell (Fig. [320] C, D, E); or the pollen-grains, formed in each of the two anther-halves, remain united and form one or a few wax-like masses (pollen-masses, pollinia). The 3 carpels form a unilocular ovary with 3 parietal, deeply bifid placentæ (except Apostasieæ, Selenipedilum). Only the two lateral carpels are prolonged and developed into the stigma (Fig. [318] B, s), while the one lying in the median line, which is situated just within the anther (Fig. [318] A), becomes either rudimentary or developed into the “rostellum” (“a small beak”), on which the sticky bodies (glandulæ) arise; by aid of these the heavy, connected pollen-masses may be glued to the insects which visit the flower, and pollination is thus secured (in Apostasieæ and Cypripedileæ the 3 carpels each contribute to the formation of the stigma). The fruit is a capsule which most often dehisces by 6 valves, 3 of which are broader and bear the placentæ, and 3 alternating with them are narrower and barren (except Vanilla). The very numerous and exceedingly small seeds have no endosperm, and have a somewhat spherical embryo without any trace of external organs. The testa is membranous and loose.
The Orchids are all perennial herbs with diverse habits and varying morphological structure (see the genera); the leaves are scattered, of the usual Liliaceous form, and the inflorescences in all cases are racemes or spikes (sometimes branched), with subtending bracts, but without bracteoles.
The forms which are the least modified are described first.
1. Apostasieæ. The perianth-leaves are almost alike and free. The column is straight, with 3 equally-developed stigmas. Neuwiedia has 3 perfect stamens (1 median of the outer whorl, and 2 lateral of the inner whorl); Apostasia has only 2 perfect (inner lateral) and one barren (the median of the outer whorl), which however may be entirely wanting. The 3 posterior stamens are entirely suppressed. The pollen is powdery. The ovary is 3-locular with axile placenta. 7 species (Tropical East India, Australia).
Fig. 319.—Cypripedilum calceolus: 1 front view of the flower; 2 lateral view, after the removal of all the perianth-leaves with the exception of the labellum, which has been divided longitudinally; 3 the stylar-column; ov ovary; s-s exterior, p interior perianth; p’ the labellum; a the two fertile stamens; a’ the staminode; st the stigma; i entrance for the insects; ex exit.
2. Cypripedileæ.[31] The flower is strongly zygomorphic with a large boat-shaped labellum. There are two perfect stamens belonging to the inner whorl, and the median anterior (later on the posterior) stamen of the outer whorl is transformed into a large, barren, shield-shaped body (Fig. [319]). Selenipedilum has a 3-locular ovary, but Cypripedilum (Ladies’-slipper) has a unilocular ovary with 3 parietal placentæ—the typical structure for the Orchids. The pollen-grains are separate (not in tetrads) and all the 3 lobes of the stigma are constructed to receive them. This group is therefore, next to the Apostasieæ, the least modified among the Orchids; in all the following groups, one of the lobes of the stigma is differently developed from the others, and there is only one stamen.—Terrestial Orchids.—The pollination of C. calceolus is effected by the forcible entrance of insects into the boat-shaped labellum (Fig. [319] p’) at i, and their escape at ex (in 2) where the anthers are situated; in this way the stigmas will first be touched and then the anthers. The pollen-grains are surrounded by a sticky mass in order that they may adhere to the insects.
3. Neottieæ. The majority are terrestrial Orchids with creeping, sympodial rhizomes; the blades of the leaves are not detached from the stem at joints, and have convolute vernation. The anthers do not drop off, but persist in the withered condition; their apex is brought in contact with the rostellum (acrotonous Orchids). The pollen-grains are united in tetrads, which, however, often hang loosely together in pollinia, attached to a sticky part of the rostellum (“adhesive disc”), so that they adhere to the insects, and are by them transferred to the stigmas. Spiranthes. Listera; Neottia. N. nidus-avis (Bird’s-nest) is brown (it has little chlorophyll) in colour, has no foliage-leaves, and lives mainly as a saprophyte; the rhizome is studded with unbranched, fleshy roots which may form buds at their extremities.—Vanilla climbs by aerial roots. The fruit is fleshy and hardly opens, or does so irregularly.—Epipactis, Cephalanthera.—Epipogon and Limodorum are saprophytes without chlorophyll.
