Fig. 320.—A Flower of Orchis maculata (front view): a stamen; b the cup; n the stigmas; x staminodes; sp the spur; spe the entrance to it; sm-sl-sl exterior perianth-leaves; pm the labellum, and pl-pl the other 2 interior perianth-leaves. B-E Orchis mascula: B lateral view of the column; C a pollinium with massulæ (p), caudicle (c) and adhesive disc (d); D caudicles with the cup (r), front view; the latter is depressed so that the adhesive disc is seen lying inside it; E a pollinium, more highly magnified; some massulæ are removed. F Ophrys aranifera: rostellum and the base of the anther-loculus; an adhesive disc is seen on the right.
4. Ophrydeæ. Anthers 2-locular, not falling off, on a very short column. The anther is united at its base with the rostellum (basitonous Orchids, Fig. [320] A, B), while in all other Orchids it is connected at the apex (acrotonous Orchids). The pollen-grains in each loculus are united into small “masses” (massulæ), each of which corresponds to a pollen-mother-cell in the anther, and which hang together by elastic threads (Fig. [320] C, E). Each pollinium is attached at the base by a stalk (caudicle) to an adhesive disc, formed by the modified stigma (rostellum), and is easily liberated from it (Fig. [320] C, D, F). The pollinium, which is formed in an anther-loculus, together with its caudicle and adhesive disc, is termed “pollinarium” (Fig. [320] C).—All Ophrydeæ are terrestrial with tuberous roots, two of which are present in the flowering period, an older one (from the preceding year) containing the nourishment for the flowering-shoot of the year, and a young one which is intended to contain the reserve material for the following year. Inflorescence terminal.
Orchis. The lip has a spur; each of the club-like pollinia is attached to its own adhesive disc, the discs being enclosed in a common pouch formed by the rostellum (Fig. [320] C, D). Tubers ovate, undivided: O. morio, mascula; tubers palmate: O. incarnata, maculata, majalis.—Ophrys; no spur, the two adhesive discs are each enclosed in a separate pouch (Fig. [320] F).—Anacamptis and Serapias have one adhesive disc.—Habenaria, Gymnadenia, Platanthera, Herminium, Nigritella, Cœloglossum, etc., have naked adhesive discs (no rostellum).
5. Epidendreæ. Acrotonous Orchids with deciduous anthers (except Malaxis); 2-8 wax-like pollinia, with or without caudicles; generally no adhesive discs. Malaxis (the flower is twisted through a complete circle, causing the labellum to be turned upwards), Sturmia and Corallorhiza[32] (Coral-root); the latter has a creeping, coral-like rhizome without roots, and is destitute of chlorophyll except in the ovary. The other two somewhat resemble the tropical Orchids in having the lower internodes of the axis of the inflorescence tuberous. Liparis; Calypso. Most of the genera are tropical epiphytes and many have aerial, green tubers formed from one or more stem-internodes; Dendrobium, Eria, Phaius, Bletia, Epidendrum, Cattleya, Lælia, Pleurothallis, Restrepia, Masdevallia, Bulbophyllum, etc.
6. Vandeæ. These resemble the preceding but have only 2 wax-like pollinia in each anther, which are attached by a caudicle to the adhesive disc of the rostellum. Nearly all are tropical epiphytes. Stanhopea, Catasetum, Maxillaria, Oncidium, Vanda, Polystachya, etc.
6,000 (10,000?) species. The majority live in the Tropics and occur, especially, as epiphytes on trees or in the crevices of rocks, to which they are attached by aerial roots. These aerial roots, like those of Araceæ, are covered by several layers of spirally-thickened cells (tracheides) which contain air and form the velamen—an apparatus to absorb moisture from the air. The roots have a white appearance when the cells are filled with air, which changes to a greenish hue when they are filled with water, the chlorophyll then shining through. They generally have horizontal rhizomes; the ascending shoots, which bear the foliage-leaves, may vary, but they very often swell and assume the form of a tuber, which persists for several years fresh and green after the leaves have fallen off (Fig. [321]). Vanilla is an exception (see above). Our Orchids are all terrestrial (or marsh-plants); the largest number of species is found in calcareous soils.
Pollination takes place principally by means of insects, but self-pollination occurs in some. The lip serves as a landing-stage for the insect visitors, which, on sucking the honey, cause the adhesive discs, with the pollinia attached to them, to adhere to their bodies (generally to the probosces) and so carry them away to other flowers. In some species parts of the flower are sensitive or irritable, which has some connection with the pollination. Without doubt there are a great many biological differences which are closely connected with the infinite multiplicity of forms; Darwin (1862) has already shown an enormous variety, never even dreamt of before, in the European species. The genus Catasetum has ♂-♀-and ☿-plants with flowers of such different appearances that they have been classed in various genera (Myanthus, Monacanthus). Platanthera is pollinated by hawk-moths; Ophrys, by flies; Epipactis latifolia, by wasps; Orchis, by bees, especially humble-bees, etc.
Fig. 321.—Chysis bractescens.
The DISTRIBUTION OF SEEDS is effected by the wind, the seeds being so exceedingly small and light. Many species moreover have peculiar, elater-like, fine, hygroscopic hairs in the ovary, which eject the seeds in a manner similar to the elaters of the Liverworts.