_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.