Fls bisexual, zygomorphic, in spikes, racemes or panicles, or solitary. Per. mostly petaloid, of 6 segs in 2 whorls, free or variously connate, occ. joined to form a tube; the middle seg. in each whorl us. different from the 2 lateral ones, the middle petal or labellum often differing greatly in size, shape or outgrowths (calli) from the others and us. directed downwards; nectary sts contained within a spur or sac from one or more of the per.-segs. Anthers and stigmas borne on a ± erect, us. fleshy structure, the column. Anthers 1 or 2, 2-locular, introrse, opening by longitudinal slits; pollen grains rarely free, us. aggregated in tetrads or larger groups (pollinia) that are mealy, waxy or quite hard and sts terminate in a sterile caudicle. Ovary inferior, 1-locular with 3 parietal placentae (rarely 3-locular); stigma borne on the part of the column facing the labellum, of 3 or 2 lobes, the median lobe sts represented by a small process, the rostellum, between the stigma and the anther; viscid areas (viscidia) are sts associated with the rostellum and to these the pollinia become attached either directly or by a connecting strip of non-viscid rostellar or columnar tissue known as the stipe. Fr. us. a capsule dehiscing by 3 or 6 longitudinal slits. Seeds very ∞, minute, often elongated, without endosperm or differentiated embryo. Perennial, terrestrial, epiphytic or saprophytic herbs with rhizomes, tubers or thickened rootstocks. Aerial stem often swollen at base into pseudobulb, and often with adventitious or aerial roots. Lvs us. entire, often distichous, sheathing at base, sts reduced to scales. Worldwide, of 500-800 genera and perhaps 30,000 spp. The 21 N.Z. genera are all except 3 (, and ) recorded also from Australia, and at least one third of the spp. are common to the two countries.