Orchids have three sepals-a dorsal and two lateral-and three petals, but the middle petal, the labellum, is usually quite different from the others. In most orchids, the flowers rotate 180° as they develop, so that the labellum is the lowest of the petals, facing upward.
Instead of separate stamens and pistils, orchids have a combined reproductive organ-the column-in which two stigmas are merged with a single stamen, and the third stigma is modified into a tiny structure, the rostellum, which prevents a flower from pollinating itself, although some orchids, e.g., forms of Thelymitra longifolia agg. are self-pollinating. Differences in the sizes, shapes and colours of the column help us to distinguish one species from another.
However, in some species the flower rotates full circle so that the labellum sits above the other petals, and the dorsal sepal is the lowermost part of the flower-examples in the New Zealand orchid flora include Gastrodia and Corunastylis spp. Photos by Jeremy Rolfe.