The development of leaf stomata in species of Orchidoideae sensu Garay is agenous or hemimesogenous with a single mesogene cell. Both kinds of development occur in all 26 species studied and are found in a characteristic proportion which may differ markedly even between related species. Leaf and stem stomata develop similarly but surrounding cells may divide obliquely in the latter, so that perigene cells are formed. At maturity, mesogene and perigene cells resemble other epidermal cells, the stomata being anomocytic. In monocotyledons, the hemimesogenous development of stomata is previously known only from five orchid genera of the neottioid tribe Cranichideae sensu Dressler. This kind of stomatal development in monocotyledons is documented by micrographs for the first time. Mesogene and perigene cells are recorded for the first time in the Orchidoideae. The diversity of types of stomatal development in this group is emphasized.