Male sterility has long been known in the grass genus Cortaderia and the species have traditionally been described as dioecious (Stapf, 1897; Pilger, 1906; Hitchcock, 1927; Conert, 1961). However, Connor and Penny (1960) described gynodioecism in one New Zealand species, and it was later shown that all New Zealand Cortaderias are gynodioecious, as well as the South American C. selloana, C. araucana and C. pilosa (Connor 1963, 1965a). Further investigation reveals that gynodioecism is the basic breeding system in Cortaderia. Interest in gynodioecism as a breeding system dates from Darwin's work on the different forms of flowers on plants of the same species (1877). Gynodioecism is an outbreeding system of interbreeding females and hermaphrodites, though hermaphrodites may often be self-compatible as well. As a system it is recorded from many genera of flowering plants but most significantly in the Labiatae. In the Gramineae it would be the least common among the breeding systems reported there. Special interest in gynodioecism centers around the genetics of the control of male sterility and the means by which females are maintained at equilibrium values in natural populations. Lewis and Crowe (1956) for Origanum vulgare, Ross (1969) for Plantago lanceolata, and Connor (1965b) for Cortaderia richardii give genetic explanations for male sterility, but Lewis and Crowe alone offer a solution to the problem of maintenance of females. These problems are examined here relative to the species studied experimentally. In the gynomonoecious species Chenopodium quinoa, Simmonds (1971) reports male sterility that has the effect of producing a population of female plants and gynomonoecious plants. Simmonds refers to this state as gynodioecism; such a classification is functionally correct though not one that would be accepted as gynodioecism in a classical sense. Male sterility there is interpreted on a gene-plus-cytoplasm basis. Floral morphology is of equal interest in gynodioecism, particularly so where there is wide variation in sex expression. This paper describes for Cortaderia the floral morphology of all but two or three species, the development of flowers and other features associated with gynodioecism. Cortaderia, a genus of 24 species, is primarily South American, but there are four species in New Zealand and one in New Guinea. The basis for the taxonomy used here is in Connor and Edgar (in press).