The Schoenus spikelet: a Rhipidium? A floral ontogenetic answer

The inflorescence unit of Schoenus nigricans and S. ferrugineus consists of a zigzag axis and distichously arranged bracts, each of which may or may not subtend a bisexual flower. Each flower seems to terminate a lateral axis. These features have led to a controversy about the nature of the inflorescence unit, particularly whether it is monopodial or sympodial. It was often seen as a pseudospikelet composed of a succession of lateral axes, each subtended by the prophyll of the previous axis, as in a rhipidium. Many authors, however, consider the inflorescence units of all Cyperaceae to be indeterminate, racemose, actual spikelets. In our study, we present new SEM observations on the floral ontogeny of S. nigricans, corroborating a monopodial interpretation of the spikelet. Concaulescent growth of the flower primordium and the spikelet apex explains: (1) the presence of a peduncle under the flower, (2) the advanced development of the subtending glume compared to its own flower primordium, and (3) the position of the distal glume with regard to the distal flower primordium.