Gynostemium structure and development in subtribe Spiranthinae (Cranichideae, Orchidaceae)

Abstract

We analysed gynostemium development and morphology of 13 species of Spiranthinae to understand the structure and homology of this organ. Flowers and flower buds in different developmental stages were examined with scanning electron and light microscopy. Gynostemium ontogeny is similar in the early stages among the analysed species, and most of the differences arose at later developmental stages. Gynostemium development starts with the appearance of the anther primordium, followed by the median carpel and finally by the lateral carpel apices. The last emerges as two congenitally united, crescent-shaped prominences located between the median carpel apex and the labellum, eventually forming a rim of tissue on the proximal margin of the stigma. The rim can be receptive or not. The base of the median carpel apex contributes mostly to the receptive stigmatic zone and its apical region develops into the viscidium. The entrance of the stylar canal is located between the two partially non-receptive lateral carpel apices and the receptive base of the median carpel apex, refuting the idea that in Sarcoglottis it is located above the stigmatic area. There are no staminodal primordia, and the membranaceous appendages at each side of the column apex represent extensions of the clinandrium margins.