Floral Morphology of<i>Apuleia leiocarpa</i>(Dialiinae: Leguminosae), an Unusual Andromonoecious Legume

Abstract

Apuleia leiocarpa (Vogel) J. F. Macbr. is a South American tree species belonging to the caesalpinioid subfamily of the Leguminosae. Phylogenetic analyses have placed Apuleia into the Dialiinae s.l. clade, with which it shares numerous morphological similarities, including dichasial cymose inflorescences. The genus has also been reported to be andromonoecious, a trait not seen elsewhere in its clade and only infrequently in the Caesalpinioideae. Scanning electron microscopy and stereomicroscopy were used to investigate the floral morphology of A. leiocarpa. It was found that while hermaphrodite flowers produced a single carpel and two stamens, staminate flowers developed three stamens but showed no signs of any carpel development. The third stamen fills the volume of the missing carpel within the bud as it grows. Inflorescences also appear to produce approximately four times as many staminate as hermaphrodite flowers. Both this high male-to-bisexual ratio and the early determination of gender seen in Apuleia are rare in the Caesalpinioideae and suggest that andromonoecy developed in this genus as a means to increase pollen dispersal rather than in response to resource limitations.