FLORAL ONTOGENY IN SOPHOREAE (LEGUMINOSAE: PAPILIONOIDEAE). I. MYROXYLON (MYROXYLON GROUP) AND CASTANOSPERMUM (ANGYLOCALYX GROUP)

Abstract

Floral ontogeny is described and documented using scanning electron microscopy in Myroxylon balsamum and Castanospermum australe, representatives respectively of Polhill's Myroxylon and Angylocalyx groups (Leguminosae: Papilionoideae), groups exhibiting relatively unspecialized flower structure for the tribe Sophoreae. Both are woody tropical trees with axillary or terminal racemes or panicles. Bracteoles are present in both Myroxylon and Castanospermum. Flowers are initiated singly in bract axils, which are produced in acropetal order by the inflorescence apical meristem. The flower structure of both includes a broad calyx tube, five petals lacking any fusion (only the vexillary distinctive), ten free homogeneous stamens in two whorls, and a long‐stipitate carpel. The two taxa are alike in early organogenetic stages with essentially acropetal order of initiation: sepals, petals, outer stamens plus carpel, inner stamens. Within each whorl the order is unidirectional from the abaxial side. They are alike through middevelopment with one exception. There is accelerated vexillar enlargement in Castanospermum by middevelopment, not found in Myroxylon. Both have a hypanthium, which forms late in development. In both, large flower size, exserted stamens, and hypanthium are adaptations to bird‐pollination. Differences between the two that are manifested in late development include strongly zygomorphic calyx and petal color change over time (Castanospermum), stamens sagittate and apiculate with some basal filament fusion (Myroxylon), stigma form differences, and fruit form.