Floral ontogeny in Swartzia (Leguminosae: Papilionoideae: Swartzieae): distribution and role of the ring meristem.

The anomalous systematic position of Swartzieae at the base of Papilionoideae is correlated with unusual developmental features: one petal or none; a ring meristem; polystemony; heterostemony; little or no alignment of stamens as antesepalous or antepetalous; multicarpely; and absence of unidirectional order of organs except in the calyx. Symmetry is zygomorphic throughout development. Floral ontogeny of four species of Swartzia reveals five sepals are initiated successively, beginning abaxially, but intercalary growth below the separate sepals forms a tubular calyx that splits irregularly, a feature typifying the genus. A single petal is initiated adaxially or may be missing altogether (in S. sericea). The apex enlarges and forms a ring meristem concurrently with carpel initiation. Several large-stamen primordia (2-15, according to the species) initiate first on the ring, followed by 40-150 small-stamen primordia. The latter initiate in centrifugal order in S. aureosericea and S. laurifolia or in acropetal order in S. sericea and S. madagascariensis. While ring meristems are considered to be homologous among Neotropical species studied as well as in the African S. madagascariensis, they vary in extent, duration, order of initiation, and productivity. Swartzieae is unlikely to be ancestral to the rest of Papilionoideae, based on radically differing floral ontogeny in the two groups.