Development of the cymose inflorescence, cupulum and flower of Psoralea pinnata (Leguminosae: Papilionoideae: Psoraleeae)

The inflorescence of Psoralea pinnata sensu lato is composed of dichasial cymes of three to seven flowers in each bract axil. However, the main inflorescence terminal is indeterminate. Cymes are very unusual among legumes, occurring in perhaps six or seven genera. The ‘cupulum’ is a unique structure found only in Psoralea among legumes. While highly variable in form, it serves as a protective structure around the calyx during development. Developmentally the cupulum results from fusion by intercalary growth of three to four successive bracts on the pedicel. Paired bracteoles are also produced above the cupulum and below the calyx, but they are very reduced and easily overlooked. Their presence is evidence that the cupulum is not part of the flower proper. Organogeny in P. pinnata is unidirectional in each floral whorl, starting on the abaxia! side, except for a minor exception in precocity of the vexillary (adaxial) stamen arising with the two lateral stamens of the same inner whorl. There is no overlap between whorls. The carpel is initiated early, during or just after petal initiation and before stamen initiation.