UNIDIRECTIONAL ORGAN INITIATION IN LEGUMINOUS FLOWERS

The order of initiation of floral organs is compared in several legumes. In Bauhinia fassoglensis, a caesalpinioid, the sepals are initiated helically, with the first one forming abaxially. In Genista tinctoria and Lupinus affinis (both papilionoids) the sepals are initiated unidirectionally, with the first forming on the abaxial side of the floral apex and subsequent sepals initiating laterally and then adaxially. All three taxa show unidirectional order of initiation for petals, first-whorl stamens, and second-whorl stamens. In each whorl, the first member or members form on the abaxial side, next to the subtending bract, then the lateral ones, and lastly the member(s) on the adaxial side, next to the axis. In Lupinus and Genista there are overlaps in time of initiation between organs in different whorls; for instance, the first stamens begin initiating before the last petals appear. Size differences among members of a whorl are evident in early stages, but may disappear after organogeny ceases, when the members become equal in size in each whorl. This precocious onset of dorsiventrality in floral development is viewed as a specialized feature.