Trends in evolution of floral ontogeny in Cassia sensu stricto, Senna, and Chamaecrista (Leguminosae: Caesalpinioideae: Cassieae: Cassiinae); a study in convergence

Abstract

Distinctions in floral ontogeny among three segregate genera (Cassia sensu stricto, Chamaecrista, and Senna) of Cassia L. support their separation. In all species studied, the order of floral organ initiation is: sepals, petals, antesepalous stamens plus carpel, and lastly antepetalous stamens. Sepal initiation is helical in all three genera, which however differ in whether the first sepal is initiated in median abaxial position (Senna), or abaxial and off‐median (Cassia javanica), a rare character state among legumes. Order of petal initiation varies: helical in Senna vs. unidirectional in Cassia and Chamaecrista. Both stamen whorls are uniformly unidirectional. Intergeneric ontogenetic differences occur in phyllotaxy, inflorescence architecture, bracteole formation, overlap of initiation among organ whorls (calyx/corolla in Cassia; two stamen whorls in Chamaecrista), eccentric initiation on one side of a flower, anther attachment, anther pore structure, and precocious carpel initiation in Senna. The asymmetric corolla and androecium in Chamaecrista arise by precocious organ initiation on one side (left or right). The poricidal anther character can result from differing developmental pathways: lateral slits vs. sealing of lateral sutures; clasping hairs vs. sutural ridges; terminal pores (one or two) vs. none; and clamp layer formation internally that prevents lateral dehiscence. Genera differ in corolla aestivation patterns and in stigma type. Convergence is shown among the three genera, based on intergeneric dissimilarities in early floral ontogeny (floral position in the inflorescence, bracteole presence, position of the first sepal initiated, order of petal initiation, asymmetric initiation, overlap between whorls, anther morphology, and time of carpel initiation) resulting in similarities at anthesis (showy, mostly yellow salverform flowers, heteromorphic stamens, poricidal anther dehiscence, bee pollination, and chambered stigma).