Comparative floral development and evolution in tribe Caesalpinieae (Leguminosae: Caesalpinioideae). Haematoxylum.

Floral organogenesis and development of the tropical legume trees Haematoxylum campechianum (logwood) and H. brasiletto (brazilwood) were studied using scanning electron microscopy. The aims were to compare ontogenies, and to elucidate the relationships of Haematoxylum with other genera of Caesalpinieae, the basal tribe of Caesalpinioideae. Flowers of Haematoxylum are in racemes or fascicles, lack bracteoles, and are pentamerous, hermaphroditic, and either actinomorphic or zygomorphic. Whorls arise in acropetal order except for the carpel, which arises concurrently with the outer stamens. Sepal order is bidirectional (a rare condition) within the whorl in both. Petals and outer stamens are initiated bidirectionally in H. campechianum, and unidirectionally in H. brasiletto. Inner stamens are initiated unidirectionally in both. In H. campechianum, time of petal initiation overlaps with that of outer stamens, and initiation of the two stamen whorls overlap. In both, the gynoecium becomes stipitate, and a hypanthium forms late in development. Both show many plesiomorphic states at anthesis; H. brasiletto alone shows several specialized states (expressed late in development), including a fused, gibbous calyx cup, a zygomorphic corolla, lightly aggregated filaments held together by hairs, and fenestrations in the stamen column. Ontogenetic divergence late in ontogeny characterizes differences at anthesis between related species.