Palynology and pollen dimorphism in the genus lagerstroemia (Lythraceae)

Pollen morphology is described for 39 (of the ca. 56) species of Lagerstroemia (Lythraceae). Different pollen types are associated with different stamen whorls, and a complete characterization of the pollen, as a contribution to an overall taxonomic data base for the genus, requires recognition of both variants within individual species. The majority of species (82%) display dimorphic stamens with dimorphic pollen. Dimorphic stamens with monomorphic pollen occur in five species and monoinorphic stamens with monomorphic pollen in two species. intuitive groupings and phenetic analysis give results of taxonomic interest, including the following: 1) the shared monomorphic stamen/monomorphic pollen condition of L speciosa and L macrocarpa has not been considered indicative of close relationship, and this is supported by the distinctiveness of their pollen, 2) there is a general correlation of pollen groups with the present three-section classification, 3) pollen of three species of sect. Lagerstroemia differ from the rest by their small size, thin exines, and indistinct margo, whereas at the same time pollen, of 10 species is extremely similar indicating exceptionally close species relationships, 4) L duperreatui is misplaced in sect. Adambea and on the basis of similar pollen and other features, should be transferred to sect. Trichocarpidium. Suggested evolutionary trends are toward the presence of pseudocolpi, a more uniformly psilate tectum, a more spherical shape, a distinct margo, reduction of the colpus, and monomorphic pollen (paralleling reduction from dimorphic to monomorphic stamens).