Cassia sensu lato (Caesalpiniaceae) is one of the largest genera of flowering plants, consists of about 600 species. Recent works on floral morphology proposed a division of this genus into three genera namely, Cassia sensu stricto, Senna, and Chamaecrista. This research was the comparative anatomy using light microscopes of stem, leaf, and flower of Cassia s.l. grew in Thailand. Seventeen species and 3 subspecies out 22 species 4 subspecies of Cassia s.l. in Thailand were employed. All of studied species shared the common characters :- the tunica-corpus meristems; the solitary diffuse and circular large vessels; the simple perforation plates; the heterocellular xylem rays; the nonstoried wood; the bifacial leaves with paracytic stomata; and the marginal placental ovaries with anatropous ovules. However, there were some useful discriminative anatomical features, i.e.- the characters of anthers, the distribution of stomata, the types and positions of crystals and trichomes, and the xylem rays. Accordingly, the studied wpecies could be classified into four group i) Cassia s.s., ii) Senna (tree), iii) Senna (shrub), and iv) Chamaecrista. The endothecial thickenings and the deposition of druse crystals in stem and leaf were found in all groups, but not in Chamaecrista. The Cassia s.s. group could be separated from Senna by its presence of trichomes on anther. Moreover, the Senna group could be subdivided into two groups according to the stomatal distribution on leaves. It was also found that Senna (tree) possessed the hypostomatic leaf whilst the amphistomatic leaf were observed in Senna (shrub). Full anatomical descriptions based on the discoverable anatomical characters of all studied species, a key to the groups and a key to the species were prepared.
