Taxonomic implications of the leaf anatomy of twelve species of Senna ser. Bacillares (Leguminosae) from northeastern Brazil

Senna ser. Bacillares is a Neotropical group with about 50 species, being the largest and taxonomically more complex series of Senna. The leaf anatomy of twelve species of Senna ser. Bacillares (Leguminosae) from northeastern Brazil was examined to investigate the importance of anatomical characters in delimiting species within the series. Forty-three specimens were analyzed using light microscopy, following the usual techniques in plant anatomy. Four different petiole profiles were observed: circular, semicircular, hexagonal, and pentagonal; most petioles showed adaxial projections, those being strongly prominent in S. chrysocarpa and S. quinquangulata var. quinquangulata. Six different leaf rachis profiles were observed: pentagonal, elliptic, obovate, circular, semicircular, and irregularly triangular. The vascular systems of the petioles and leaf rachis showed similar patterns, with 4–7 central beams and two accessories. The midrib was plane-convex with an arched vascular bundle. The dorsiventral mesophyll has uniseriate palisade parenchyma, and 4–7-seriate spongy parenchyma. Five different leaflet edges outlines were observed: rounded, recurved, acute, acuminate, and truncate; the truncate and acuminate types were exclusive to S. angulata var. miscadena and S. gardneri, respectively. The dendrogram generated by cluster analysis showed the formation of two groups differentiated by the presence/absence of trichomes on the petiole and leaf rachis, and by the outline of the petiole. An identification key, based on morphoanatomical characters, is presented. Anatomical characters like the contour and number of vascular bundles of the petiole and leaf rachis, and shape of leaflet edges, provided additional support to the taxonomy of species of Senna ser. Bacillares, reinforcing the importance of anatomical studies to the taxonomy and systematics of Leguminosae.