Leaflet anatomy of 14 taxa in Mimosa sect. Batocaulon, series Quadrivalves is described for the first time, using cleared leaflets and light microscopy. Members of the series have obliquely asymmetric, linear-oblong, narrowly elliptic, or narrowly oblanceolate leaflets with an eccentric primary vein. The venation pattern is brochidodromous with basal lateral veins. In nine of the 14 taxa the lateral loops become notably narrower in the distal portion (anisonerved loops). On the basis of primary vein size and width, and the nature of the loops, the taxa of the series are divided into two groups, one with isonerved loops and a notably thickened primary vein (massive) and the other with anisonerved loops and a stout or moderately thickened primary vein; the primary vein and secondary veins can be sinuate or straight. Idioblasts were observed in association with the ends of the veinlets. These are of three types: polymorphic sclereids, osteosclereids, and tracheoidal elements. Calcium oxalate prisms and druses are present in most taxa of the series. The first are associated with the vascular bundles, while druses are found in mesophyll cells. The systematic value of leaflet anatomical characters is discussed, and leaflet anatomy is found to provide significant characters for distinguishing groups and individual taxa in series Quadrivalves.