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By their wide morphological diversity, succulent leaves have aroused the interest of many researchers. Nonetheless, comparative anatomical studies of unifacial, bifacial, and subunifacial leaves are scarce. To address this gap, our study examines the growth and differentiation of vascular tissues in succulent leaves of 12 families of angiosperms. Altogether, we identified six arrangements arising as variations of three basic vascularization patterns. In addition, cases of convergent evolution were identified in (1) terete leaves showing both endoscopic peripheral bundles and a central bundle in some Crassulaceae (Saxifragales, eudicots) and Aizoaceae (Caryophyllales, eudicots) and (2) terete leaves showing exclusively endoscopic peripheral bundles in some Asteraceae (eudicots) and Orchidaceae (monocots). Both endoscopic and exoscopic bundles originate from a peripheral region, and in all species with endoscopic peripheral bundles, the differentiation of the vascular bundles begins from abaxial domains, wh...

Unifacial leaves are described in different angiosperm groups, especially in monocotyledons, which show anatomical and molecular evidence of an abaxialization process. Among the structural characters, the anatomy of epidermis and growth tissues has been reported. Special focus has been given to vascularization patterns, such as the presence and organization of peripheral vascular bundles. In the Aizoaceae, peripheral vascular bundles exhibit two patterns: exoscopic and endoscopic bundles; however, studies including an ontogenetic approach with evidence for a unifacial condition in the group are lacking. We investigated eleven Ruschioideae species, including members of two tribes (Ruschieae and Dorotheantheae), which present flat and terete leaves. Our results show that all leaves present a bifacial leaf base, with xylem cells oriented toward the adaxial face present in collateral vascular bundles. However, the occurrence of peripheral endoscopic vascular bundles in the leaf blade in all terete leaves demonstrates a unifacial condition which is present only at the leaf blades. Based on ontogenetics studies, we demonstrate that the peripheral endoscopic bundles begin their differentiation in the abaxial side of the leaf primordia, suggesting the occurrence of an abaxialization process during leaf blade development.