Growth Patterns and Different Arrangements of Vascular Tissues in Succulent Leaves

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...