Ontogeny of the multiple cambial variants and secondary growth in some species of Merremia and related genera (Convolvulaceae)

Abstract

The present study investigates the stem anatomy and development of cambial variants in 10 species of Merremia and related genera (Convolvulaceae) were compared histologically. Among them, Camonea kingii, C. umbellata, Distimake aegyptius, D. quinquefolius, D. vitifolius and M. hederacea increased their stem diameter by initiating successive cambia. In contrast, remaining species showed regular secondary growth. These species also demonstrated inversely oriented vascular elements, phloem wedges, intraxylary cambia, parenchyma proliferation and development of radial vascular cylinders in the large rays of xylem. Phloem wedges developed by the differential activity of regular cambium in small segments by depositing only unlignified parenchyma internally whereas nearby sectors of the cambium behaved regularly. After the development of intraxylary protophloem from procambial derivatives, additional phloem was formed from the pith cells and by initiating intraxylary cambium. Initially, intraxylary cambium was unifacial in all species, but eventually became bifacial (except M. emarginata) and deposited both xylem and phloem. In conclusion, Merremia species use all feasible methods to generate new vascular elements to compensate for the supply of water, nutrients and photosynthate. Inter-and intraxylary phloem development appears to provide an additional and safe conduit for long-distance vertical conduction, whereas radial vascular cylinders facilitate radial conduction.