Stem anatomy of Grewia caffra (Malvaceae): an uncommon cambial variant in the order Malvales

Abstract

The genus Grewia comprises about 300 species widely distributed in Africa, Asia, and Australia. Most of its species are shrubs and small trees, but three species are climbers having stems with four prominent wings. We studied the structure of juvenile and mature stems of Grewia caffra to clarify the anatomical background of the wing formation. Four wings arise in the stem sectors oriented at an angle 45° to the distichy plane. G. caffra shows irregular activity of a single cambium with alternation of zones of normal bifacial cambial activity and dormant zones. This condition, that had also been reported in some Fabaceae and Urticaceae, is interpreted here as a new type of cambial variant. This is the first report of cambial variant for the plant order Malvales based on anatomical evidence. The stiffness of the young self-supporting stems of G. caffra is presumably supplied not only by wood, but also by clusters of secondary phloem fibers. The increase in stem flexibility at the climbing stage is thought to be the result of a shift from circular to lobed stem transection. G. caffra shows very uneven deposition of secondary phloem around the stem. Sufficient strength and flexibility are probably conferred to secondary phloem strands in wings by the tangential bands of fibers. The presence of conspicuous mucilage cavities in secondary phloem suggests that this tissue is involved in accumulation of water and carbohydrates making the stems particularly succulent.