Successive cambia revisited: ontogeny, histology, diversity, and functional significance

Abstract

Plant anatomists are generally agreed on the histological products of successive cambia: bands of secondary phloem and associated secondary xylem (“vascular increments”) embedded in a background of conjunctive tissue (parenchyma, sometimes fibers). Interpretations have varied widely on the ontogeny of this plan. Studies have usually involved one or a few species. The study of numerous taxa, especially in centrospermoid families, leads to the conclusion that there is a common plan, although variations on it are manifold. A master cambium produces secondary cortex externally and, internally, rays, conjunctive tissue, vascular cambia, secondary phloem, and secondary xylem. Secondary phloem and secondary xylem are formed from the vascular cambium in each vascular increment. Vascular cambia function indefinitely, so that a master cambium and a series of vascular cambia (each in a vascular increment) function indefinitely. The master cambium either remains active as long as an axis is actively growing (although it may become quiescent following the initiation of each vascular increment and associated conjunctive tissue), or, less commonly, may be reinvented in the secondary cortex. In order to establish a framework for interpretations, the varied appearances of each of these tissues in genera that have successive cambia is discussed. Themes that particular genera represent are then examined: diversification in ray types and raylessness (Nyctaginaceae); diversification in conjunctive tissue (Aizoacaeae); rays as key structural elements (Gnetaceae); successive cambia as an apomorphy (Chrysanthemoides); protraction of cambial activity (Menispermaceae); and cambial fracture and parenchyma proliferation (Bauhinia, Mendoncia). In some taxa, such as Gnetum africanum and the large tropical lianas (Bauhinia, Menispermaceae) a master cambium is absent; new vascular cambia arise by one or a few cell divisions in cortical parenchyma, followed by rapid tangential widening of the vascular cambium. Pervasive ecophysiological themes in plants with successive cambia are examined: storage and retrieval; promotion of mechanical strength and longevity of vascular tissues; and modes of lianoid structure. Because the background tissue in plants with successive cambia is conjunctive tissue, not secondary xylem, the terms “included phloem: and “interxylary phloem” are inapplicable. The term “lateral meristem” is abandoned in favor of “master cambium.”