Multiple cambia and secondary xylem of Ipomoea pes-caprae (L.) R. Br. (Convolvulaceae)

摘要

Structure of secondary xylem and pattern of secondary thickening in climbing species are different from those in self-supporting plants. In many climbing species, stem diameter increases by forming more than one ring of cambium (referred to as multiple/successive cambia), while their secondary xylem usually contains abundant parenchyma, large vessels and wide rays. In beach morning glory (Ipomoea pes-caprae (L.) R. Br., Convolvulaceae), stem thickness increases by forming multiple rings of cambia. After a short period of normal secondary growth, the first successive cambium ensues from the pericyclic parenchyma. Thereafter, subsequent cambial rings originate from parenchyma cells produced initially by the previous cambium. In stems that are 15-20 mm thick, parenchymal cells produced by the initial activity of the previous cambia become meristematic and form small arcs of functionally inverse cambia that produce secondary xylem centrifugally and secondary phloem centripetally. Unequal production of secondary xylem by these cambia gives the stem various shapes other than cylindrical. Besides successive cambia, some cambial variants also develop in the stem which are: (1) irregularly distributed patches of thin-walled xylem parenchyma becoming meristematic and differentiating into interxylary phloem islands; (2) xylem ray cells acquiring meristematic character and behaving like cambium (referred to as ray cambium); and (3) in thick stems, internal cambium deriving from marginal pith cells, which are functionally bidirectional and producing secondary xylem centripetally and phloem centrifugally. Structure and development of successive cambia, ray cambia and internal cambium are discussed here.