Successive cambia in Aizoaceae: products and process

Abstract

The transverse and longitudinal sections of the stems and roots of 11 genera of Aizoaceae. representing a wide range of growth forms from hard fibrous stems to fibre-free roots, were studied using light microscopy and scanning elec tron microscop Inmost of the genera, fibres are the first xylary product of each vascular cambium. followed by ves sels in a parenchyma background. Variations on this pattern help to prove that fibres are produced by vascular cambia, except in Ruschia and Stayneria, in which both the lateral meristem and the vascular cambia produce fibres. Cylinders of conjunctive tissue parenchyma that alternate with the vascular cylinders are produced by the lateral meristem. The concept that the lateral meristem gives rise to the vascular cambia and secondary cortex is supported by photographic evidence. Radial divisions occur in the origin of the lateral meristem, and then again as vascular cambia arise from the lateral meristem; these radial divisions account for storeying in fibres and conjunctive tissue. Raylessness characterizes all Aizoaceae studied, with the exception of Tetragonia, which also differs from the remaining genera by having vasicentric axial parenchvma, a scattering of vessels amongst fibres, and the presence of druses instead of raphides. Several vascular cambia axe typically formed per year. Several vascular cambia are active simultaneously in a given stem or root. Roots have fewer fibres and more abundant conjunctive tissue paren chyma than stems. Successive cambia result in an ideal dispersion of vascular tissue with respect to water and pho tosynthate storage and retrieval capabilities of the paxenchyma, and to liana stem plans. The distribution and relative abundance of fibres, vessels, secondary phloem, and conjunctive tissue parenchyma relate primarily to habit and are not a good source of systematic data, with the probable exception of Tetragortia. The general pattern of lat eral meristem and vascular cambial ontogeny is the same as in other families of the core Caryophyllales, although the patterns of the tissues produced are diverse. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153, 141—155.