Ontogeny of multiple variants in the stems of Phaseolus lunatus L. (Fabaceae)

Development of the cambial variant is characteristic to most of the climbing members of the Fabaceae and is considered as an alternative path to increase their stem diameter for efficient supply of minerals and photosynthate within limited stem diameter. These alternative forms of secondary growths are ascribed as an adaptive character to increase stem flexibility and most often they are found to be taxon-specific. In the present study, histological investigation on the main stems of Phaseolus lunatus L. (Fabaceae) showed multiple combinations of variant secondary growth like the formation of vascular cylinders external to the secondary phloem and proliferation of parenchyma (axial and ray) followed by the development of interxylary phloem, interxylary cambia (functionally regular and inverse) and ray cambia. The secondary xylem produced by regular cambium was diffuse-porous with indistinct growth rings and composed of an abundance of thin-walled unlignified parenchyma. With the increase in age, phloem rays underwent dilatation, subsequently became meristematic and formed oval to circular vascular cylinders/units with irregular orientation. Concomitantly, dedifferentiation xylem parenchyma not only formed small islands of interxylary phloem but also led to the origin of interxylary cambial segments that differentiated bidirectionally and deposited xylem and phloem having inverse, diagonal, tangential or radial orientations. Similar cell division activity was also observed in marginal xylem ray cells (ray cambium) that produced both the xylem and phloem on either side. All these cambial variants are described in detail and their possible significance in the climbing habit is discussed.