Tissue-Dependent Heterogeneity of Cell Growth in the Root Apex of Pisum sativum

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Abstract
Cell division, expansion, and differentiation occur at all levels in the apical 20 mm of the root apex of Pisum sativum, frequently occurring simultaneously in one cell. Cells display strongly tissue-dependent characteristics with respect to polarity, rate, and developmental timing of both division and expansion. Most tissue-dependent changes in cell shape and size are associated with the frequency and polarity of cell division; however, some changes in cell shape in the cross-sectional plane occur in the absence of cell division, indicating polarity of expansion. During expansion, proportions of the root cross-sectional area occupied by different tissues change, demonstrating tissue-dependent differences in rates of growth. Cell development is thus far less uniform, both between tissues and during development within a tissue, than implied in the classical zonation model of root development. Longitudinal zones of strictly coordinated and uniform developmental activities do not occur, while division, expansion, and differentiation are clearly not necessarily incompatible or spatially or temporally separated during cell development. The root does not grow strictly as a unit; tissues develop relatively independently of each other. Cellular growth and development appear to proceed in a manner primarily dependent on tissue type from a very early stage of development.