Steady Form from Changing Cells

The analysis of longitudinal growth is reviewed in terms of growth trajectories and fields of velocity and relative elemental growth (REG) rate. Then the relationships among cell division rate, cell size, and growth rate are reviewed. A careful distinction is made between spatial (site specific) and material (cellparticle specific) aspects of growth and cell division. The equations indicate the experimental design and numerical analysis needed to assess the physiological importance of gene products recently proposed to regulate cell cycle activity. Effects of temperature and water stress on growth and cell division in seedling roots of maize are described. Temperature changes REG rates and cell production rates in synchrony, so the cell length pattern is unchanged by temperature perturbation between 19 and 29 C. In contrast, water stress has no effect on REG rate in apical regions but shortens the length of the growth zone. Water stress also causes a decrease in the cell production rate. Thus water stress uncouples the synchrony between growth and cell division to produce longer cells at the base of the meristem and shorter cells at the base of the growth zone. The longer cells near the root apex probably facilitate transport of metabolites from the phloem to the dividing cells in stressed roots. In general, patterns of cell size may affect the selection of plants for environmental adaptation.