The Changing Shape of Plant Cells: Transformations During Cell Proliferation

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Abstract
Shapes of ideal cells can be inspected for the dynamic, or gnomonic, feature of producing daughter cells of the same shape. Such features can be found for (a) elongating epidermal cells, (b) isdiametrically enlarging epidermal cells, (c) elongating parenchymatous cells and (d) parenchymatous cells enlarging in three dimensions. Since each cell passes through a series of changes to finally assume the form of the parental cell, a gnomonic cell must pass through a gnomonic sequence of shapes during the cell cycle. A model tissue composed of gnomonic cells has complete stability of form through subsequent generations. Each of six parameters of ideal cells can be inspected in real cells in order to evaluate the effects of deviations from the ideal on the stability of tissue pattern. (1) Cell plates of real and ideal cells do not expand for one generation. (2) The angles in vertices of real cells shift over three cell cycles from 170.1° to 137.3° to 124.0°, values close to the expected set of 163°, 133° and 120° (3) Cell plates of real cells are not perpendicular to the longitudinal axis of the cell. (4) Real cells do not divide synchronously as do ideal cells. (5) Real cells do not divide equally in half as do ideal cells. (6) Finally, ideal cells have the same duration of the cell cycle whereas real cells have cycle times inversely related to the initial size of the cell. It appears that a population of meristematic cells do not adhere to the restrictions of ideal cells, and consequently a significant amount of variance of form is added at each generation. There are two compensating mechanisms, one to hold size variation in check and one to keep shape deviations under control. Because of the probabilistic nature of cell division, cells increase in volume at various rates while the cell edges of all cells expand at a constant rate, indicating that the latter is the primary element of growth while facet area and cell volume increase in dimension only for accommodation.