PLASTID DEVELOPMENT IN THE PRIMARY LEAF OF PHASEOLUS VULGARIS: VARIATIONS BETWEEN DIFFERENT TYPES OF CELL

Summary During the period of cellular differentiation, plastids in all cells throughout the primary leaf of Phaseolus vulgaris undergo the same sequence of structural changes. However plastids in different types of cell can, at an early stage, be distinguished from each other quantitatively, i.e. in size, in the amount of starch accumulated, and, later, in the extent of their thylakoid systems. These quantitative differences depend in part on the position of the cell within the lamina and in part on the type of cell involved, and they persist throughout development. Further modifications in plastid structure affect only a few specialized types of cell. These later modifications in plastid structure take place only as the cells become fully differentiated. Plastid division occurs in two separate phases, during seed development on the parent plant and during germination. It is probable that differing rates of plastid division and cell division in epidermal, palisade and mesophyll cells during early stages of germination are responsible for the characteristically different numbers of plastids present in each type of cell in the mature leaf.