The distribution of the radioactivity from [14C]proline that is bound in cultured cells of Acer has been determined by electron microscope autoradiography. In this way proline may be related to the cell wall as a morphological entity rather than as a fraction in a biochemical separation of a heterogeneous crop of cells. The cells in culture may vary greatly. Some are active growing, turgid cells, with thin protoplasts tightly pressed against their walls; in others the protoplasts may spontaneously withdraw from the wall; in still others the protoplasts disorganize, and walls thicken and become sculptured as the cells differentiate and even senesce. Different culturing practices may affect the status of the cells, and this, in turn, affects the distribution of radioactivity from proline in the cells. Cells which are actively growing, turgid, and nucleated have the highest grain density in their protoplasts and nuclei; as the protoplasts of such cells withdraw from their walls, they retain the bulk of the radioactivity. On the other hand, in cells which have thickened walls and sparse protoplast contents, the radioactivity is accumulated in their walls. A high content of proline and hydroxyproline-rich protein is, therefore, not a necessary or invariable feature of the cell walls of cultured Acer cells but depends on the state of development of these cells.

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