The proliferation of amyloplasts in meristematic cells of developing stolons of potato and apple callus: progenitors of proplastids.

To monitor the events in the proliferation of amyloplasts, the ultrastructure of relevant structures in the cytosol has to be studied. For this investigation, photographs of cellular ultrastructures in developing potato stolons and apple callus were taken and examined. The images indicated that the contribution to proliferation of the division of mature amyloplasts was extremely low and that the major pathway involved the generation of the proplastids from "mother" amyloplasts. The generation of proplastids was followed either by division into small bodies of 1microm or less in diameter or by growth to slender proplastids of 5microm in length. The elongated proplastids multiplied by splitting at random sites, with subsequent enlargement to mature sizes. The latter process contributed to the massive accumulation of amyloplasts in cells but has not previously been adequately emphasized. With respect to the putative "mother" amyloplasts, numerous divergent amyloplasts were observed with a considerably different ultrastructure compared to the normal types, and with a characteristically extended and constricted stroma. Various lines of evidence indicated that the divergent amyloplasts were the "mother" amyloplasts of the proplastids. No other plastidic organelles with features that suggest the generation of proplastids were detected in the cytosol.