Abstract: The plasmalemma of cells within meristematic regions was observed to possess invaginations in cultured callus of Stevia rebaudiana under differentiation. The ultrastructure and acid phosphatase (AcPase) ultracytochemistry Of these invaginations were studied. The plasmalemma invaginations occurred in the cells at various stages of vacuolation. In cells with dense protoplasm, plasmalemma appeared undulated but occasionally spherical and variable in size with conspicuous invaginations that projected into the peripheral cytoplasm. In the partially vacuolated cells, plasmalemma invagination became voluminously enlarged with increased contents and structurally complexed. In vacuolated cells, the enlarged invaginations protruded into the central vacuole but were delimitted from the tonoplast by an intermembrane zone continuous with the peripheral cytoplasm. Complex accumulations of membranes consisting of vesicular and coiled membranous Structures might develop within the plasmalemma invaginations. AcPase localization demonstrated high enzymic activity in the plasmalemma and its associated invagination. It seemed likely that these invaginations were functionally analogous to the vacuoles and therefore constituted part of the lytic compartment in these cells.

Key words: Plasmalemma invagination, Ultrastructure, Acid phosphatase ultracytoche-mical localization, Callus tissue, Stevia rebaudiana