FURTHER OBSERVATIONS ON THE PHENOMENON OF SECONDARY VACUOLATION IN LIVING CELLS

Invagination of the plasma membrane in plant cells forms peripheral or endocytic structures which often contain a complement of membrane‐bound vesicles. These structures, or secondary vacuoles, move with the streaming cytoplasm although their velocities are somewhat slower than that for the various organelles within the cytoplasm. They glide over the nucleus or flow from the peripheral cytoplasm onto a transvacuolar strand and continue unabated along the length of a strand. These structures may detach from the plasma membrane as sacs to become positioned in the cytoplasm directly under the tonoplast and project into the primary vacuole. Some endocytic vacuoles may separate from the peripheral cytoplasm and remain free within the primary vacuole; subsequently they can re‐associate with the cytoplasm. While the content and function of these vacuoles are yet to be determined, indirect evidence indicates that they are pinocytic in character since the content of an invagination is confined to the sac upon its detachment from the plasma membrane and is subsequently transported throughout the cell by cyclosis.