The plasma membrane in immature cells is often irregular in contour. Some irregularities become conspicuous folds that continue to enlarge into the cytoplasm. These invaginations may continue to increase in size and typically expand into the central vacuole. Sections show two closely parallel membranes in areas where the invagination projects into the vacuole. A narrow layer of cytoplasm may traverse the intermembrane zone between the membranes. The interior of an invagination may lack obvious content, or may be occupied by a fibrous material, or vesicular and tubular structures. The small vesicles bound by a single membrane appear to be derived from a projection formed most frequently near the orifice of the invagination. The origin of large vesicles and tubules possessing one membrane is not certain although they may arise by the fusion of several small vesicles. Alternatively, tubules, once formed, often possess constrictions along their length which suggest that these structures may become divided into a series of smaller vesicles. There is some evidence that cytoplasmic vesicles may fold into the membrane of the invagination and subsequently be pinched off into the interior of the sac. These vesicles are bounded by two membranes. Vesicles frequently contain an electron‐dense content more or less homogeneous in composition and unlike the typical ribosomal character of the cytoplasm. The function of the invaginations or their content remains to be elucidated.

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The vacuolar apparatus of various plant cells consists of two distinct features: the large central vacuole and peripheral vacuoles which are derived from invaginations of the plasma membrane. Peripheral vacuoles are conspicuous structures in both living and fixed hair or filament cells of Tradescantia virginiana. They occur as spherical structures along the inner boundary of the peripheral cytoplasm and can be recognized as projections into the central vacuole. These structures are variable in size and number within a cell and can represent a significant proportion of the volume of the vacuole. Peripheral vacuoles most frequently are observed in motion with the streaming cytoplasm although their velocity is usually somewhat slower that that of the cytoplasmic organelles. Ultrastructural studies show two closely approximated membranes, one for each vacuole, in areas where a peripheral vacuole projects into the central vacuole. These are separated by an intermembrane zone continuous with the peripheral cytoplasm. The movement of organelles over the perimeter of the peripheral vacuole is presumed to occur along this intermembrane zone. The internal area of the peripheral vacuoles may appear empty although some contain a vesicular content of unknown origin and function.