| Abstract: | The cysts (statospores) of Ochromonas tuberculata Hibberd are produced within a cytoplasmic silica deposition vesicle (SDV) whose membrane (silicalemma) appears to be formed by the coalescence of golgi vesicles. Silica is first deposited as small nodules and the collar and spines develop by centrifugal growth only after a complete but still thin wall has been laid down. Small vesicles appear to be attached to the SDV only in the region overlying the developing collar; a cap of radially arranged, moderately electron-dense material occurs at the tip of the growing spines. The cyst pore is formed at the anterior end of the flagellate cell, by lack of silica deposition over a small region of the SDV and rupture of the SDV and other membranes crossing this region. When the cyst wall is complete, an extracystic plug is formed in the pore, resulting in the loss of some extracystic cytoplasm and the plasmalemma, and the inner SDV membrane becomes the functional plasmalemma. The plug develops first by coalescence with the cell membrane of golgi-derived vesicles containing dense but apparently nonsiliceous spicules surrounded by amorphous material. During later stages of plug formation only fibrous material is deposited, some of which may be extruded through the pore forcing out some of the spiculate component. Scanning electron micrographs of the mature wall show it is smooth except for the concentrically wrinkled inner face of the flared collar and that the real pore diameter is only ca. half that of the collar. At germination the plug completely disappears in an unknown way and a single cell, similar to a normal vegetative cell emerges through the pore. Chrysophycean cyst formation generally resembles cell wall formation in diatoms, but differs in some details. |
