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Adequate ultrastructural preservation of cells of the green alga Trebouxia aggregata is achieved by immersion freeze fixation using liquid propane followed by freeze substitution and resin embedding at ambient temperature. Despite differential staining of membranes, using this method we have been able to study plasma membrane biogenesis during cellular division. Daughter protoplasts are separated by an ingrowing septum of plasma membrane that extends into the cell from a particular site at the peripheral plasma membrane marked by centrioles. Septum development involves tip growth followed by lateral growth. This growth seems to involve transfer of membrane from an adjacent partially coated reticulum to the septum plasma membrane. The reticulum which extends from nearby Golgi stacks to the area of septum growth is associated with an extensive array of microtubules. After daughter protoplasts are completely separated, each one becomes surrounded by a cell wall which is distinct from the persisting mother wall. The ultrastructural evidence suggests that cells of T. aggregata are autospores rather than vegetative cells. [ABSTRACT FROM AUTHOR]

A detailed description of the zoospore and its flagellar apparatus (including their absolute configuration) of the filamentous, branched green alga Coleochaete pulvinata is given. It shares several features with the previously studied swarmers of Coleochaete scutata, such as the presence of a multi-layered structure (MLS) - associated microtubular root, and diamond-type body and flagellar scales. New observations include a T-shaped striated fiber linking the two basal bodies to the MLS, four additional fibrous connections between individual basal bodies and the MLS, and a second rootlet consisting of three microtubules. The organization and absolute configuration of the entire flagellar apparatus of the Coleochaete pulvinata zoospore is compared to swarmers of other green algae belonging to the Charophyceae sensu Stewart and Mattox and to bryophyte spermatids. Some new aspects of their phylogenetic relationships are discussed. [ABSTRACT FROM AUTHOR]

The stephanokont flagellar apparatus of the zoospores of Derbesia tenuissima (De Not.) Crouan is examined and compared to the flagellar apparatuses of other green algae. The flagella of Derbesia are attached to two of three bands which lie at the junction of the body and papilla. Serial longitudinal and cross sections reveal that the basal bodies are attached to the bands along their sides and at their proximal ends. The bands are not striated in any plane. The lack of striation in the bands and the partial covering of the proximal end of the basal bodies by one of the bands closely resemble the type of flagellar connection system described as the ' Bryopsis-type' by Melkonian (1980). Zoospores of ulvalean green algae also possess these features, suggesting that green siphons are phylogenetically related to the Ulvales. It is proposed that green siphons be tentatively classified in the Ulvaphyceae rather than in the Chlorophyceae or Charophyceae. [ABSTRACT FROM AUTHOR]

Cell division in Cylindrocapsa geminella, in particular the mode of septum membrane biogenesis, has been studied with the transmission electron microscope. Septum formation takes place in a narrow layer of cytoplasm separating post-mitotic nuclei. First, each daughter nucleus develops a wide cytoplasmic pocket (invagination) containing numerous strands of rough endoplasmic reticulum (ER). Next, a proliferation of rough ER is observed in the equatorial zone of cytoplasm, which invariably contains a small number of widely scattered microtubules. The equatorially aligned cisternae of rough ER produce smooth-membraned vesicles, interpreted as smooth ER, which subsequently coalesce to form the membranous transverse septum. Thus, primary septum formation does not follow any of the two previously known basic cytokinetic patterns in green plants (i.e. plasma membrane furrowing and cell-plate formation), but instead represents a novel type of membrane flow, which effectively bypasses the Golgi apparatus. This pathway of membrane flow has remained largely ignored in current concepts of endomembrane structure and function in eukaryotes. However, it appears to be more widespread than has previously been recognized, especially in autospore-producing green algae and in red algae during the formation of tetraspores. It may represent an evolutionary intermediate type of cell division between the supposedly primitive method of plasma membrane furrowing and the more advanced cell-plate system.