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Studies on the ultrastructure of desmids and its relation to their taxonomy Gerrath, Joseph Fredrick

Abstract

The cell wall was studied in 13 species in 10 genera of the Desmidiaceae in order to determine whether ultrastructural differences exist among the three subfamilies of the Desmidiaceae: Penieae. Closterieae, and Cosmarieae. The cell wall of the Penieae comprises two distinct layers, an inner layer of cellulose microfibrils and an outer electron-dense layer. The semicell walls do not overlap at the isthmus region and the lack of pores in the wall is confirmed. The cell wall of the Closterieae is one-layered without pores and the semicell walls overlap where they join. The cell wall of the Cosmarieae is one-layered with pores through which a pectic fibrillar sheath is secreted. The semicell walls overlap at the isthmus. In the unicellular Cosmarieae an initial wall lacking pores is present during elongation of the new semicell following cell division. This initial wall is discarded after formation of the mature cell wall containing pores. In the filamentous Cosmarieae both initial and mature cell walls are formed before semicell elongation occurs. In the colonial desmid, Cosmocladium saxonicum, the connecting strands between cells comprise several fibrillar subunit strands each associated with one of a group of pores near the isthmus of the cell. Cytoplasmic microtubules are present immediately below the plasma membrane in the isthmus region of cells fixed with glutaraldehyde. These microtubules are oriented perpendicular to the cell axis. A single cell at metaphase of Spondylosium nulchrum was examined. Evidence for the existence of localized centromeres in the chromosomes of this species is presented. Bundles of spindle microtubules attach to the metaphase chromosomes at localized regions and not at any other part of the chromosome. The rarely collected species Pleurotaenium spinulosum (Wolle) Brunei (Cosmarieae) has a cell wall with the same fine structure as in the genus Penium (Penieae). The transfer of this species to the genus Penium as Penium spinulosum (Wolle) comb. nov. is therefore proposed.

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