Penium spinulosum (Wolle) comb. nov. (Desmidiaceae): a taxonomic correction based on cell wall ultrastructure

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Abstract
Electron microscope studies on the desmid Pleurotaenium spinulosum (Wolle) Brunel show that it should be transferred to the genus Penium. The cell wall ultrastructure is similar to that of P. margaritaceum and P. spirostriolatum. Two definite wall layers are present in all three species, an outer electron-dense layer and an inner layer of interwoven bands of parallel microfibrils. The inner portion of the outer wall layer is embedded among the microfibrils of the inner wall layer. In all three species the outer wall layer is perforated but the perforations do not extend through the inner wall layer. The spines of P. spinulosum are extensions of the outer wall layer. At the isthmus region there is a furrow in the outer wall layer but the inner wall layer and the embedded portion of the outer wall layer both appear continuous across the isthmus. In Pleurotaenium nodosum and other Cosmarium-type desmids the wall appears homogeneous in the electron microscope and has many pores that extend completely through the cell wall. A fibrillar sheath is present. At the isthmus region there is a conspicuous overlap of the semicell walls.

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