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Abstract
Sexual reproduction in N. protracta is isogamous, although one gametangium often dehisces, releasing its gametes, before the other gametangium has completed meiosis. Sexualized cells produce an envelope of mucilage, into which the gametes are released and within which the zygotes begin their development. Two gametes are produced per gametangium and the supernumerary nuclei begin to degenerate very quickly after meiosis II. The young auxospores lie between the empty gametangia and are ellipsoidal. Expansion of the auxospores takes place in directions bearing no fixed relation to the orientation of the gametangia. The initial epivalve lacks pseudosepta, while the rostrate poles of the vegetative cells must develop during size reduction, being absent from the initial cell. Comparisons with Navicula sensu stricto show no significant similarities in sexual reproduction or frustule structure. Thus, in spite of their superficially similar protoplast structure, N. protracta is probably not closely related to Navicula sensu stricto; instead its affinities seem to lie with Parlibellus.
