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Abstract
Auxospore formation in Nitzschia recta and N. sigmoidea is associated with allogamous sexual reproduction, in which two vegetative cells pair and function as gametangia. Each gametangium produces two morphologically identical gametes, one of which is active and the other passive (trans-behavioural anisogamy). The gametes become re-arranged within the gametangia and then the active gametes migrate through a narrow copulation tube to effect plasmogamy. The copulation tube has an organic wall and is produced by fusion of two papillae, whch grow out through localized splits in the gametangial frustules during meiotic prophase. The auxospores do not have an obvious perizonium and expand parallel to the apical axes of the gametangia. The same kind of reproduction occurs in N. dissipata and N. flexoides, supporting the idea that all these species form a natural group, also characterized by the presence of conopea on the valves. Reproductive and other characters suggest that Nitzschia and its allies may have evolved from an amphoroid ancestor.