Megasporogenesis in a heterosporous fern; features of the organelles in meiotic cells and young megaspores.

Megasporogenesis in the heterosporous fern Marsilea (Hydropterideae) shows features intermediate between sporogenesis in homosporous ferns and that in heterosporous seed plants. The plastids in the dyads and young spores were associated with tubules 30-35 nm in diameter, probably a specialized form of endoplasmic reticulum. No consistent differences in size or cytoplasmic components could be found between the megaspores of a tetrad that might account for differential survival. The view that megaspore viability within the tetrad is genetically determined is thereby strengthened.