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Hypothetical stages in the evolution of seeds.

Description
The parallel evolution of heterospory and endosporic gametophytes in some lycophytes and pteridophytes, as well as the seed plants, suggests that these features were acquired early in the evolution of seeds. Later-occurring events in the origin of seeds were the reduction in the number of megaspores to one per megasporangium and the enclosure of the megasporangium by protective, leaflike integuments.
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