Heterospory

Heterospory is the production of two distinct spore types: microspores develop into male gametophytes, and megaspores develop into female gametophytes.

Microspore: A small spore that develops into a male gametophyte in heterosporous plants. It is the male-destined spore type produced alongside the megaspore.
Selaginella: A genus of lycophytes that includes living heterosporous plants. Its species show heterospory outside the seed-plant lineage.
Evolution of seeds: The evolutionary origin of seeds from reproductive structures of ancient vascular plants. Heterospory was a prerequisite for the seed lineage's reproductive innovations.
Marsilea: A genus of aquatic ferns that produces microspores and megaspores in specialized sporangia. This fern demonstrates heterospory in a lineage distinct from lycophytes and seed plants.
Megaspore: A large spore that develops into a female gametophyte in heterosporous plants. Its greater size supports female gametophyte development and often nourishes it.
Isoetes: A living genus of lycophytes whose species produce microspores and megaspores. It provides another surviving example of heterospory among lycophytes.
Ovule: A plant structure containing the megasporangium and female gametophyte that can develop into a seed after fertilization. The ovule retains the megaspore and female gametophyte within the parent sporophyte.
Azolla: A genus of small aquatic ferns with microspores and megaspores enclosed in sporocarps. Its compact reproductive structures make heterospory conspicuous in living ferns.
Gametophyte: The haploid life-cycle stage that produces gametes in plants and some algae. The two spore types differ because each produces a distinct-sex gametophyte.
Archaeopteris: An extinct Devonian tree genus known for large forests and early seed-like reproductive features. Its reproductive history helps trace the transition toward seed plants.

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