Heterospory

Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the ferns including the arborescent horsetails,[1] and progymnosperms.

[4][5] Heterospory evolved due to natural selection that favoured an increase in propagule size compared with the smaller spores of homosporous plants.

This may first have led to an increase in spore size and ultimately resulted in the species producing larger megaspores as well as smaller microspores.

[9][10] Heterospory is advantageous in that having two different types of spores increases the likeliness that plants would successfully produce offspring.

[7] Heterosporous spores can respond independently to selection by ecological conditions in order to strengthen male and female reproductive function.

[8] Microspores are haploid spores that in endosporic species contain the male gametophyte, which is carried to the megaspores by wind, water currents or animal vectors.

[11] The morphology of the microspore consists of an outer double walled structures surrounding the dense cytoplasm and central nucleus.

[8] This specific type of self-fertilization is termed as sporophytic selfing, and in extant plants it occurs most commonly among angiosperms.