Heterospory

Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the ferns including the arborescent horsetails, and progymnosperms. This occurred as part of the process of evolution of the timing of sex differentiation.