heterospory

thumb|A female pinecone (Pinophyta) produces the megaspores of this heterosporic plant. thumb|A male pinecone (Pinophyta) produces the microspores of this heterosporic plant. Heterospory is the production of spores of two distinct sizes and sexes by the sporophytes of vascular plants (in bryophytes, spore dimorphism is referred to as anisospory). 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. Four extant groups of plants are heterosporous; Selaginella, Isoetes, Salviniales and seed plants.

==Origin of heterospory== Heterospory evolved due to natural selection that favoured an increase in propagule size compared with the smaller spores of homosporous plants. Heterosporous plants produce two different sized spores in separate sporangia that develop into separate male and female gametophytes. It is proposed that the emergence of heterosporous plants started with the separation of sporangia, which allowed for the development of two different spore types; numerous small spores that are easily dispersed, and fewer, larger spores that contain adequate resources to support the developing seedling. During the Devonian period there were many species that utilized vertical growth to capture more sunlight. Heterospory and separate sporangia probably evolved in response to competition for light. Disruptive selection within species resulted in there being two separate sexes of gamete or even the whole plant. 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.