Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. Heterospory was evolved from isospory independently by several plant groups; the clubmosses, the arborescent horsetails, and progymnosperms in the Devonian period as part of the process of evolution of the timing of sex differentiation. Heterosporic plants produce small spores called microspores which either germinate to become free-living male gametophytes or have reduced male gametophytes packaged within them, and larger spores called megaspores that either germinate into free-living female gametophytes, or which have a female gametophyte packaged within them which is retained in and nurtured by the sporophyte phase, a condition referred to as endospory. Heterosporous species are thus usually dioicous, a condition that promotes outcrossing. Some heterosporous species produce micro- and megaspores in the same sporangium, a condition known as homoangy, while in others the micro- and megaspores are produced in separate sporangia (heterangy). These may both be borne on the same monoecious sporophyte or on different sporophytes in dioecious species.
- Stewart, W.N.; Rothwell, G.W. (1993). Paleobotany and the evolution of plants (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-38294-7.
- Bateman, R.M.; Dimichele, W.A. (1994). "Heterospory - the most iterative key innovation in the evolutionary history of the plant kingdom" (PDF). Biological Reviews of the Cambridge Philosophical Society. 69: 315–417. Retrieved 2010-12-30.
- Sussex, I.M. (1966) The origin and development of heterospory in vascular plants. Chapter 9 in Trends in Plant morphogenesis, ed. by E.G. Cutter, Longmans.
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