Heterospory

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,^[1] and progymnosperms^[1] in the Devonian period^[2] as part of the process of evolution of the timing of sex differentiation.^[3] 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.

References

1. Stewart, W.N.; Rothwell, G.W. (1993). Paleobotany and the evolution of plants (2nd ed.). Cambridge: Cambridge University Press. ISBN 0-521-38294-7.
2. 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.
3. 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.