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Selaginella is a heterosporous pteridophyte, producing both microspores and megaspores.
Ref: NCERT Class 11 Biology Chapter 3: Plant Kingdom Pteridophytes - Structure and Life Cycle
Heterospory marks pivotal evolutionary innovation directly underlying origin and success of seed habit in vascular plants. Production of large megaspore retained within megasporangium ensures female gametophyte develops endosporically, drastically reduced to few cells, remains protected and nourished by sporophyte tissue. Coevolution of integument surrounding megasporangium forming ovule, pollen grain delivering male gamete, and endosporic growth established seed habit providing protection against desiccation. Vessel formation relates to xylem efficiency, loss of alternation contradicts plant cycles, xylem reduction unrelated. Heterospory appears in Selaginellales.
Ref: Bateman & DiMichele 1994 Origin of Seed; Stewart & Rothwell Paleobotany Ch Heterospory
Heterospory denotes reproductive differentiation involving production of two morphologically and functionally distinct spore size classes by same sporophyte. Microspores, small and numerous produced in microsporangia, germinate into reduced male gametophytes producing spermatozoids. Megaspores, larger with stored starch, lipids and proteins, develop into female gametophytes retaining archegonia and supporting embryo growth matrotrophically. This separation improves resource allocation, outcrossing potential and early embryo provision. Selaginella, Isoetes, Marsilea, Salvinia and all seed plants exhibit heterospory; homospory produces one spore type. This structural framework illustrates phylogenetic relationships and adaptive significance highlighted in competitive examinations focusing on comparative plant morphology and systematics.
Ref: Bateman & DiMichele 1994 Heterospory; Campbell Ch 30; Raven Biology of Plants
Heterospory describes production two distinct spore sizes differing resource allocation: numerous small microspores from microsporangia giving rise to male gametophytes pollen grains with few cells, and fewer larger megaspores with abundant reserves from megasporangia developing into female gametophytes embryo sacs retained. This division labor contrasts with homospory where single spore type yields bisexual free-living gametophyte like most ferns and bryophytes. Evolution toward heterospory in progymnosperms like Archaeopteris enabled retention of megagametophyte within sporophyte tissues, protection via integuments, reduction in gametophyte size and eventual origin seed habit enhancing survival under arid terrestrial conditions reducing water dependence.
Ref: Campbell Biology, 12th ed., Chapter 30: Heterospory microspores megaspores evolution seed habit
