Practice question

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.