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The advent ofreproduction by seeds was one of the most essential evolutionary steps in planthistory: the vast majority of living plants are seed plants (spermatophytes). Theseed habit includes the following set of defining characteristics: (1)heterospory, (2) occurrence of a single megaspore that germinates within anindehiscent megasporangium (nucellus) retained on the sporophyte, (3) enclosureof the megasporangium in an integument, and (4) capture of pollen before seeddispersal. Contrasting hypotheses about the single / multiple, saltational / gradualorigin of the seed habit, and identification of the closest relatives of seed plants (aneurophytalean or archaeopteridaleanprogymnosperms) are still matters of considerabledebate. Early seeds did not possess the whole set of characters that definemodern seeds. They lacked a true micropyle and an entire integument, and nonehas yet been discovered containing an embryo. Hence they are called preovules. Asuite of characters observed in the Late Devonian preovules Moresnetia and Elkinsiaand in a number of younger taxa define hydrasperman reproduction. In hydrasperman preovules, prepollen capture wasrealized by the modified apex of the nucellus. Prepollen were directed into ahollow lagenostome, then retained in a pollen chamber. A central columnattached to the pollen chamber floor sealed the chamber to provide optimalconditions for prepollen germination. The hydrasperman syndrome has been presentedas shared by all basal members of the seed plant clade. Discovery of the Lower Carboniferous preovule Coumiasperma challengedthis view. Galtier and Rowe (1989, 1991) suggested that the possession of amassive nucellar tip and the lack of pollen chamber characterized anotherprimitive seed organization that either preceded and was ancestral to the hydraspermantype, or evolved independently and was adaptated to wet habitats.

The significance of Runcaria (Middle Devonian, Belgium) in the evolution of seed plants