The advent of reproduction by seeds was one of the most essential evolutionary steps in plant history: the vast majority of living plants are seed plants (spermatophytes). The seed habit includes the following set of defining characteristics: (1) heterospory, (2) occurrence of a single megaspore that germinates within an indehiscent megasporangium (nucellus) retained on the sporophyte, (3) enclosure of the megasporangium in an integument, and (4) capture of pollen before seed dispersal. Contrasting hypotheses about the single / multiple, saltational / gradual origin of the seed habit, and identification of the closest relatives of seed plants (aneurophytalean or archaeopteridalean progymnosperms) are still matters of considerable debate. Early seeds did not possess the whole set of characters that define modern seeds. They lacked a true micropyle and an entire integument, and none has yet been discovered containing an embryo. Hence they are called preovules.