Genera like Selaginella and Salvinia which produce two kinds of spores, are known as heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes. This event is the precursor to the seed habit which is considered as an important step in evolution.