Like other members of the division Lycophyta, the spike mosses have an asexual reproductive structure, the strobilus. Selaginella strobilus, commonly used in slide preparations, is scientifically named for this characteristic structure, which is spike-like. Strobili (plural of strobilus) form at the uppermost portion of fertile branches and are composed of sporophylls bearing a single sporangum. All species in the genus Selaginella are heterosporous; that is, they make two different types of reproductive spores. Megasporangia and microsporangia are produced in the same strobilus with spore mother cells present in both. They each divide by meiosis to produce megaspores and microspores, respectively. Megaspores develop into megagametophytes or females while microspores grow into microgametophyte or male plants. Fertilization of the megagametophyte takes place when the microgametophytes release their biflagellated sperm and they swim to reach the eggs during a rain shower or in a thin film of dew.