female gametophytes and hence both male and female gametes. For example, a single flowering plant of a monoecious species has both functional stamens and carpels, either in separate flowers or in the same flower. The sporophyte generation of seed plants is called "dioecious" when each sporophyte plant has only one kind of spore-producing organ, all of whose spores give rise either to male gametophytes, which produce only male gametes (sperm), or to female gametophytes, which produce only female gametes (egg cells). For example, a single flowering plant sporophyte of a fully dioecious species has either flowers with functional stamens producing pollen containing male gametes (staminate or 'male' flowers), or flowers with functional carpels producing female gametes (carpellate or 'female' flowers), but not both. (See Plant reproductive morphology for further details, including more complex cases.) Slightly different terms, dioicous and monoicous, may be used for the gametophyte generation, although dioecious and monoecious are also used. A dioicous gametophyte either produces only male gametes (sperm) or produces only female gametes (egg cells). About 60% of liverworts are dioicous. In order for the distinction between dioecious and monoecious sporophytes to make sense, the plant must exhibit hetrospory. Thus, all dioecious plants must produce heterospores. Furthermore, all heteresporous plants must be dioicous. Dioecy occurs in a wide variety of plant groups. Examples of dioecious plant species include ginkgos, willows, cannabis and African teak. As its specific name implies, the perennial stinging nettle Urtica dioica is dioecious, while the annual nettle Urtica urens is monoecious. Dioecious flora are predominant in tropical environments. About 65% of gymnosperm species are dioecious, but almost all conifers are monoecious. In gymnosperms, the sexual systems dioecy and monoecy are strongly correlated with the mode of pollen dispersal, monoecious species are predominantly wind dispersed (anemophily) and dioecious species animal-dispersed (zoophily). About 6 percent of flowering plant species are entirely dioecious and about 7% of angiosperm genera contain some dioecious species. Dioecy is more common in woody plants, and heterotrophic species. In most dioecious plants, whether male or female gametophytes are produced is determined genetically, but in some cases it can be determined by the environment, as in Arisaema species. Certain algae are dioecious. Dioecy is prevalent in the brown algae (Phaeophyceae) and may have been the ancestral state in that group. Evolution of dioecy In plants, dioecy has evolved independently multiple times generally either from hermaphroditic species or from monoecious species.