Dioicous

Monoicous plants are those species that bear both sperm and eggs on the same gametophyte. Dioicous plants are those that have gametophytes that produce only sperm or eggs but never both. The terms are used largely but not exclusively in the context of bryophytes. Both monoicous and dioicous gametophytes produce gametes in gametangia by mitosis rather than meiosis, so that sperm and eggs are genetically identical with their parent gametophyte. The states of being monoicous or dioicous are called monoicy and dioicy respectively.

The word monoicous and the related forms mon(o)ecious are derived from the Greek mόνος (mónos), single, and οἶκος (oîkos) or οἰκία (oikía), house. The words dioicous and di(o)ecious are derived from οἶκος or οἰκία and δι- (di-), twice, double. ((o)e is the Latin way of transliterating Greek οι, whereas oi is a more straightforward modern way.) Generally, the terms "monoicous" and "dioicous" have been restricted to description of haploid sexuality (gametophytic sexuality), and are thus used primarily to describe bryophytes in which the gametophyte is the dominant generation. Meanwhile, "monoecious" and "dioecious" are used to describe diploid sexuality (sporophytic sexuality), and thus are used to describe tracheophytes (vascular plants) in which the sporophyte is the dominant generation. However, this usage, although precise, is not universal, and "monoecious" and "dioecious" are still used by some bryologists for the gametophyte.

Bryophytes have life cycles that are gametophyte dominated. The longer lived, more prominent autotrophic plant is the gametophyte. The sporophyte in mosses and liverworts consists of an unbranched stalk (a seta) bearing a single sporangium or spore-producing capsule. Even when capable of photosynthesis, as in mosses and hornworts, bryophyte sporophytes require additional photosynthate from the gametophyte to sustain growth and spore development and are dependent on the gametophyte for their supplies of water, mineral nutrients and nitrogen.

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