in certain plants and algae, nonsexual phase (or an individual representing the phase) in the alternation of generations—a phenomenon in which two distinct phases (a haploid and a diploid phase) occur in the life history of a plant, each phase producing the other. (The alternate, sexual phase is the gametophyte.) In the sporophyte phase, a diploid plant body grows and eventually produces spores through meiosis. These spores divide mitotically to produce haploid, gamete-producing bodies called gametophytes. The union of two gametes during fertilization produces a diploid zygote, which divides mitotically to form a new sporophyte.

The character and relative extent of the two phases vary greatly among different groups of plants and algae. During the course of evolution, the gametophyte stage is progressively reduced; thus, in the higher (i.e., vascular) plants, the sporophyte is the dominant phase in the life cycle, whereas in the more primitive nonvascular plants the gametophyte remains dominant. See also gametophyte.