Alternation of Generations
Alternation of Generations, occurrence of two or more alternating forms in the life cycle of plants, algae, and among a small number of invertebrate animals. All plants take two generations to complete one life cycle. Among some algae, the two generations are similar in structure and appearance. In other plants, the two generations are fundamentally different. Among hydrozoa and other cnidarians the two phases are morphologically quite different. Alternation of generations always includes a sexual phase alternating with an asexual phase. In all plant and algae species, the sexual phase is called the gametophyte and the asexual phase is called the sporophyte. The gametophyte generation produces gametes (eggs and sperm) that fuse, giving rise to the sporophyte generation. The essential characteristic of the sporophyte is that its reproductive cells or spores are asexual; each spore germinates to produce a gametophyte. Among the mosses and liverworts, the gametophytic generation is the co