Alternation of Generations is the 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 conspicuous form; the sporophyte cannot exist independently. The sporophyte of the moss is composed of a capsule, which is the center of spore formation; a stalk; and a foot that attaches the sporophyte body to the tip of the gametophyte.
The gametophyte and the gametes that it produces are haploid, that is, they contain half the number of chromosomes that is characteristic of the species. When the egg and sperm fuse, they form a sporophyte that is diploid; it has the complete number of chromosomes. When cell division occurs within the spore-bearing structures (sporangia) of the sporophyte, the diploid chromosome number is reduced again to the haploid state.
Among plants more advanced in evolutionary development than the ferns, the gametophyte does not occur as an independent plant. The sporophyte is the conspicuous generation, and the vestigial gametophytes are reduced to a few nuclei that can be seen only with a microscope. Among the flowering plants, the pollen grain is the microspore, within which are produced male gametophytes that contain the sperm. The egg sac, or female gametophyte, is produced by germination of a megaspore within the ovary or pistil of the flower. Microspores and megaspores are produced within the anther sacs of stamens and within the ovulary tissues of the pistil, respectively.
For some years scientists have known that some sporophytes spontaneously give rise to gametophytes, which are therefore diploid. Experimentally disturbed gametophytes may also give rise to sporophytes, which may then be haploid. These unusual conditions have caused scientists to question the validity of earlier conclusions about the significance of the life cycle of plants. Animals do not undergo alternation of generations that differ in chromosome number. The hydrozoa and other cnidarians undergo an alternation of generations between a colonial polyp form and a free-swimming medusa or other jellyfish form. Both forms are diploid. This cycle is also known as metagenesis.