alternation of generations

Typical life cycle of terrestrial plants and some seaweeds, in which there are two distinct forms occurring alternately: diploid (having two sets of chromosomes) and haploid (one set of chromosomes). The diploid generation produces haploid spores by meiosis, and is called the sporophyte, while the haploid generation produces gametes (sex cells), and is called the gametophyte. The gametes fuse to form a diploid zygote which develops into a new sporophyte; thus the sporophyte and gametophyte alternate.

In mosses, the familiar green moss plant is gametophyte, while the long-stalked spore capsules growing from it are sporophyte. In ferns the familiar plant is the sporophyte and the gametophyte, which grows separately from it, is very small and inconspicuous. All higher plants are sporophytes, and the gametophyte is not seen because it completes its life within the body of the sporophyte. The life cycles of certain animals (such as the jellyfish) are sometimes said to show alternation of generations, but this is rarely as regular and clearly defined as in plants.