gametophyte

gametophyte , phase of plant life cycles in which the gametes, i.e., egg and sperm, are produced. The gametophyte is haploid, that is, each cell contains a single complete set of chromosomes, and arises from the germination of a haploid spore. In many lower plants, the gametophyte phase is the dominant plant form; for example, the familiar mosses are the gametophyte form of the plants. The alternate phase of the plant life cycle is the sporophyte, the diploid plant form, with each cell containing two complete sets of chromosomes. For example, in mosses the sporophyte is a capsule atop a slender stalk that grows out of the top of the gametophyte. The sporophyte develops from the union of two gametes, such as an egg fertilized by a sperm; in turn, the sporophyte forms spores that develop into gametophytes. The alternation between haploid gametophyte and diploid sporophyte phases, known as alternation of generations, occurs in all multicellular plants. As plants advanced in evolutionary development, the sporophyte became the increasingly dominant plant form and the gametophyte form has been correspondingly reduced. In contrast to mosses, for example, in the advanced angiosperms the male and female gametophytes are reduced to three-celled and seven-celled structures, respectively, found within the reproductive organs of the familiar flowering plant (the sporophyte). See also fertilization ; reproduction .

gametophyte The generation in the life cycle of a plant that bears the gamete-producing sex organs. The gametophyte is haploid. It is the dominant phase in the life cycle of mosses and liverworts, the sporophyte generation depending on it either partially or completely. In clubmosses, horsetails, and ferns it is the prothallus. In seed plants it is very much reduced. For example, in angiosperms the pollen grain is the male gametophyte and the embryo sac is the female gametophyte. See also alternation of generations.