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alternation of generations
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alternation of generations - alternation of generations
alternation of generations - In life cycle, alternation of a generation having sexual reproduction with a generation having asexual reproduction. The sexually and asexually reproducing forms are often very different from each other. Occurs among animals in, e.g. hydroids, jelly fish, tapeworms; in these both generations are diploid. In plants seen most clearly in, e.g., ferns where the two generations are independent. The fern plant is a diploid sporophyte and reproduces asexually by formation of haploid spores following meiosis. Germination of the spores initiates the gametophyte generation, a small prothallus, which reproduces sexually. Male and female gametes fuse together to form a zygote which develops into a new fern plant. Great differences exist between the plant groups with respect to the relative prominence and degree of independence shown by gametophyte and sporophyte generations. In many members of the Thallophyta, spores are not produced by the diploid generation, which cannot therefore be termed 'sporophyte'. Nevertheless, one can recognize, as in all sexually reproducing plants, an alternation between haploid and diploid phases in the life history. The mycelium of many Phycomycete and Ascomycete fungi is haploid and commonly gives rise to asexually produced haploid spores as well as to gametes. The diploid phase is confined to the zygote, meiosis occurring with the first division of the zygote nucleus. Other members of these groups are diploid in the vegetative condition, while in the Basidiomycetes the mycelium consists of dikaryon cells, a condition usually considered equivalent to the diploid condition. The haploid phase, initiated by the basidiospore, is brought to an end early in the life cycle by union of two basidio-spores, or anastomosis of hyphae produced by them, to form a dikaryon. Amongst algae, various conditions exist, e.g. dominant gametophyte with diploid phase confined to the zygote; dominant diploid plant bearing dependent gametophyte; distinct gametophyte and true sporophyte plants. In mosses and liverworts the dominant generation, the moss or liverwort plant, is the gametophyte; the sporophyte, the capsule, is small, nutritionally dependent on the gametophyte. In flowering plants, there are separate male and female gametophytes reduced to microscopic proportions. The male gametophyte is shed as the pollen grain; the female gametophyte, the embryo sac, is retained on the sporophyte in the ovule. The sporophyte generation is the plant itself (herb, shrub, tree).
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