The typical fern, a sporophyte, consists of stem, leaf, and root; it produces spores; and its cells each have two sets of chromosomes, one set from the egg and one from the sperm. The sporophyte of most ferns is perennial (it lives for several years) and reproduces vegetatively by branching of the rootlike underground stem, or rhizome, often forming large, genetically uniform colonies, or clones. A few ferns propagate by root proliferations, and some, especially in the wet tropics, reproduce by leaf proliferations.
The spores are haploid; that is, they have one set of chromosomes. They are produced in specialized organs—the spore cases, or sporangia—on the fern leaves (fronds). Once released, the spores are carried by wind currents, and a small percentage of them fall in appropriate germination sites to form the sexual plants, or gametophytes. In ferns the gametophytes are commonly referred to as prothallia, and they are best known to biologists as laboratory objects in artificial culture. They are rarely observed in nature without arduous searching, and the gametophyte stage of the majority of fern species has never been seen in the wild.
The prothallia are tiny—usually less than 8 mm (0.3 inch) long—and kidney-shaped in the majority of species. They grow only until the new sporophyte has been formed by fertilization; then they wither and die in most species. The process of fertilization is accomplished by sperm and eggs produced upon the same or more commonly different gametophytes, and both the fertilized egg (zygote) and the resultant embryo are held within the tissues of the prothallium until the embryo grows out as an independent plant. Exceptions to this life cycle include several aquatic genera with separate megaspores and microspores, in which the gametophytic phase is reduced and remains largely within the spore walls. Also, many ferns are apomictic; that is, they produce spores with the same number of chromosomes as found in the sporophyte, and new sporophytes arise directly from cells of the gametophyte without the need for gametes or fertilization.
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