GAMETOPHYTE, Reduc tion of. The gametophyte, as the name implies, is the gamete-bearing plant. In all plants which have reached the level of sexuality there is a gamete-bearing plant in the life history, alter nating with another phase which may or may not bear spores. This other phase, in some alga and fungi .and in all plants above these two groups, does bear spores and, consequently is called the sporophyte. The gametophyte pro duces gametes, usually called sperms and eggs; a sperm unites with an egg, and this fertilized egg is the first cell of the sporophyte. The sporophyte, at maturity, produces spores; a spore produces the gamete-bearing plant and so the gametophyte and sporophyte generations alternate. See ALTERNATION OF GENERATIONS.
In the liverworts, the gametophyte genera tion is the conspicuous one, and the only one likely to be seen, except by the botanist, the sporophyte being comparatively small and para sitic upon the gametophyte (Fig. I, A).
In mosses, the matted cushions, which every one calls moss, are masses of gametophyte plants. The sporophytes are the capsules, often nodding on slender stalks, which are seen at tached to the gametophytes, upon which they are parasitic (B). In the common ferns, the gametophyte — called the prothallium — is a small thin, prostrate plant, seldom as large as one's finger nail. It produces eggs and sperms, a sperm fertilizes an egg, which then grows into the familiar fern plant. As the fern plant —the sporophyte — develops roots and becomes independent, the gametophyte dies (C). While this gametophyte is small and evanescent, it is green and independent as long as it lives. In some of the fern allies — Selaginella, Isoetes and the water ferns — the sporophyte produces two kinds of spore cases containing two kinds of spores, small spores, called microspores, and large spores, called megaspores. The micro spores develop microscopically small prothallia (male gametophytes) which produce a few sperms; the large spores develop female game tophytes which produces one or more eggs. Both male and female gametophytes are con tained within the spore, so that they are ex posed only through cracks in the spore coats (D.) They are not green and consequently, are practically parasitic. The male gametophyte dies as soon as it has discharged its sperms"; the female gametophyte, with a larger supply of stored food material, not only matures its eggs, but nourishes the young embryo until it develops roots and leaves and thus becomes independent. The gametophyte then dies. In the seed plants, which include the gymnosperms and angio sperms, there are also two kinds of spores, microspores and megaspores, which develop parasitic gametophytet. The male gametophyte
consists of the microspore and a long tube, the pollen tube, which contains two sperms. The female gametophyte is not only entirely eluded within the megaspore but the megaspore itself is entirely included within the sporangium (ovule) (E). This inclusion has resulted in a more and more pronounced dependence, ac companied by reduction. In the gymnosperms, e.g., in the pine, the female gametophyte still bears considerable resemblance to that of some fern allies, but in the angiosperms, e.g., in the sunflower, the reduction has gone so far that a megaspore and female gametophyte are identi fied only by the evidence of comparative mor phology. In this case, which represents the majority of the flowering plants, the female gametophyte consists of only seven nuclei, or loosely organized cells, so specialized that they have received individual names. The egg with two accompanying cells occupies one end of the gametophyte while three antipodal cells occupy the other extremity. Between these two groups is the endosperm nucleus, formed by the fusion of two polar nuclei (F). The final stage of reduction is found in an unfamiliar plant, Plumbaginella, in which four megaspore nuclei form a female gametophyte of only four nuclei, one of which organizes the egg, two unite to form the endosperm nucleus, while the fourth disorganizes; thus, at the time of fertilization, the female gametophyte has only two nuclei, the egg to start the embryo and the endosperm nucleus to start the endosperm, or nutritive tissue which support the embryo until it de velops root and leaves (G.) Summarizing for the female gametophyte, it will be seen that in the lower plants, the game tophyte is green and independent and is the phase which we recognize as the plant, the sporophyte being parasitic upon it. In the flowering plants, the sporophyte is the con spicuous phase which we recognize as the plant, while the gametophyte has become en tirely parasitic. A mean, between these two *extremes, is illustrated by the mosses, in which both gametophyte and sporophyte are green, so that the sporophyte, although still attached to the gametophyte during its entire life, never theless is partially independent' The reduc tion, however, is very gradual and numerous intergrades could be cited to make the series complete. The reduction of the male gameto phyte is similar, but there is less difference be tween the extremes.
Bibliography.—Atkinson. G. F., 'College Botany' ; Campbell, D. H., 'Mosses and Ferns' Coulter, Barnes and Cowles, 'A Text-Book of Botany.'