ALTERNATION OF GENERATIONS. The term, alternation of generations, is applied to the alternation of sexual and asexual indi viduals in the life history of a plant ; e. g., in the fern, a small sexual plant, called the prothal lium, which most people never see, bears the eggs and the sperms. A sperm fertilizes an egg, which then develops into the familiar fern plant. The fern plant produces spores, which in turn develop into prothallia, so that the fern plant produces the prothallium and the, pro thallium produces the fern plant, the two phases, or generations alternating regularly.
Alternation of generations, in a form most easily understood, is seen in the true ferns. The principal features of the life history of a common fern will make the subject clear the prothallium. Thus the fertilized egg, pro duced by the prothallium, develops into the fern plant and the spore produced by the fern plant develops into the prothallium. This regular alternation of prothallium and fern plant illustrates alternation of generations in its most easily understood form. The nuclei. during division, show a feature of great im portance; the nuclei of the sporophyte have twice as many chromosomes as those of the gametophyte (See CELL and CHROMOSOMES). The number of chromosomes is doubled when the sperm and the egg nuclei fuse at fertiliza tion, and during the two divisions by which four spores are formed from a spore mother cell in the sporangium, the number of chromosomes is reduced one-half, so that the gametophyte num ber is restored. This and "2x" condition of chromosomes is characteristic of the two gen erations in all plants which have reached the level of sexuality.
(Fig. 2). A spore from the fern plant, falling on moist ground or rotten wood, germinates and develops into the prothallium, a thin, Rat, prostrate body, which seldom reaches more than a quarter of an inch in length or more than one-thirty-second of an inch in thickness. It is green and therefore independent (Fig. 2, A). Upon the under side of the prothallium the eggs and sperms are developed, the eggs being developed singly in organs called archegonia, while the sperms are developed in considerable numbers in organs called antheridia. A vertical section of a portion of a prothallium showing an archegonium containing one egg and an antheridium containing several sperms is shown in Fig. 2, B. Since the prothallium produces eggs and sperms, which are commonly called gametes, it has been named the gametophyte (q. v.). The sperm escapes from the antherid ium, enters the archegonium and unites with the egg. This egg, thus fertilized, is the first cell of the fern plant which ultimately pro duces spores and is therefore called the sporo phyte. After fertilization, the egg divides rapidly, forming a more or less spherical mass of cells (Fig. 2, C). It then breaks out from the archegonium and becomes erect (Fig. 2, D).
The first leaves are small and simple, but the little plant develops roots and larger leaves, while the prothallium disintegrates and soon disappears. completely, so that the sporophyte, now fully independent, is the only phase re maining. As the leaves approach maturity, sori, or °fruit dots,/ easily visible to the naked eye, appear upon the under surface (Fig. 2, E, f). A vertical section of a sorus is shown in Fig. 2, F. In this case, the sorus is covered by an umbrella-like indusium, underneath which are the essential structures, the sporangia. Within the sporangium, the shift from the sporophyte generation to the gametophyte gen eration takes place. As the sporangium ap proaches maturity, spore mother cells are formed (Fig. 2, G) each of which by two characteristic divisions called the reduction divisions gives rise to four spores (Fig. 2 H). The spore is the first cell of the gametophyte generation. Upon germination, it develops into Alternation of generations is just as defi nitely present in plants above and below the ferns, but it is not so easily recognized, because the gametophyte, in the higher plants, becomes parasitic upon the sporophyte, while in the mosses and liverworts the sporophyte is para sitic upon the gametophyte, not yet having at tained the independent condition illustrated by the ferns.
In the higher plants, as the gametophyte be came dependent upon the sporophyte and even became included within the tissues of the sporo phyte, it became more and more reduced until it lost all resemblance to an independent gameto phyte and is recognizable as such only by the evidence of comparative morphology. A com plete series of forms illustrating the gradual reduction of the gametophyte would require pages of description and illustration, but a pine and a sunflower will indicate the general trend of the reduction. The pine tree and the sun flower plant are sporophytes. The pine (also the fir, spruce, hemlock, etc.) bears two kinds of cones, a rather small one producing an abundance of pollen and a comparatively large one producing seeds. The small cone, called the male cone or the staminate cone, consists of an axis with a large number of very small and much modified leaves, each bearing on its under surface two sporangia which are equiv alents of the sporangia of the ferns. The sporangium contains several spore mother cells, each of which produces four spores, called microspores or pollen grains. The microspore is the first cell of the male gametophyte. The larger cone, called the female or ovulate cone, consists of an axis bearing much reduced leaves, in the axil of each of which is a "scale') with two sporangia or ovules. Within the sporangium one spore mother cell appears and divides into four spores, called megaspores, three of which disintegrate while the other germinates within the tissue of the sporangium, forming a pro