. Heredity and evolution in plants . is composed of cells of a single or haploid nature. 26. Alternation of Generations.—Our study of thefern also brought out another fact of very fundamentalimportance. Sporophytes do not produce sporophytes,nor gametophytes, gametophytes; but there is alwaysan alternation of generations, sporophytes producinggametophytes, and gametophytes, sporophytes. The order of sequence in the life-cycle is as follows: sporophyte—>spore—>gametophyte—gametes—K)6sperm—»sporophyte. The order of structures and processes involved in thelife-cycle is as follows: OUTLINE O
. Heredity and evolution in plants . is composed of cells of a single or haploid nature. 26. Alternation of Generations.—Our study of thefern also brought out another fact of very fundamentalimportance. Sporophytes do not produce sporophytes,nor gametophytes, gametophytes; but there is alwaysan alternation of generations, sporophytes producinggametophytes, and gametophytes, sporophytes. The order of sequence in the life-cycle is as follows: sporophyte—>spore—>gametophyte—gametes—K)6sperm—»sporophyte. The order of structures and processes involved in thelife-cycle is as follows: OUTLINE OF LIFE HISTORY OF A FERN Gametophyte (prothallus) Antheridium Archegonium 4 4 Sperm (male gamete) Egg (female gamete) Fertilisation ^* ^ Oosperm (zygote) 44 Embryo 44 Mature sporophyte (mature zygote) 44 Sporophyll 44 Sporangium 44 Spore-mot her-ccl I 4 ill \Rfduciim Spoxe S{)ore Spore Spore 4 Gametophyte PRINCiPLKS 35 The fact of a cycle in the life history is brought outclearly in the following diagram:. FIG. 29.—Diagram of life-cycle of a fern. 27. Reduction.—Since the sporophyte (descended fromthe diploid oosperm) has cells of a double nature, resultingfrom fertilization, and since the spores which give rise tothe gametophyte are of a single (or haploid) nature,there must occur, at some stage in the life of the sporo-phyte, a process of reduction, restoring the cells, madediploid by fertilization, to the haploid condition. Pains-taking studies of cellular structure and processes hasdisclosed the fact that this reduction takes place duringthe two successive divisions (tetrad-divisions) of the spore-mother-cell, resulting in the formation of four diploid condition persists in all the cells of thesporophyte, and through every cell-division, up to the twodivisions preceding spore-formation, just as the single orhaploid condition persists in all the cells of the gameto-phyte, up to the very act of fertilization. 36 IIKREDITY AND EVOLUTIO
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