. Introduction to botany. Botany. Mosses, Ferns, and Horsetails. 295 thallium bears the sex organs, it is rightly called the sexual genera- tion, or gametophyte. The egg, having been fertilized by fusion with a sperm (see paragraph 180), undergoes segmentation; and the cellular division thus begun con- tinues indeiinitely, resulting in a conspicuous fern plant (Fig. 157 B, e) which bears spores asexually. As has been said, the fern plant has no sex organs, and is there- fore called the asexual generation, or sporophyte. During the cell divisions which are immediately concerned in the formation
. Introduction to botany. Botany. Mosses, Ferns, and Horsetails. 295 thallium bears the sex organs, it is rightly called the sexual genera- tion, or gametophyte. The egg, having been fertilized by fusion with a sperm (see paragraph 180), undergoes segmentation; and the cellular division thus begun con- tinues indeiinitely, resulting in a conspicuous fern plant (Fig. 157 B, e) which bears spores asexually. As has been said, the fern plant has no sex organs, and is there- fore called the asexual generation, or sporophyte. During the cell divisions which are immediately concerned in the formation of the spore, the number of the chromosomes entering into the constitution of the nucleus is reduced by one-half (see page 109 for a description of cell and nuclear division), so that the spore con- tains only one-half as many chro- mosomes as do the cells which make up the body of the fern plant, but precisely the same number as the cells of the prothallium or gameto- phyte. Therefore we look upon the spore as the one-celled stage of the gametophyte. When the nucleus of the sperm cell fuses with that of the egg cell, the number of chromosomes now entering into the. Diagrams of the sporophyte and gametophyte of a fern. The spore d produced asexually on the fern plant e (sporophyte) is the beginning of the gameto- phyte. All of the gametophyte produced by the germination of the spore is shaded; all of thesporophyte is leflunshaded. A, the prothallium (gameto- phyte) bearing an archego- nium (a) with its egg, and an antheridium(^)with its sperms. The fern plant f grows fro m the fertiUzed egg. B, a later stage than Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Stevens, William Chase, 1861-. Boston, D. C. Heath & Co.
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