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 indefinitely, 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 left unshaded. 

 A, the prothallium (gameto- 

 phyte) bearing an archego- 

 nium (a) with its egg, and an 

 antheridium (<$)with its sperms. 

 The fern plant e grows from the 

 fertilized egg. B t a later stage 

 than A. 



