CHAPTEE III 



THE EVOLUTION OF SEX 



12. The general problem. — In the last chapter it was re- 

 marked that the simplest Algae reproduce only by vegetative 

 multiplication, the ordinary cell division (fission) of nutri- 

 tive cells multiplying cells and hence individuals. Among 

 other low Algae asexual spores are added to fission as a 

 method of reproduction, the spores being also formed by 

 cell division, generally internal division. In higher forms 

 gametes appear, and a new method of reproduction, the 

 sexual, is added to the other two. 



Sexual reproduction is so important a process in all 

 plants except the lowest, that it is of interest to discover 

 how it may have originated, and how it developed into its 

 highest form. Among the Algse the origin and develop- 

 ment of the sexual process seems to be plainly suggested ; 

 and as all other plant groups have probably been derived 

 more or less directly from Algae, what has been accom- 

 plished for the sexual process in this lowest group was 

 probably done for the whole plant kingdom. 



13. The origin of gametes.— One of the best Alg^ to 

 illustrate the possible origin of gametes is a common fresh- 

 water form known as Ulothrix (Fig. 2). The body consists 

 of a simple filament composed of a single row of short 

 cells (Fig. 2, A). Each cell contjiins a nucleus, and a 

 single large chloroplast which has the form of a thick cyl- 

 inder investing the rest of the cell contents. Through the 

 microscope, if the focus is upon the center of the cell, 

 an optical section of the cylinder is obtained, the chloro- 



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