172 



CAUSES OF VARIATION 



true polar bodies are formed, 1 but that the chromosomes sud- 

 denly appear in reduced number at the first division, which 

 results in four daughter cells, any one or all of which may be 

 functional, according to the species. It is not the purpose here 

 to discuss the detailed behavior of various plant forms in matu- 

 ration. It is sufficient to say that botanists recognize two stages 

 in the development of the female germ cell of the plant, neither 

 of which is identical with maturation in animals, though the first 

 is fairly comparable thereto. The first stage, or sporogenesis, 

 follows after that active massing of food material which marks, 

 in both plant and animal, the preparation for reduction. At this 

 time the nucleus divides quickly into four daughter nuclei, each 

 of which is supplied with half the number of chromosomes that 

 characterizes the species. The significant fact is that reduction 

 is accomplished at this stage. 



Of these four daughter nuclei none are extruded, but three 

 of them degenerate in the cytoplasm, while the fourth increases 

 in size to form the embryo sac, which, without waiting for ferti- 

 lization as among animals, continues to divide, commonly 

 three times, giving rise to eight sub-nuclei, which arrange 

 themselves in definite positions. Two of these sub-nuclei re- 

 main near the center of the embryo sac and give rise to the 

 endosperm ; three migrate to the extremity nearest the point 

 of attachment with the pistil, and one of these (and one only) 

 the so-called egg nucleus unites with the nucleus of the 

 pollen grain to form the fertilized germ ; the three remaining 

 migrate to the other extremity of the embryo sac and concern 

 themselves with establishing a food supply with the parent plant. 



On the male side the process is simpler. The pollen nucleus 

 divides, one half forming the pollen tube, along which the other 

 half travels, dividing again at some point before uniting with the 

 egg nucleus of the embryo sac. These divisions are evidently 

 not reducing divisions, as reduction occurs previously during 

 the division of the pollen mother cell. 



1 Disputed by Chamberlain, who believes that "the egg with its three polar 

 bodies constitutes a generation directly comparable with the gametophytic genera- 

 tion in plants." See Botanical Gazette, XXXIX, 139; see also under " Xenia," in 

 this text. 



