296 General Botany 



Reduction division. In the reduction division the chromo- 

 somes become arranged in pairs in the early stages of nuclear 

 division and group themselves at the equator of the spindle, 

 as double chromosomes. Then, instead of splitting, as in vege- 

 tative cell division, the two chromosomes of each pair separate 

 and migrate to the opposite poles of the spindle, forming the chro- 

 mosomes of the new daughter nuclei. As a result each daughter 

 nucleus contains one half the number of chromosomes contained 

 in the vegetative cells of the plant. 



For example, the vegetative cells of the white lily contain 

 twenty-four chromosomes. When the mother cells of the pollen 

 and the embryo sac divide, the daughter cells each contain twelve 

 chromosomes. These daughter cells divide again and again, 

 until the sperms and eggs are finally formed and each nucleus 

 contains twelve, the reduced number of chromosomes. When 

 fertilization takes place and the sperm and egg unite, each carries 

 twelve chromosomes. The resulting zygote nucleus contains 

 twenty-four. The zygote is the first cell of the plant of the 

 next generation, and in the further development of the embryo 

 and plant all the cells carry twenty-four chromosomes. 



Explanation of Mendel's laws. There are many reasons for 

 concluding that the chromosomes are the principal carriers of the 

 factors of heredity, and that each individual chromosome carries 

 certain particular factors. We can explain the Mendelian be- 

 havior of hybrids if we assume that in the reduction division 

 each of the two chromosomes that are paired in the early stages 

 contains one of two contrasting factors. Then, when they 

 separate, one of these factors is carried to one of the daughter 

 nuclei and the contrasting factor to the other daughter nucleus. 

 In this way each sperm and each egg will contain only one mem- 

 ber of each pair of contrasting factors, and the plant that 

 develops from each zygote has its characteristics determined by 

 the factors contained in the chromosomes of the sperm and tgg 

 that united to produce it. 



