Hybridization and Selection 295 



The number of chromosomes in the vegetative cells of any 

 species of plant is definite, and in the cell divisions that occur 



Fig. 185. Diagram showing behavior of chromosomes in the reduction division. The 

 daughter cells have half as many chromosomes as the mother cell. {Afkr Sharp.) 



during vegetative growth each chromosome splits longitudinally 

 and one half goes to each of the daughter cells. So every cell has 

 a set of chromosomes similar to that of every other cell. Each 

 chromosome is the bearer of certain hereditary factors. In 

 vegetative multiplication the chromosome complement of cut- 

 tings, cions, and vegetative offshoots is the same as that of the 

 parent plant. Hence, the hereditary qualities of vegetative prop- 

 agating shoots are similar to those of the parent plant. 



Behavior of chromosomes in sexual reproduction. In Chapter 

 XXVI (page 249) the process of fertilization is discussed and at- 

 tention is called to the fact that fertilization consists of the union 

 of a sperm and egg, forming a zygote. This fusion would lead to a 

 doubling of the number of chromosomes in each successive gener- 

 ation if it were not for the fact that in the formation of the pollen 

 and the embryo sac which precedes fertilization a cell division 

 takes place in which the behavior of the chromosomes is differ- 

 ent from that described for vegetative growth. 



In all the complex plants the mother cells, which give rise to 

 the pollen and embryo sac, and ultimately the sperms and eggs 

 (gametes) divide by a method called the reduction division. The 

 word ''reduction" refers to the fact that in this division the num- 

 ber of chromosomes is reduced to half the number that occur in 

 the vegetative cells. 



