Hybridization and Selection 293 



have been extensively studied and their hereditary behavior dis- 

 covered. With these studies as a foundation further progress in 

 improving, modifying, and combining qualities in new ways is 

 rapidly being made. 



Cell structures and Mendel's laws. Since Mendel's time 

 much has been learned concerning the physiological explanation 

 of these laws. It is now well established that the explanation 

 rests on the behavior of certain nuclear structures, the chromo- 

 somes, during cell division, during the formation of pollen and 

 embryo sac, and when fertilization occurs. 



Fig. 183. Root tip of onion, showing cell division and enlargement. A series of stages 

 showing behavior of the chromosomes during vegetative cell division is labeled a to g. 



