184 



GENERAL BOTANY 



MENDEL'S PRINCIPLES OF HEREDITY 



It will be evident to the student that in the crossing of plants 

 for increased vigor and for new combinations of characters there 

 is a large element of chance in the methods of Darwin, Burbank, 

 and other plant breeders. The results attained are indeed re- 

 markable and are of 

 the greatest value 

 in the improvement 

 of plants for man's 

 use, but they lack 

 the certainty and the 

 definiteness of a sci- 

 entific method. The 

 breeder of plants 

 needs to be in the 

 position of the chem- 

 ist with reference 

 to the characters of 

 the plants which he 

 wishes to combine 

 by crossing. The 

 chemist can com- 

 bine the atoms and 

 molecules of two 



Grandchildren, F 2 

 "Segregation " of SS 



and ww and 

 " mathematical ratio " 



FIG. 96. Diagram illustrating Mendel's results in 

 crosses between smooth and wrinkled peas 



Great-grandchildren, F 

 Purity of SS and w w 

 demonstrated 



substances in a 

 chemical flask, and 

 by reason of known 

 laws of chemistry 

 he can predict with 

 certainty the new compound or compounds that will result. In 

 a similiar manner the breeder of plants needs to know both the 

 nature of the characters of the plants with which he deals and 

 the laws by which these characters will combine in the zygote 

 to form the kind of offspring which will meet his need or his 

 desire. Mendel's principles of heredity now supply something 

 like this scientific basis for plant breeding and improvement. 



