142 PLANT BREEDING 



certain stage of cell division a separation or segregation. These 

 two principles of non-mixing and of segregation of factors were 

 discovered by Mendel and they are among the most important 

 and fundamental facts in biological science. As a single illustra- 

 tion of this note that the success in breeding is solely based upon 

 Mendel's principles of heredity. Desirable characters of one 

 individual may by crossing be associated with the desirable 

 characters of another individual and in the same way undesirable 

 traits may be eliminated. These new combinations of char- 

 acters may be effected in some cases with such certainty that 

 horticulturists have advertised new forms, describing their 

 characters'and qualities before they have been bred. Late fruit- 

 ing grains have been made to mature earlier, species susceptible 

 to cold have become hardy, as in the case of wheat, which may 

 now be cultivated in Canada many miles north of its former range, 

 and non-resistant forms have become immune to disease. The 

 annual loss from a single plant disease, one of the rusts, is esti- 

 mated at 500 million dollars. In the same way the percentage 

 composition of the reserve food in grains has been changed and 

 the resulting yield greatly increased. 



Another important feature in the transmission of the hereditary 

 factors is illustrated in the crossing of a blue-flowered tall sweet 

 pea with a red-flowered dwarf sweet pea. In this case we have 

 two pairs of allelomorphic factors instead of one as in the four- 

 o'clock. One member of a pair of chromosomes contains the 

 factor for blue and the other homologous member of this pair 

 contains the factor for red. In another pair of chromosomes 

 there is lodged in the same way the factors for tallness and 

 dwarfness. When these two plants are crossed we find that the 

 factors segregate as in the first example but the hybrids resulting 

 from the cross are never intermediate between the two parents. 

 It will be noticed in a hybrid from this cross containing the factors 

 for tallness and dwarfness that the plant is always tall like the 

 tall parent; so in a hybrid where blue and red are associated the 

 flower is always blue. It would look as though the factors for 

 dwarfness and redness in these cases had been obliterated. 

 Such, however, is not the case. They have been inhibited in 



