HYBRIDIZING PLANTS 137 



tions are resistant. This requires a special technique, but 

 other desirable characters besides disease resistance can be 

 introduced by the same process of backcross and selection. 



Some characteristics are conditioned by two or more 

 genes. Certain purple-colored sweet peas require a domi- 

 nant gene for the color chromogen and another dominant 

 gene for the enzyme to activate the color gene. When either 

 or both of these dominant genes are absent, the flowers are 

 white, but if both are present they are purple. The result is 

 nine purple to seven white. If the. sixteen individuals in the 

 above dihybrid experiment are examined, the 9 : 7 ratio can 

 be seen. Nine are smooth yellow showing the presence of 

 both dominant genes; the other seven lack one or both of the 

 dominant genes. 



Genes have been shown to control chemical reactions in 

 the developing organism in such complex structures as the 

 anthocyanin pigment and the chlorophyll formation, which 

 are physiological Mendelian characters. Recently the natu- 

 ral prostrate growth of corn, called lazy corn, has been shown 

 to be the result of a gene causing the growth hormone to be 

 more concentrated at the upper side of the stalk. Transmis- 

 sion of physiological characters has been studied much less 

 than the structural characters, but when we think of the 

 advancing knowledge of physiology, the field for breeding 

 becomes almost unlimited. 



Luther Burbank made thousands of crosses and produced 

 a number of valuable plants. He was not primarily inter- 

 ested in the parents of a new plant, and, therefore, kept few 

 records. He used grafting and other vegetative means of 

 propagation to grow his creations long enough to decide on 

 their merit. He might graft pieces from six hundred seed- 

 lings on a single tree where they would grow fruit in a year 

 or two to demonstrate their value. He had an unusual 

 ability to select desirable plants from millions of undesirable 

 ones, while they were young, and before other people could 



