CHAPTER THIRTY-ONE 



HYBRIDIZATION AND SELECTION 



The crossing of two species or varieties of plants is known as 

 hybridization. It is brought about by transferring the pollen 

 from one plant to the stigma of the other plant, which ultimately 

 results in cross-fertilization. The plants grown from seed pro- 

 duced in this way are called hybrids. Hybrids may resemble 

 one of the parents, or they may have some characters of both 

 parents. In the second generation derived from crosses some 

 plants show a wide range of variations, with all possible combina- 

 tions of the characteristics of the parent plants. Successful hy- 

 bridization, therefore, increases the number of variations avail- 

 able for selection by the plant breeder. 



Hybrid vigor. Hybridizing often has a physiological effect 

 which is of importance, for in many plants it increases the vigor 

 of the offspring. This may result in increased yield of grain, or 

 in greater resistance to disease organisms, to drought, and to the 

 effects of high or low temperature. Sometimes the vigor is ex- 

 pressed merely in the size attained by the hybrids. For example, 

 the hybrids secured by crossing the American sunflower and the 

 Russian sunflower, neither of which is over lo feet in height, 

 grow to a height of 15 feet. In a series of experiments with corn, 

 hybrids gave an average yield more than 50 per cent greater 

 than the average of their parents. 



Mendel's experiments. To see more definitely what may hap- 

 pen in hybridization, we may review an experiment performed by 

 Gregor Mendel about the middle of the last century. Mendel 

 crossed a tall variety of the common garden pea with a dwarf 

 variety. In this way he secured hybrids which had received some 

 characters from each of two different parent plants. One of 

 the most important discoveries he made was that in plant breed- 

 ing every feature of a plant must be studied separately. Consid- 

 ering height growth alone, he found that when he planted the 

 seeds secured from the cross all the plants grew tall. This first 



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