CHAPTER VI 



THE APPLICATION OF BIOLOGICAL PRINCIPLES TO 



PLANT BREEDING 



In this chapter I shall take up the commercial appli- 

 cation of some of the principles of plant genetics previously 

 discussed. 



The fact must again be emphasized that there are two 

 kinds of variation: 



1. Fluctuating variations, which are due solely to sur- 

 rounding influences such as better position for develop- 

 ment or varying fertility of the soil. Such variations are 

 not inherited. 



2. Inherited variations, which are due to some structural 

 change in the reproductive cells. These variations may 

 depend upon environmental conditions for their full devel- 

 opment but not for their transmission. 



Inherited variations possess the only value to the plant 

 breeder, yet the work of improving plants is rendered a 

 great deal more irksome by the presence of fluctuations and 

 by the fact that one cannot tell the gametic constitution of 

 a plant, that is, its breeding capacity, by its appearance. 

 The whole problem of the plant breeder is to find, to fix, and 

 to recombine desirable inherited variations, and to do this 

 in spite of their tendency to be obscured by fluctuations. 

 The methods used to accomplish these results will be taken 

 up presently. First, attention must be called to a physio- 

 logical phenomenon that may be made a tool of such high 

 value to the plant breeder that the statement just made 

 concerning his problem is apparently untrue. This matter 



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