32 BREEDING CROP PLANTS 



a conception of the mode of transmission of these size characters 

 and there seems no good reason for believing that a different 

 mechanism is involved than in the inheritance of color characters. 

 Environmental conditions probably play a larger role in the 

 modification of the appearance of size characters than for color 

 characters. 



Stability of Inherited Factors. That sudden changes in the 

 appearance of a character are sometimes found is a well-known 

 fact. The causes of these sudden changes are not so easily 

 determined. Whether these are more logically explained as due 

 to changes in certain inherited factors or due to a new recombina- 

 tion of factors or by other causes is an unanswered question. 

 The pure-line theory of Johannsen was a result of an experimental 

 attack on the question of the stability of a character. A few 

 sudden changes in characters have been observed. Nevertheless, 

 plant characters of self-fertilized crops exhibit remarkable 

 uniformity. Many of the inherited sudden changes which 

 have been noted are most logically explained as the result of a 

 natural cross. Others appear to be due to a sudden change in 

 the hereditary factors of the organism or to the loss of a genetic 

 factor. 



The view of factor stability which seems most helpful for the 

 plant breeder has been clearly stated by East and Jones (1919). 



"For these and other reasons which might be given, could further 

 space be devoted to the subject, we believe there should be no hesitation 

 in identifying the hypothetical factor unit with the physical unit factor 

 of the germ cells. Occasional changes in the constitution of these 

 factors, changes which may have great or small effects on the characters 

 of the organism, do occur; but their frequency is not such as to make 

 necessary any change in our theory of the factor as a permanent entity. 

 In this conception biology is on a par with chemistry, for the practical 

 usefulness of the conception of stability in the atom is not affected by the 

 knowledge that the atoms of at least one element, radium, are breaking 

 down rapidly enough to make measurement of the process possible." 



