MUTATIONS IN PLANT BREEDING 369 



new characters. Surface and Zinn consider this sufficient evidence to 

 make it "almost certain that similarly inherited variations may occur in 

 respect to physiological characters such as yield." That they were 

 justified in making this inference is shown by the success of their ex- 

 periments on pure line selections for yield. 



Other self-fertilized crop plants in which mutations have been reported 

 are barley, wheat, tomato and potato. 



In maize the sudden appearance of new characters in established vari- 

 eties or strains has been reported by a number of investigators. The 

 remarkable diversity between inbred strains as discovered by Shull and 

 by East indicates the extent to which germinal variations occur in this 

 plant. Each author obtained one strain which was so nearly sterile as to 

 be in danger of complete extinction while other strains appeared to be 

 capable of maintaining fairly good annual yields indefinitely. Abundant 

 evidence of the occurrence of factor mutations in maize is also found in the 

 numerous pairs of contrasted characters which are inherited in Mendelian 

 fashion. Besides those referred to in earlier chapters we may mention 

 multiple stems (suckers) as dominant over single stems (no suckers), 

 normal stature (tall) as dominant (usually) over dwarf stature, normal 

 green leaves as dominant to striped leaves, presence and absence of aerial 

 roots, hairy and glabrous stems, branched and unbranched tassel, normal 

 ear and branched ear, normal anthers and fasciated anthers, presence of 

 normal reproductive organs and absence of the same (barrenness). Most 

 of these allelomorphs behave as unit characters. Various quantitative 

 differences such as stature, ear-length and number of rows to the ear are 

 either conditioned by several pairs of factors or by single pairs of factors 

 which are subject to a wide range of variability in expression. Con- 

 stitutional vigor and productivity are doubtless conditioned by the in- 

 teraction of very many factors and a mutation in a single one would alter 

 the end result. In short the inherent individuality of corn plants, which 

 makes possible the successful application of selection methods, must be 

 referred to factor mutations. 



Sugar beet improvement, particularly increase in sugar content 

 of the roots, depends directly upon the occurrence and selection of 

 mutations, according to Pritchard. As a result of statistical investiga- 

 tions of variation, correlation, inheritance and selection in the sugar 

 beet, he concludes that although sugar beet improvement has been ac- 

 complished, continuous selection is not necessarily the determining 

 factor in attaining the present high sugar content of the best varieties. 

 His statistics show that the best roots transmit no better qualities 

 than do the mediocre roots because the differences are merely "fluctua- 

 tions" (modifications). The real differences between sugar beet families 

 are usually very slight and are greatly exceeded by their "fluctuations." 



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