190 MUTATION AND PLANT BREEDING 



ed from a difference of 54 days in blooming. This difference in size 

 is apparently the result of the time element alone and not a differ- 

 ence in rate of growth because no appreciable difference occurred 

 in the time that corresponding successive leaves appeared on the 

 four genotypes. The fact that larger leaves appeared on the later- 

 maturing genotypes indicates only that the growing point con- 

 tinued to grow in circumference from day to day in the early life 

 of the plant, and a leaf arising from a higher node would be expected 

 to be larger than one arising from a lower node. Some additional 

 data are presented in Table 3 to show the morphological relation- 

 ships of the various genotypes. As would be expected, there is a 

 positive correlation between leaf number and number of days to 

 anthesis, height, stalk diameter, and leaf size. The four genotypes 

 measured are similar to one another in appearance, except that 

 each genotype is a larger counterpart of the one immediately pre- 

 ceding it in earliness of maturity. Early maturing varieties are used 

 by farmers because the later maturing varieties exhaust the soil 

 moisture before maturity or need additional irrigations. 



A logical explanation of the nature of the maturity genes in 

 milo would be that Ma 1} Ma 2 , and Ma 3 are dominant inhibitors 

 that block some essential reaction leading to floral initiation. This 

 assumption would mean that the "wild type" was small and very 

 early in maturing because of early head initiation. Sorghum must 

 have been domesticated by the preservation in the tropics of larg- 

 er and later maturing plants that carried inhibitors that prevented 

 early floral initiation. When tropical varieties came to the United 

 States and were grown at latitudes above 30 degrees, they were 

 too late in maturity to be of greatest use and man again preserved 

 mutations, but this time those that inactivated the inhibitors 

 to some degree. 



-o 



Height Genes 



In most of the sorghum-producing areas of the world, tall 

 stature is preferred; but, in the United States, farmers have used 

 shorter and shorter varieties as they became available. The dwarf- 

 ing genes that have been useful in sorghum improvement shorten 

 only the internodes. Four such dwarfing genes are known in sor- 

 ghum and are inherited independently (28). 



Recessive r/w 4 was in Standard and Early White Milos and 



