FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 175 



and consequently continue growth until checked by unfavorable external 

 conditions or by the drain of seed production. It is, therefore, possible 

 in this case to state definitely upon what a size difference depends. 



In sweet peas there are two distinct dwarf forms which display a 

 simple type of inheritance when crossed with tall forms. One of these 

 types is the Cupid sweet pea which originally arose as a mutation from 

 Emily Henderson, a tall white variety. The Cupid sweet pea is a very 

 dwarf procumbent type which produces no erect stems. When crossed 

 with tall varieties it gives tall plants in Fi and in FZ segregation into 



FIG. 82. (right) Young bush bean plant showing determinate habit of growth. The 

 axis is terminated by a flower cluster; (left) young pole bean plant showing indeterminate 

 habit of growth. The flowers are all in the axils of the leaves. (After Emerson.) 



tall and dwarf in the ratio 3:1. The other restricted form is the bush 

 sweet pea. It is characterized by a profuse production of thin, wiry 

 branches which intertwine and form a bush, sometimes three and a 

 half feet high. Bush crossed with tall gives in F\ tall plants and in F 2 , 3 

 tall : 1 bush. The bush sweet pea, therefore, like the Cupid differs from 

 the tall sweet pea in a single genetic factor, and in both cases the tall 

 form is completely dominant. 



An interesting situation arises when bush sweet peas are crossed with 

 Cupid sweet peas. From our knowledge of the inheritance exhibited 

 by these two forms when crossed with tall, we may assume as in previous 

 cases that in the appearance of the Cupid mutation there was a change 



