108 White. 



abnormal expression of A was small. Contabescent anthers occurred 

 in this cross, as well as in all the others, but their number per flower 

 or plant depended always on the degree of dominance expressed by 

 the factor A. 



Fg, F3 and F^ progreny. 



F2 generation from three different varietal crosses have been 

 grown, these being 301—1—5 X 373, 301—1—1 X 353 and reciprocal, 

 and 304X402. 



N. iabacum fasciata (304) X N. tabacum (402) (Fig. 20). 



The experimental results and data from this cross have been given 

 in detail in an earlier paper (White, 1913). The F2 population con- 

 tained abnormals (AA), intermediates (Aa), and normals (aa) in the 

 proportion of 1:2:1. All the apparent classes were easily and accu- 

 rately separated. The abnormal segregates bred asolutely true in Fs 

 and Fi, while the intermediates continued to produce AA, Aa and aa 

 progeny in these later generations. Such a population is represented 

 by Tables 15 — 16. By glancing at it, one may see how sharply the 

 three classes are differentiated, and this contrast becomes more marked 

 when one sees the plants themselves rather than mathematical charac- 

 terizations of them. 



Table 14 shows the ration of AA, Aa und aa segregates among 

 the progeny of six different selfed heterozygotes. In some cases, where 

 the families are small, the actual and expected ratio are far from 

 agreement, but in the case of (304X402) — 1 — 34), one could not 

 possibly expect a closer approximation between the two. The total 

 ratio from all the heterozygous families also closely approaches that 

 demanded by theory. 



In this cross, so far as one could determine by observation, no 

 segregation in other factors took place, and one would expect none to 

 occur if the two races differ only by the genetic factor A, as I strongly 

 believe. 



Character of aa and AA segregates. 



Table 17 shows the character of an F3 population from a selfed 

 AA Fa segregate, while Tables 18 — 19 represent two families from 

 selfed aa F2 segregates. A glance at the latter may cast doubt upon 

 my statement that pure aa segregates occur in F2 and breed absolutely 

 true, because these tables show there is a slight variability in the 

 number of parts per flower. But the normal (402) itself, so far as 



