516 University of California Publications in Botany [A'^ol. 5 



long established diverse types, evidently related to one another in 

 fundamentally the same manner as are the various Drosophila mutants, 

 but more complexly, and from these complex assemblages he must 

 unravel the tangled skein of heredity. 



There are, however, other and perhaps quicker ways of establishing 

 a uniform and favorable residual heredity than that of securing and 

 testing homozygous extractives, and these may be employed in certain 

 special cases. Thus, if it be desired to study the relationship of the 

 pair of factors Ss for the petioled versus sessile condition, it should 

 be possible to proceed by crossing back the F^ of angustifolia x macro- 

 pJiylla, for example, to macrophylla, selecting the petioled forms from 

 the back cross for again crossing back to macrophylla, and continuing 

 the process until clear-cut segregation was obtained. Such a mode 

 of procedure should establish a residual genotype equivalent to that 

 of macrophylla itself, and should thereby enable the student event- 

 ually to study the effect of substituting SS for ss in the macrophylla 

 genotype. In tobaccos technical details make it particularly easy to 

 adopt such a procedure, but it is useless to speculate further upon its 

 results until it shall have been attempted. 



3. MENDELIAN HEREDITY IN TABACUM 



From the standpoint of factor analysis, we have demonstrated 

 clearly in the foregoing pages, the existence of a number of distinct 

 pairs of factors. Two of these affect flower color, one flower form, 

 and three affect the character of the leaf base. The particular effects 

 of the opposing members of these pairs of factors and the interrela- 

 tions which they exhibit so far as these have been investigated have 

 been set forth in the discussions which follow the description of each 

 of the three series of hybrids. Although evidently many other factor 

 differences were concerned in these studies, and remain for further 

 investigation, the results which we have described make a beginning 

 tow^ard a more accurate knowledge of Mendelian heredity in Tabacum. 



So far as our results furnish any data on the question, the six 

 pairs of factors isolated exhibit no linkage relations. The data here 

 are far from complete, but the results are in accordance w'ith theory. 

 According to White (1912), there are twenty-four pairs of chromo- 

 somes in Nicotiana. Assuming for the sake of discussion that each 

 of these pairs of chromosomes bears a set of factors comparable in 

 numbers to any other pair, then the chances of finding linkage when 

 only six pairs of factors are studied is very slight. This large number 



