1917] Goodspeed-Clausen : F^ Species Hyhrids i)i Nicotiana 313 



of a number of factor differences even in relatively simple character 

 contrasts such as colored as against white flowers. Blending inheri- 

 tance followed by complex, intergrading segregations of this kind is 

 characteristic particularly of the size and form relations in the various 

 organs, as for instance in the leaves. The frequent production in 

 such segregation of extremes lying beyond those exhibited by either 

 parent is further evidence that this condition may be largely the 

 expression of the inter-relations of a relatively large number of factors 

 derived from both parents. It is all the more significant, therefore, 

 that tliese partially sterile hybrids of sylvestris and varieties of 

 Tabacum furnish such faithful reproductions of the particular 

 Tahacum variety concerned in the cross, for it is hardly conceivable 

 that all the Tahacum factors should be dominant to the corresponding 

 factors in sylvestris in the same sense that a certain Tahacum factor 

 is dominant to an allelomorph in the Tabacum series of factors. Thus, 

 to take a specific instance, when a red-flowered Tahacum, macroph ylla 

 for example, is crossed with sylvestris the F^ hybrids as grown in the 

 field show practically the some flower color as the red-flowered 

 Tahacum parent. There is perhaps a slightly lower intensity of 

 coloration in the flowers of the F^ hybrid which may well be dependent 

 upon their larger size. The color changes in fading are remarkably 

 sinnlar in the two forms. On the other hand, when such a red-flowered 

 Tahacum is crossed with a white-flowered Tahacum, "Cuba" for ex- 

 ample, the flowers are strictly intermediate in color. This different 

 behavior in the two cases might of course be attributed to factor 

 differences in the two whites used, for as a matter of fact the white 

 of the flower of sylvestris is distinctly different from the white of 

 the flower of "Cuba." Nevertheless, when the relations shown by 

 other character complexes, such as size and form of flower, size and 

 form of leaf, method of branching, etc., are considered, it appears 

 more reasonable to interpret the reproduction of the Tahacum flower 

 color in the F^ hybrid with sylvestris as dependent upon the directive 

 action of the set of flower-color factors brought in by the Tahacum 

 parent acting by virtue of their position in the general Tahacum 

 reaction system. The F^ hybrids, therefore, are to be considered as 

 depending for their developmental expression upon the Tahacum re- 

 action system of Mendelian factors acting as a unit in contrast to tlie 

 sylvestris reaction system which appears to renuiin in a latent con- 

 dition while the developmental processes are expressing themselves. 

 The yiodified physiological relations which result from the presence 



