1922] Setchell-Goodspeed-Clausen: NicoUana Tahacum 513 



would have a real significance, and the relationships indicated by it 

 would be fundamental ones. It is, however, necessary to have a much 

 more extensive knowledge of Mendelian heredity in Tahacum than we 

 have at present before such a system can be formulated. 



2. METHODOLOGY OF MENDELIAN ANALYSIS IN TABACUM 



From the Mendelian side there are certain obvious facts associated 

 with Tahacum as a species. In the first place, as we have stated before, 

 the species is highly polymorphic. A large and striking assemblage 

 of varieties exists, the most extreme of which hybridize readily and 

 give fully fertile hybrids and full fertility in their derivatives. A 

 few teratological forms are known in which fertility is somewhat re- 

 duced, but the above generalization does not far overstate the facts. 

 The species is, moreover, so highly polymorphic that with respect to 

 any given character a representative collection of varieties may be 

 arranged in a series connecting the most extreme expressions of that 

 character by imperceptible steps. Thus in flower color we have repre- 

 sented in the collection of varieties of the University of California 

 Botanical Garden dark red, red, light pink, pinkish, and white, and 

 descriptions occur in the literature which indicate the existence of 

 further shades of red connecting these. Now flower color is a rather 

 definite character, comparatively speaking, for it appears to be little 

 affected by ordinary environmental conditions. In many polymorphic 

 forms, such for example as maize, there are a large number of such 

 definite characters, and as a consequence studies of inheritance in 

 these forms have resulted in definite Mendelian analysis of many char- 

 acter differences. But in Tahacum unfortunately most of the char- 

 acters involve quantitative elements, and these with few exceptions 

 depend so largely for their particular expression upon environmental 

 conditions that it becomes a difficult matter in a segregating population 

 to distinguish between those differences which are inherent and refer- 

 able to the genotype and those which have come about through the 

 action of extrinsic forces. And yet our assemblage of tobacco varie- 

 ties indicates clearly that there are genotypes which give rise to all 

 possible expressions in these characters. Here we find the reason for 

 the present backward state of knowledge of inheritance in Tahacum, 

 for while there have been numerous investigations which indicate 

 clearly that the Mendelian mode of transmission may be followed in 

 all these character differences, yet there are very few investigations 

 which have resulted in the precise type of factor analysis characteristic 



