118 University of California Publications in Botany [Vol. 5 



I. INTRODUCTION 



Probably the most recent discussion of plant and animal 

 breeding, especially from the point of view of the Mendelian 

 discovery, contains the following statement in reference to 

 dominance, that "unessential feature of the Mendelian phenom- 

 enon" — "The fact that the dominant and hybrid tall (in Men- 

 del's classic experiment with Pisum) appear to us identical is 

 probably no more than a measure of the crudeness of the means 

 which hitherto have been adopted to distinguish between them — " 

 (Darbishire, 1911, p. 38). Thus, leaving aside the question 

 whether or not the condition and appearance of the hybrid in F^ 

 is an "unessential feature," the report on the quantitative es- 

 timation of the condition in which certain parental characters 

 are inherited in F^ seems of interest and value. 



Despite such assurances as the above in reference to the rather 

 unessential nature of dominance (Bateson, 1909, pp. 13 and 53; 

 see, also, Stockberger, 1912, p. 152, and Baur, 1911, p. 52), 

 a considerable number of references to the "Law of Dominance" 

 are to be found in the literature on plant-breeding which has 

 appeared since the rediscovery of Mendel's experiments (East, 

 1907, p. 40, and Brainerd, 1907). With either point of view, the 

 Fi generation, throughout the literature as it has been accessible 

 to me, does not receive any great measure of critical attention. 

 When dominance can be reported, the ground is cleared for a 

 direct advance toward the hoped-for Mendelian segregation in 

 Fo and, on the other hand, when a strikingly intermediate hybrid 

 appears in F^ we look up the situation and find that we are 

 justified in merely making a note of this relatively unimportant 

 stage in our experiment and in reserving our efforts in anticipa- 

 tion of cliificulties in i^roperly interpreting the Fo generation. 



The present paper contains the results of a strictly quanti- 

 tative investigation of an easily measured character — corolla 

 diameter, in the hybrids of three varieties of Nicotiana acuminata 

 which are to be distinguished from one another in respect to this 

 character alone. 



