No. 601] MENDELIAN FACTOR DIFFERENCES 39 



planation, if we regard them as the outcome of a contrast 

 of two distinct Mendelian reaction systems the elements 

 of which can not be freely interchanged without pro- 

 foundly affecting the general functions of the reaction 

 systems thereby resulting. We shall take up in very gen- 

 eral fashion the points which have inclined us to this view, 

 reserving for a later treatment the discussion of the hy- 

 pothesis in detail and also the presentation of the ex- 

 tended data. 



When hybrids are obtained between sylvestris and the 

 various varieties of Tahacum they agree throughout in 

 Fj in presenting the entire set of characters of the Taha- 

 cum parent to the exclusion of those of sylvestris. This 

 behavior may be definitely accounted for as a dominance 

 of the Tahacum reaction system as such over the sylves- 

 tris reaction system. For point by point and character 

 by character throughout, the correspondence between the 

 Tahacum variety and its F^ sijlvestris hybrid may be 

 demonstrated in a remarkable fashion and this irrespec- 

 tive of whether the factors concerned in these character 

 expressions in the Tahacum varieties are dominant or 

 recessive in varietal crosses. This correspondence is not 

 only apparent in general appearance, but it extends to 

 minute details of form and structure, and it is displayed 

 even in the more intangible characteristics generally in- 

 cluded under the term habit— i. e., such characteristic 

 varietal peculiarities of expression as the method of 

 branching, insertion and inclination of the leaves, the type 

 of inflorescence, and so on through a whole series of de- 

 tails. For example, when N. Tahacum var macrophylla 

 (Setchell, I. c, p. 8) is the Tahacum parent, the F^ hybrids 

 display the particular appearance and also the particular 

 characteristics of macrophylla, except that throughout 

 they are expressed on a very much enlarged scale (c/. 

 East and Hayes, 1912). The broad clasping leaf of mac- 

 rophylla with its distinctly pointed tip is faithfully repro- 

 duced in the hybrids. The flowers show no effect of the 

 very much elongated corolla tube and the lobing of the 

 limb peculiar to sylvestris, but display the macrophylla 



