No. 601] MENDELIAN FACTOR DIFFERENCES 41 



When particular tagged Mendelian factors are con- 

 sidered the same behavior is displayed. Perhaps there is 

 no more striking instance of this than that shown by the 

 expression of the calycine flower type in these sylvestris 

 hybrids. When N. Tahacum var. calycina (Setchell, I. c, 

 p. 6) with its peculiar split, hose-in-hose flowers is crossed 

 with Tahacum varieties of the normal flower type, the Fj 

 hybrids display the normal flower form and segregation 

 occurs in into normal and calycine in accordance with 

 simple Mendelian expectations. But when the Tahacum 

 reaction system carries the recessive calycine flower 

 factor into these species hybrids with sylvestris then 

 every flower on the Fj plants displays a more or less caly- 

 cine structure. Similarly when the parthenocarpic char- 

 acteristics of A^. Tahacum var. "Cuba" (Goodspeed, 

 1915) are carried in by the Tahacum parent then the F^ 

 hybrid, instead of shedding its capsules soon after anthe- 

 sis as is the case in all the other Tahacvm-sylvestris hy- 

 brids, retains them indefinitely, in spite of the fact that no 

 good pollen is produced, and thus non-fertilization, the 

 stimulus for fruit abscission in Nicotiana, here also is the 

 rule. So far as present evidence indicates this character- 

 istic is rather strictly confined in Nicotiana to the variety 

 " Cuba." This behavior of recessive factors of Tahacum 

 varieties in hybrids with sylvestris is a striking confirma- 

 tion of the conception that in such cases there is a con- 

 trast between distinct reaction systems rather than be- 

 tween certain factors as opposed to each other. In gen- 

 eral when Tahacum varieties of the type mentioned above 

 are crossed with each other the hybrids, especially with 

 respect to flower color, leaf shape, etc., are intermediate. 

 The contrast in this case is not one between two distinct 

 Mendelian reaction systems, but it is merely a contrast 

 of certain differences within a oonmion system, and the 

 segregation in subsequent ^eii.Tal ions, nltliough complex, 

 indicates a general accordaiu'c with luiriiial Mendelian 

 expectation. But in the case of s])eeios liyhrids between 

 Tahacum and sylvestris the contrast is between distinct 



