302 University of California Puhlications in Botany [Vol. 5 



may conceive in a varietal cross that the reaction occurs between sets 

 of factors common to a given system and that the particular type of 

 behavior exhibited is dependent upon hardly more than a superficial 

 difference in the case of relatively few of the factors involved. On 

 the other hand, evidence has been obtained from the results of the 

 Tahacum-sylvestris hybridizations which indicate that in such wide 

 crosses there are concerned not contrasts between factors of a com- 

 mon reaction system, but fundamentally distinct reaction systems 

 functioning to a certain extent as units in themselves. In presenting 

 this conception these Nicotian a hybrids were briefly mentioned as 

 evidence bearing out the suggestions made, and it is the purpose here 

 to emphasize and amplify in descriptive manner the condition of 

 affairs elsewhere only touched upon. 



The data presented in what follows seem to demonstrate that 

 when varieties of Nicotiana Tahacum are crossed with .V. sylvestris 

 the Tahacum reaction system dominates the course of somatogenesis 

 nearly or quite to the exclusion of the sylvestris reaction system. 

 In addition to the evidence on this point herein to be presented 

 we have available a considerable ma&s of quantitative data dealing 

 with the size and form of vegetative and floral organs (cf. Good- 

 speed and Clausen, 1915) which, though pertinent and bearing out 

 our main contention, is not essential to establish the conclusion here 

 emphasized. An attempt has been made to elaborate and at the 

 same time cut down the amount of purely descriptive matter by 

 the inclusion of a considerable number of photographs and draw- 

 ings of the experimental material. Most of the photographs were 

 not taken to give evidence in the present connection, and thus it 

 has not been possible in some cases to secure illustrations of plants 

 of parent and hybrid in exactly or even approximately the same 

 stage of development. In the main the photographs of plants or 

 portions of populations indicate simply the general habit character- 

 istics of parents and hybrids, while the photographs of flowers and 

 drawings of leaves give more specific evidence as to the relative 

 size and form of parent and hybrid organs. All the drawings were 

 made from fresh material. The leaves drawn in all cases represent 

 the fourth significant leaf up from the base of the plant, and on a 

 given plate are all drawn to the same scale. 



The following hybrids in F,^ are discussed in this paper. The garden 

 number in the University of California Botanical Garden is given in 

 each case. 



