24 BOTANICAL GAZETTE [januasy 



will be found to comply with the views proposed in this article. 

 It would lead us too far, however, to reproduce these calculations 

 here. 



In all these cases the conception that mass mutation is the chief 

 cause of the production of twin hybrids evidently makes the sup- 

 position of a labile condition of the factor for laeta superfluous. It 

 seems desirable, therefore, to lay stress on the fact that this sup- 

 position does not rest on the phenomena observed in the produc- 

 tion of these twins. It is mainly derived from other observations, 

 and some of them may be briefly repeated here in order to make 

 this point clear. They refer to the brittleness of O. rubrinervis and 

 O. deserens and to the dwarfish stature of O. nanella. 



In crosses brittleness behaves in three different ways. With 

 O. biennis Chicago and O. Cockerelli it is recessive to the tough 

 structure of the fibers, since it fails in the first generation and 

 reappears in the second in ratios corresponding to Mendel's 

 law. In crosses with O. Lamarckiana it is sometimes dominant 

 and sometimes recessive, as has been shown. In O. rubrinervis 

 and O. deserens the toughness is wholly absent. From these and 

 other facts it is clear that at least three conditions of this 

 factor are possible. I call them active, labile, and inactive. 

 Whether the labile condition is due to linkage or to some other 

 cause is as yet an open question, which, however, has no influence 

 upon the main contention. The combination '' active X inactive" 

 is assumed to be responsible for Mendelian crosses, but the com- 

 bination " labile X inactive" may cause a splitting in the first gen- 

 eration and produces, as a rule, constant hybrids. The two types 

 of first generation hybrids appear in variable numerical propor- 

 tions according to different circumstances. If one of the groups 

 is so small as not to be represented in every loo specimens, the 

 splitting may seem to fail, and such extremes are of common occur- 

 rence. This would explain the dominance of an evidently recessive 

 character. 



The case is exactly the same for the dwarfish stature. The 

 factor for tallness must be in the inactive condition in the dwarfs, 

 but in the active condition in O. rubrinervis, since the crosses 

 between these two types follow Mendel's law. In O. Lamarckiana, 



