BRITTLE RACES OF OENOTHERA LAMARCKIANA. 



Brittleness is a quality which belongs to the central group of 

 characters in Oenothera, as distinguished by Boedijn and the writer 

 (9). Like almost all the members of this class, it is not accompanied 

 by the doubling of a chromosome. Moreover, it is recessive and is- 

 ogamic, and therefore can be combined in the mutant races with 

 any other mark. This article reports the study of its relations to 

 the lethal factors, which are also situated in that group. This study 

 starts from two races, one of which (0. similis) has been found in 

 the original field at Hilversum, lately described by Uphof (2), while 

 the other (0. scindens) originated in my experimental garden at 

 Amsterdam. These races are to be considered as due to recombina- 

 tions of mutated qualities, the first origin or premutation of which 

 is probably as old as the parent species itself. Secondary changes 

 may accompany them, but they are not of paramount interest for 

 the present study. 



Oenothera Lamarckiana similis. 



In crosses of individuals of the same family of 0. rubrinervis with 

 Lamarckiana plants from various sources, and with different mu- 

 tant strains, I observed that in the first generation sometimes brittle 

 rubrinervis are produced, but in other cases tough hybrids of the 

 type of subrobusta instead of rubrinervis (3). From a single cross be- 

 tween two individual parents both types are never seen to arise in 

 the first generation. Moreover, the two contrasting cases have oc- 

 curred mainly in strains derived from different initial plants (3). 

 Rubrinervis is only a brittle form of subrobusta, and this latter 

 usually splits off brittle specimens in the second generation. On the 

 other hand, we have to consider 0. erythrina (5), which is a mutant 

 externally very like the hybrid subrobusta. It never arose by way 

 of mutation from the main strain of 0. Lamarckiana, which I am 

 still using for my cultures, and which produces regularly brittle 

 mutants of the type of rubrinervis. From rosettes collected at the 

 original station near Hilversum in 1905, however, it sprung in one 

 instance in two specimens in 1907, and from another rosette in 



