OENOTHERA RUBRINERVIS, A HALF MUTANT. 355 



to be a fact of paramount interest in the discussion concerning the 

 probable causes of this phenomenon. 



The details of the following experiments will give proof of the 

 proposed conception. I shall first give those relating to self-fertili- 

 zations and afterward deal with the crosses. 



Oenothera rubrinervis originated in my garden from 0. Lamarcki- 

 ana quite regularly in a percentage of about 0.1. Every time the 

 visible characters were exactly the same. Between 1890 and 1900 

 the mutation was repeated 66 times among 66,000 plants 1 ). In 1905 

 I introduced new rosettes of 0. Lamarckiana from the original loca- 

 lity near Hilversum into my garden, and among their offspring 

 I observed also repeated mutations into rubrinervis. The characters 

 were always the same, namely, a pale reddish tinge, narrow and 

 longitudinally folded leaves, a hairy epidermis, cup-shaped flowers, 

 but above all the brittleness of the stems, branches, and petioles, 

 due to the incomplete development of the cell walls in the fibers 

 of the bark and wood. Until now I cultivated mainly two strains, 

 derived from two different mutants of 1895. One of them has given 

 the material for all my crosses, and I shall designate it as the main 

 line. The second line of 1895 was originally destined for control 

 experiments only, but in 1913 it produced the first observed case 

 of 0. deserens, as previously mentioned, and since then it has been 

 studied carefully in this respect. 



After repeated cultures of pure 0. deserens had been made and 

 compared with 0. rubrinervis, the characteristic marks of the two 

 forms became quite clear and reliable, although very smal. In 

 mixed cultures the types may even be separated when very yolung, 

 but some dubious specimens may remain. At the time of flowering 

 these have almost always been shown to belong to the new type. 

 Among the very young rosettes, with only 3—5 leaves, those of 

 deserens are broader and more flattened and of a deeper and purer 

 green, resembling therein young plants of 0. Lamarckiana. These 

 differences increase slowly until the time when the rosettes must 

 be planted out from the boxes into the garden, about the middle 

 of April. The leaves of 0. deserens have now a broader base and a 

 less pointed top than those of 0. rubrinervis, besides the marks 

 already given. In July the differences remain very small, the two 

 types reaching the same height at the same period, but the rubriner- 

 vis begin to flower one or two weeks earlier than the deserens. Seen 



J ) The mutation theory, English ed. 1 : 331. 1909. 



