30 OENOTHERA GIGAS NANELLA, A MENDEL1AN MUTANT. 



If we compare these figures with the results of the crosses be- 

 tween O.rubrinervis and 0. nanella itself, as described in my Gruppen- 

 weise Artbildung (p. 215), we find a complete analogy, since these 

 crosses give no dwarfs in the first generation, and in the second 

 about 10-14 per cent from the self -fertilized specimens of 0. sub- 

 robusta. It is evident, therefore, that the exceptionally high yield 

 of dwarfs in these crosses of 0. Lamarckiana and 0. rubrinervis 

 must be the product of latent mutations which occurred in some of 

 the sexual cells of one of the parents. And since 0. Lamarckiana 

 is known to produce ordinarily 1-2 per cent dwarfs, while 0. ru- 

 brinervis does not show signs of such a mutability, we may con- 

 fidently assume that our figures indicate latent mutations of sexual 

 cells of 0. Lamarckiana. 



Bartlett 1 ) recently described a similar instance of an unexpect- 

 edly high mutability, and proposed for it the same explanation, on 

 the assumption of a latent mutation of a sexual cell in a previous 

 generation. This case is of the greatest interest since it relates to 

 a pure species and not to the discovery of mutated gametes by 

 means of crosses as in the experiments just described. The mutating 

 species was 0. Reynoldsii Bartlett, one of the forms of the old 

 0. biennis. It produced in 1913 three marked types, one repeating 

 the parental form, and the two others being dwarfs and called 

 mut. semialta and mut. debilis. The latter is, on the average, about 

 half as high as the former. This curious segregation repeated itself 

 in the next generation in 1914, not from all the individuals, but 

 from only one of the two whose offspring have been tried in this 

 respect. 



Similar proofs of latent mutations of sexual cells may evidently 

 be expected to occur in other strains also and will have to be looked 

 for in all cases of an unexpectedly high degree of mutability. 



I will now return to my experiments on the production of dwarfs 

 by 0. gigas. In order to obtain specimens of 0. gigas yielding a 

 high percentage of dwarfs from their seeds, I sowed in 1911 seeds 

 of my pure strain, cultivated the plants as biennials, and fertilized 

 them in 1912 by their own pollen, in bags. They were vigorous 

 plants of the fourth generation (Gruppenweise Artbildung, p. 175), 

 and yielded a large harvest of seed, which was sown in 1913, and 

 served as a criterion, since no essential differences were to be seen 

 on the plants themselves. Moreover, I used the seeds of some good 



i) Bartlett, H. H., Mutation en masse. Amer. Nat. 19 15. 



