238 MASS MUTATIONS AND TWIN HYBRIDS OF 



H. Analogy between the twins of 0. grandiflora and 

 0. Lamarckiana 



In concluding this article I might point out the striking analogy 

 between the splitting phenomena of 0. grandiflora and those of 

 0. Lamarckiana. Evidently they must be considered as the results 

 of the same internal causes. The chief difference is the absence of 

 a visible mass mutation in the latter species. On the contrary, the 

 amount of barren grains among its seeds is double that of 0. grandi- 

 flora. We are therefore induced to assume a second lethal factor, 

 linked with the characters of ochracea and laeta, respectively, and 

 killing the laeta germs of 0. Lamarckiana. Or, stating it in other 

 words, we may imagine the factor for weakness, which causes the 

 death of a large part of the ochracea mutants after germination, to 

 be replaced in 0. Lamarckiana by a lethal factor, which kills the 

 corresponding germs before germination. 



Renner (12) has proposed an explanation which in some respects 

 parallels the views developed in this article, but, as I have already 

 explained, it differs mainly in the conception of the first origin 

 of 0. Lamarckiana. Renner considers this species to be a hybrid 

 between two previously existing types, corresponding to laeta and 

 velutina, and sees in this hybrid condition the cause of its mutability. 

 The analogy with 0. grandiflora leads us, however, to consider 

 this "hybrid condition" not as the cause but as a result of the 

 mutability. Elsewhere I have shown that his conception leads to 

 contradictions and requires too many additional hypotheses, even 

 without considering the analogy with 0. grandiflora (4). A detailed 

 criticism of Renner's views from this latter standpoint, however, 

 must be postponed until another opportunity. 



Summary 



1. Oenothera grandiflora Aiton from Castleberry, Alabama, splits 

 in my cultures in every generation into two types. One of them 

 consists of strong, green plants of the parent type; the other of 

 weak, yellow individuals, of which only a few are vigorous enough 

 to flower and ripen their seeds. This weak type is called 0. grandi- 

 flora mut. ochracea. 



2. Besides these it produces other mutants in the ordinary pro- 

 portions of 0.1 — 1 per cent, namely, mut. lorea with almost linear 

 leaves and mut. gigas with 28 chromosomes and the corresponding 



