BOTANICAL GAZETTE 



[JULY 



4 * 



(2) hybrids between two mutants; (3) ordinary Mendelian hybrids, 

 which appear in some of the mutant crosses. These can only be con- 

 sidered as general tendencies, however, for the results vary widely in 

 many of the crosses, both in the number and the percentage of the 

 types appearing. DeVries has also been able to vary the results 

 experimentally in a number of cases. Crosses in which a mutant is 



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involved he calls mutation hybrids (Mutalionskreuzungen), and hybrids 

 of exactly this character do not occur, so far as I know, in any other 

 genus of plants or animals. 



The conspicuous types of hybridization in Oenothera are as fol- 

 lows : When O. Lamarckiana is crossed with a mutant, in the case of 

 several of the forms DeVries experimented upon, a hybrid is pro- 

 duced which splits, but differs from a typical Mendehan hybrid in 

 three respects, (i) A heterozygote, showing only the characters of the 

 dominant parent, and containing the recessive characters latent, is 

 not formed in the first hybrid generation, but instead the two parent 

 forms are produced. (2) The proportions of the offspring are not 

 always about 3:1 as in a Mendelian hybrid, but the percentage of 

 plants having the recessive characters (the mutants are recessive to the 

 parent O. Lamarckiana) often fluctuates between o and 50 per cent-, 

 or even with wider range. This distinction is believed to be less 

 important, as fluctuations in the percentage of the forms are known to 

 occur in some Mendelian hybrids. (3) A still more striking difference 

 is that, while in a Mendelian hybrid two-thirds of the dominant plants 

 continue to split in the proportions of 3:1 in successive generations, 

 both the forms produced in a mutation hybrid, or cross of O, 

 Lamarckiana with a mutant, breed true and show no sphtting. There 

 is thus no external evidence that crossing has ever occurred. This type 

 of hybrid appears when O. lata, O. nanella^ or O. rubrinervis is crossed 



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with O- Lamarckiana, The following 

 diagram will make clear the distinc- 

 tions between this and an ordinary 

 Mendehan hybrid. A represents the 

 characters of O. Lamarckiana in fig. j, 

 and of the dominant parent in fig. 2. B 

 Constarii represents the characters of a mutant in 



Fig I. —Mutation hybrid fig" -f , and of the recessive parent in fig. 2. 



A 



B 



A 



(0-50% or more) 

 Constant 



B 



I 





