156 TJniversiti) of California Publications in Agricultural Sciences [Vol.4 



Formerly (Frost, 1916) the liypothesi-s ol' I'rL'ijucnt doiuiiiant 

 mutations seemed the more probable, but there is apparently non- 

 conformable evidence. It is true that the peculiar behavior of the 

 slender type might conceivablj' depend on an occasional mutation in 

 another locus, or an exchange (Shull, 1914) or duplication of loci, 

 giving two similar or identical factors for slender. An ai)parcntly 

 fatal objection, however, is the fact that the supposed mutants seem 

 to show linkage with singleness or doubleness at their origin from 

 Snowflake as well as in subsequent generations — a fact which strongly 

 suggests segregation in the former case. 



If the apparent mutants are really due to segregation complicated 

 bj' lethal action, the origin of the complex heterozygosis indicated for 

 Snowflake is doubtful; it may be due to hybridization, but more 

 probably to a gradual accumulation of mutant factors in balanced- 

 lethal combinations. On the analogy of Muller's Drosopliila case, 

 especially, it might be expected that the latter would be the true 

 explanation, particularly since self fertilization seems to be the rule 

 in Mattliiola. On this basis the term mutant type is used with some 

 confidence in this paper, while the aberrant individuals have been 

 called apparent mutants. 



We must not forget that some of the mutant types may arise, as 

 with Oenothera gigas and 0. lata, by non-disjunction, or reduplication 

 of chromosomes, and that this fact may determine their heredity. 

 This is not to be expected with the types whose factors show apparent 

 coupling with singleness or doubleness. ])ut it might be true of the 

 apparently unlinked smooth-leaved type. A preliminary study of 

 several types shows that the usual .somatic number of chromosomes 

 is probably fourteen, but that positive counts are difficult. While it 

 might be very hard to demonstrate the regular presence of one extra 

 chromosome in an individual or a type, it should ])c easy to decide 

 between tlic diploid and triploid numbers. The large-leaved type is 

 so strongly suggestive of 0. gigas that it would not be surprising to 

 find the triploid number in the material now on hand for examination. 



In a preliminary paper on these types the writer (Frost, 191(5) 

 discussed some possible relations of nuitation, heterozygosis, and 

 partial sterility, with special reference to Oenothera, mentioning the 

 po.s.sibility that special prevalence of heterozygosis in the genus may 

 be, "in large part, a result rather than a cause of mutation." This 

 .suggestion is evidently ju.stified even if much of the supposed mutation 

 of Oenothera is really segregation, since it is highly probable that 



