FIFTEEN- AND SIXTEEN-CHROMOSOME OENOTHERA MUTANTS 77 
of 14-chromosome combinations would be possible. If the common 
types of 15-chromosome mutant offspring of 0. Lamarckiana could be 
limited to seven, one might assume that these are "half mutants" 
(borrowing de Vries's term, but applying it differently) resulting in 
each case from the union of a regular 7- with an 8-chromosome gamete, 
each of the latter entering into the union differing from every one of 
the remaining six with respect to the particular extra chromosome 
which it possesses. The union of any one of the many irregular 7- 
chromosome gametes with an 8- might produce one of the rarer types 
of 15-chromosome mutants and such a form might be regarded as a 
"whole mutant" (de Vries). The objections to these suggestions are 
obvious: irregular 7-chromosome distributions would be expected to 
occur more rarely than 6-8, and germ-cells resulting from the former 
would be expected to unite with regular 7-, producing 14-chromosome 
half mutants in the vast majority of cases, and to unite with cells having 
8 chromosomes only in extremely rare instances. We would be obliged 
to conclude that the common 14-chromosome mutant offspring of 
0. Lamarckiana result from 9 or cf regular {Lamarckiana) 7 + 9 or 
cf irregular 7. If such were the case, one of these mutants, such as 
O. nanella, for example, could reproduce itself only by means of the 
union of dissimilar gametes, probably of the same types as those which 
entered into the original combination. We have designated the 
Lamarckiana combination sts abcdefg -\- a'*h' c' d' e' f g' = aa' hh' cc' 
dd' ee' f gg' ; then if the mutant nanella resulted from abcdefg 
+ a' h'b' — d' e' j' g' = aa' hh'h' c— dd' ee' ff' gg\ and if the male 
and female gametes produced by the mutant were each represented 
by the two types of gametes entering into the original combination, 
0. Lamarckiana X 0. nanella should produce two types of offspring: 
abcdefg -\- a' b'b' — d' e' f g' = aa' bb'b' c— dd' ee' ff' gg', 0. nanella, 
and abcdefg a' b' c' d' e' f g' = aa' bb' cc' dd' ee' ff' gg', 0. La- 
marckiana. The same results should be secured from the reciprocal 
cross. As a matter of fact, de Vries ('13, p. 207) has shown that 
these are the results obtained from the two crosses ; but how shall we ex- 
plain the behavior of 0. nanella, selfed? It is well known that this 
mutant breeds true, while on the basis of our previous assumptions, 
we should expect it to produce three types of offspring: {i)a bb — d e f g 
+ a' b'b' — d' e' f g' = aa' bbb'b' — dd' ee' ff' gg', unlike both parents; 
{2) a b c d e f g -\- a' b'b' — d' e' f g' = aa' bb'b' c— dd' ee' ff' gg', 0. 
Unless apogamous development were possible. 
