FIFTEEN- AND SIXTEEN-CHROMOSOME OENOTHERA MUTANTS IO3 
for fertilization purposes is rarely obtained from 15 -chromosome forms; 
however, this statement does not necessarily exclude the latter, since 
no evidence has been produced to show positively that y -{--chromosome 
male gametes, capable of functioning, are produced by any -chromosome 
form). The results are practically the same as those derived from 
1 5-chromosome 0. lata X 14-chromosome 0. Lamar ckiana. This 
certainly indicates that the buds of 0. semilata de Vries which were 
employed in this cross, produced only 7-chromosome male gametes 
capable of uniting with the 7- and 8-chromosome eggs of 0. lata 
and producing viable seeds. If this mutant had 16 chromosomes and 
if only 7-chromosome male gametes were produced by all buds, it is 
obvious that when this form was selfed, the only eggs which united 
with the 7-chromosome male gametes and produced seeds capable of 
germinating in the time which de Vries allowed them, had 9 chromo- 
somes. 
All facts and possibilities considered, it seems quite as probable 
that 0. semilata de Vries had 14, as 16, chromosomes. The production 
of pollen by this form and the evidence of constancy, when selfed, do 
not preclude the pOvSsibility of its having had 15 chromosomes. 
We may briefly outline our conclusions regarding the factors which 
determine the constancy or inconstancy of a plant as follows: 
It has been shown that somatic chromosome number in Oenothera 
is constant; therefore, unless 1 5-chromosome offspring are produced 
apogamously or unless the chromosomes in excess of 15 are eliminated 
after fertilization takes place, it is evident that a 15-chromosome form 
can breed true, i. e., produce offspring having the somatic characters 
of the parent in every case, only when two gametes having dissimilar 
chromosome numbers, one odd and the other even, unite and produce 
viable seed. While not all offspring resulting from such combinations 
reproduce the parental characters, it is certain that, with the possible 
exceptions noted, the parental type can be duplicated in no other 
way. However, as we have seen, the constancy or inconstancy of a 
plant is not determined solely by the presence of an even number of chro- 
mosomes in the first case, and of an odd, in the second. All depends upon 
the types of male and female . germ-cells produced and the fertilization 
combinations which result in the production of seeds capable of germinat- 
ing. Thus, mutant A, having 15 chromosomes, may produce only 
8-chromosome gametes, type a, of one sex, and only 7-chromosome 
gametes, type b, of the other sex, or, although others are formed by 
