FIFTEEN- AND SIXTEEN-CHROMOSOME OENOTHERA MUTANTS 93 
of functioning, are produced by 15 -chromosome forms. So far as has 
been observed, the male gametes appear to be, with rare exceptions, 
exclusively 7-. It is possible that the embryo-sac is more frequently 
differentiated from a 7- than from an 8-chromosome group, and that 
more 7- than 8-chromosome eggs, capable of functioning, are produced.^^ 
2. Since many 14 +-chromosome forms produce both 7- and 7+- 
chromosome female gametes {9 lata 7 and 8), but are usually male- 
sterile, hence produce, ordinarily, neither 7- nor 7 +-chromosome male 
gametes, our knowledge of controlling factors is still too limited to 
warrant the suggestion that 7 -f -chromosome groups are more likely 
than 7- to fail to produce male gametes capable of functioning.^^ It is 
probable, however, that 7-chromosome gametes unite with 7- and 
produce seeds capable of germinating more readily than any other 
combination. There is also considerable evidence to indicate that, 
as a rule, gametes unite and produce viable seeds more readily (a) 
when one gamete contains 7, and the other a multiple of 7, chromo- 
somes^^ (7 +14; 2i-chromosome Lamar ckiana-gigas offspring of 
0. lata X O. gigas, 0. Lamarckiana X 0. gigas, etc.) ; {h) when both 
gametes contain a multiple of 7 chromosomes (14 + 14; 28-chromo- 
some offspring of 0. gigas, selfed, than (c) when each contains more than 
7, but fewer than 14, chromosomes.^^ Plants having more than 14, 
Gates and Miss Thomas ('14) have stated that 8-chromosome megaspores 
of 0. lata evidently have fewer prospects of functioning than 7-chromosome mega- 
spores, since the percentage of latas derived from lataX Lamarckiana usually falls 
below 20 percent, and sometimes to 4 percent. 
28 We should not overlook the fact that in absolutely male-sterile 15-chromosome 
forms, 7-chromosome male gametes are eliminated as completely as the 7-f-. 
2^ There is also some evidence to indicate that 8- unites with 14- and produces 
seed capable of germinating more readily than two gametes, both of which contain 
more than 7, but fewer than 14, chromosomes (22-chromosome offspring of 0. lata 
XO. gigas). We have no evidence whatever on which to base conclusions regarding 
7 + 13. Whether 8 + 14 combinations are less likely to occur and produce viable 
seed than 7 + 14, cannot be stated. The culture of 133 offspring which de Vries 
('13, p. 186) obtained from O.lata X 0, gigas in 1907 consisted of 65 Lamarckiana- 
gigas (presumably 7 + 14) and 68 lata-gigas (presumably 8 + 14), offspring. 
^° The writer's experience with cultures of 14- and 15-chromosome forms pol- 
linated by 28- has not been sufficiently extensive to justify the assertion that seed 
and offspring are less readily secured from these than from 28- pollinated by 28-, 
but numerous attempts were made to pollinate 2 8-chromosome 0. gigas de Vries 
with 14- and 15-chromosome forms, invariably with the same result; only flat seeds, 
or seed-like structures were secured, and these, of course, were utterly incapable 
of germinating. De Vries and Davis, however, have each grown offspring of 0. gigas 
X 0. Lamarckiana, though Davis's culture ('10) was quite small, — 12 plants. 
