544 Gates .— r Fhe Trisomic Mutations of Oenothera. 
and it was found impossible (contrary to the usual experience with this 
character in other hybrids) to classify the individuals with any certainty in 
the F 2 , and the usual ratios are not obtained. Nevertheless, there is segre¬ 
gation of this character in later generations, although the green-budded 
segregates usually retain a tinge of red on the hypanthium. Whether segre¬ 
gation actually occurs irl the other characters need not be discussed in the 
present connexion, although the first impression is that of a blend which 
essentially retains its blended condition. 
In the present strain of Oe. Heivettii , a native of New Mexico which was 
kindly sent me by its discoverer Dr. T. D. A. Cockerell ( 1913 ) in 1914, as 
well as in these hybrids, occasional aberrant forms appear, as already 
mentioned. Reference will here be made only to a type which appeared 
in 1920 in the F x of the cross Oe. rubricalyx x Hewettii. This culture 
(No. 54) numbered 53 plants, including two which were strikingly aberrant 
and precisely alike. The rosette leaves were very much broader and 
more crinkled than the typical F x hybrids. In this feature these mutants 
resembled Oe. mut. lata. The stem leaves were also more crinkled, but were 
pointed at the tip and a resemblance to the mutant lata was not at once sug¬ 
gested, although the plants were shorter than the type and the stem was 
slightly zigzag. These plants probably most resembled Oe. mut. semilata , 
Gates. For a discussion of the various occurrences of this mutation see Gates 
( 1915 , p. hi). Examination of the pollen in these plants showed that the 
grains were all three-angled. A flower of one of them was selfed and pro¬ 
duced 33 seeds (as against the usual 100-300 in typical members from 
these crosses), but the seeds failed to germinate. 
Descriptions and photographs of this type will be published elsewhere. 
Cytological material was collected from one of them and shows that this 
mutant had 15 chromosomes. I am indebted to Miss E. M. Rees, B.Sc., 
for making the microscopic preparations for this study, and to Mrs. N. 
Ferguson, B.Sc., for preparing the drawings. The remainder of this paper 
will be devoted to pointing out the main cytological peculiarities of this new 
15-chromosome type. A large number of chromosome counts both in 
pollen mother-cells and in somatic tissues established beyond doubt that 
the chromosome number is 15, but in two somatic cells 16 chromosomes 
were clearly present. The explanation of such departures from the somatic 
number has been discussed elsewhere (Gates and Thomas, 1914 ). 
In the first place, it may be pointed out that the occurrence of mutations 
with an extra chromosome in a pair of individuals in a culture is not un¬ 
common, and it furnishes clear evidence as to how they have arisen. For it 
means that the irregular 8-6 distribution of chromosomes took place on the 
heterotypic spindle in a pollen mother-cell and resulted in the production 
of two pollen grains with eight chromosomes, both of which functioned in 
fertilization to produce mutants. If the irregular division had occurred in 
