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568 Gates. — The Chromosomes of a Triploid Oenothera Hybrid. 
results. The most common chromosome number was therefore 24, 25, or 
26, when gigas was the pollen parent. The reciprocal, Lamarckiana gigas x 
semigigas , gave 3x21, 1x22, 1x27, 10x28, and 3x29. Hence 28 
chromosomes was the commonest number when gigas was seed parent. 
The exact manner of origin of the large number of 28’s and 29’s in gigas x 
semigigas is perhaps uncertain. These results confirm the writer’s early 
observations that in triploid forms the segregation of chromosomes is usually 
io-ii, but occasionally 9-12. They indicate further that the segregation 
may be occasionally 8-13 and rarely even 7-14. 
From Oe. biennis semigigas x biennis van Overeem obtained a somewhat 
narrower range of variation. There were 41 plants with 14 chromosomes, 
26 (albinervis) with 15, and 4 with 16. In biennis semigigas x gigas, on the 
other hand, the chromosome numbers were 21, 22, 23, 24, 25, 26, 27, 28, and 
one plant with 36, while Lamar ckiana gigas x biennis semigigas gave 33 
plants with 28 chromosomes, 2 with 29, and 6 undetermined. Van Overeem 
suggests with probability that the plant with 36 chromosomes arose from an 
egg cell of biennis semigigas having 8 chromosomes uniting with a pollen 
grain of gigas having 28. That such a pollen nucleus may arise In gigas 
has already been shown (Gates, 1915 , Fig. 73 f.). 
In a plant from Oe. gigas x lata rubricalyx which had 22 chromosomes 
(Gates, 1915 , p. 189) I showed that the chromosome segregation in the 
pollen heterotype was usually 10-12, but other daughter nuclei were observed 
having 11, 13, and 9 chromosomes, as well as 6-| and 9^, which points to the 
production of a range of new numbers in the next generation. 
The present study of the reduction divisions in the pollen of Oe. rnbri- 
calyx x gigas helps to make clear the chromosome behaviour of triploid 
forms, and explains the origin of the wide range of chromosome numbers 
found in their offspring. 
Summary. 
This paper deals with the distribution of the chromosomes in the micro¬ 
spore meiosis of a triploid hybrid, Oe. rubricalyx xgigas. It confirms earlier 
results with similar triploid hybrids and further explains how the variety of 
chromosome numbers found in the second generation of such hybrids arises. 
In the heterotypic mitosis the chromosome distribution is usually 10-11, 
but from 1 to 6 chromosomes may be left out of the daughter nuclei. This 
is shown by the chromosome numbers on the homotypic spindles, which 
may be 11, 10, 9, 8, or 7, and usually differ in the two homotypic spindles 
of a cell. Cases occur where these two spindles have 10 + 11, 9 4-10, or 8 + 
7. All the chromosomes regularly split and separate on the homotypic 
spindles, but certain split chromosomes frequently lag behind and are left 
out of the daughter nuclei, as on the heterotypic spindle. 
