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The Trisomic Mutations of Oenothera. 
BY 
R. RUGGLES GATES, Ph.D., F.L.S., 
Professor of Botany, University of London (. King's College'). 
With Plate XI. 
T HE purpose of the present paper is to record the occurrence and cyto- 
logical behaviour of a new mutation having 15 chromosomes, and to 
discuss various questions relating to trisomic mutations in Oenothera. The 
breeding experiments involved will be described elsewhere, but it is neces¬ 
sary to refer to them briefly in this connexion. 
Reciprocal crosses were made between a pure homozygous race of Oeno¬ 
thera rnbricalyx and Oe. Hewettii , Ckll., in 1915 at the Missouri Botanical 
Garden and the following year at the University of California. It should 
be pointed out that Oe. Hezvettii has not bred entirely true in my cultures. 
There is a green-stemmed as well as a red-stemmed form, and a second 
generation culture of 73 red-stemmed plants includes one small dwarf. Oe. 
Hezvettii differs throughout from Oe. rnbricalyx , the most conspicuous 
differences being the narrower leaves, which are practically smooth (crinkled 
in rnbricalyx ) with midribs red on the upper surface and green below. The 
petals of Oe. Hezvettii are also much narrower, so that gaps are left between 
them at the base where they fail to overlap. The buds are green and 
covered with soft pubescence, while the stems are free from the characteristic 
red papillae of the Lamarckianai orms. Four generations of hybrids have 
been grown from Oe. Hewettii x rnbricalyx , and three generations of the 
reciprocal. With the exception of one culture these have been grown at 
the Royal Botanic Gardens, Regent’s Park. The F x hybrids are intermediate 
between the parents, but in both cases more like the seed parent. They 
both have the red bud colour pattern of rnbricalyx , but it is considerably 
diluted. In later generations these differences between the reciprocal 
hybrids are maintained. As regards bud colour in F 2 and later genera¬ 
tions, there is a continuous range from full red to completely green buds, 
[Annals of Botany, Vol. XXXVII. No. CXLVIII. October, 1923 ] 
Oo 
