Ig10] ANDREWS—TWIN HYBRIDS 201 
Summary . 
1. The stigma of Oenothera Lamarckiana is much higher than the 
anthers. The anthers open both on rainy and sunny days, whereas 
some of the other forms open their anthers only on sunny days. 
2. The European species, O. biennis, differs from the American 
species. Its anthers and stigma are of the same height, instead of 
being of different height as in O. Lamarckiana. Its petals, pubes- 
cence, and most of its cells are the smallest of any of the forms 
described, while those of O. Lamarckiana tend to be the largest. 
3. The twin hybrids /aeta and velutina show themselves by their 
foliage, flowers, the greater density and character of the pubescence 
in velutina, as well as the arrangement and form of the cells, to be 
distinct, and, in so far as has yet been investigated, constant forms. 
4. On rainy days the anthers of the twin hybrids do not open, but 
only on sunny days, and then only part of them, whereas in all the 
other forms they open on both rainy and sunny days. This non- 
opening of the anther cells may probably be the beginning of 
sterility. 
In conclusion I take pleasure in expressing my thanks to Professor 
DeVries for having placed at my disposal one of his laboratories 
and material from his garden for this investigation, and also for 
his many kind suggestions and assistance. 
UNIVERSITY OF AMSTERDAM 
