1916] FORSAITH—ON AGRACEAE 485 
3. The relation of the geographical distribution in the subgenus 
Chamaenerion of Epilobium to hybridism is very pronounced. 
Whenever there is no possibility of intercrossing, the pollen is 
usually perfect. In contrast to this, there is a more general tend- 
ency to abortion in those regions where interbreeding is possible. 
4. Epilobium hirsutum, E. luteum, E. obcordatum, and Zausch- 
neria californica show conclusively that there is no relation between 
the number of microspores which may abort in apparent hybrid 
derivatives and the original number in the tetrad. 
5. The essentially monotypic Gongylocar pus, the geographically 
limited “Epilobium angustifolium, and the strictly monotypic 
Zauschneria present perfect pollen, a condition strikingly in accord 
with the general principle of hybrid sterility. 
6. It is evident, on morphological grounds at least, that natural 
species crossing is a general feature among the Onagraceae, where 
such is possible in respect to geographical distribution and other 
factors. 
In conclusion, the writer wishes to express his sincere thanks 
to Professors B. L. Roprnson and M. L. FERNALD for permission 
to obtain specimens from the Gray Herbarium; and to Miss Day, 
Librarian at the Gray Herbarium, for aid in securing literature. 
These investigations were carried on in the Laboratories of Plant 
Morphology of Harvard University, under the direction of Pro- 
fessor E. C. JeErrrey, and to him I am greatly indebted for advice. 
HARVARD UNIVERSITY 
LITERATURE CITED 
1. ANDERSON, H. J., Form of pollen grains. Hybridization. Gard. Chron. 
529. 1876 
2. i. HH. :, a study of genetic relationship. Amer. 
Jour. Bot. 2:132-155. 
- Bateson, W., Mendes le of heredity. Cambridge. 1909. 
. Beer, R., On the development of the pollen grain and anther of some 
Oungraceke: Beih. Bot. Centralbl. 19: goeg 1906. 
Darwin, C., Origin of species. London. 1859. 
Davis, B. M., Cytological studies on Oenctheré Ill. Ann. Botany 25: 
942-971. IQII. 
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