1916] HOAR—STERILITY IN RUBUS 385 
group which is a puzzle to the systematist, and whose species will, 
perhaps, never be definitely established. 
Conclusions 
1. Sterility of pollen has long been recognized as a criterion of 
hybridization. 
2. Crosses between distinct species have long been known to be 
more or less sterile and to behave differently from crosses between 
more closely related forms or varieties. 
3. True species when crossed do not, in most cases, follow the 
laws of MENDEL, but tend rather to blend to form more or less 
constant types, often systematically recognized as species. 
4. Many species of the angiosperms are species in a very dif- 
ferent sense from those of the lower plants and of the gymno- 
sperms in particular. 
5. In some cases they are natural hybrids which have external 
characteristics distinct and constant enough to have specific rank 
from a systematic standpoint. 
6. Although these species may be distinct from the systematic 
standpoint, yet they must be treated in a different manner from the 
standpoint of the evolutionist and the plant breeder. 
7. Finally, the species of the genus Rubus, as shown from the 
pollen condition and also from external characters, clearly hybridize 
very frequently in nature, giving rise to constant forms often 
recognized as true species. 
In closing I wish’to express my sincere thanks to those in charge 
of the Arnold Arboretum and of the Harvard Botanical Garden 
for the privilege of collecting. I desire, also, to express my thanks 
to Miss Day, librarian of the Gray Herbarium, for assistance in 
securing literature, and to others who have kindly aided me in 
Carrying out my purpose. This work has been carried on in the 
Laboratories of Plant Morphology of Harvard University under 
the direction of Professor E. C. Jerry, and to him I am indebted 
for advice and assistance. 
HARVARD UNIVERSITY 
