1916] HOAR—STERILITY IN RUBUS 371 
Sterility may be brought about in several different ways, both 
physiologically and morphologically. My purpose is not to enter 
into the subject of physiological sterility, which is of much more 
interest economically than it is from the evolutionary standpoint; 
and, accordingly, I shall confine my attention entirely to the 
morphological side. 
One of the most obvious types of morphological sterility is 
indicated by the sexual organs of the flower. Often as the result of 
hybridization the anthers and pistils are abnormal, and still more 
often, even when these are normal, we find a large percentage of 
sterile ovules present. The pollen shows this type of sterility 
especially well. I have records as far back as 1832 in which morpho- 
logically sterile pollen has been recognized as a criterion for hybridi- 
zation. In that year DuTRocHET (6) wrote a letter to the Academy 
in which he attributed the sterility of hybrids in part to the imper- 
fections of pollen. He refers to the flower of a hybrid cherry tree, 
resulting from the union of Prunus cerasus and P. avium, whose 
“stamina’’ formed a compact mass in which the pollen did not divide 
into pollinic or fertile dust as in the case of fruitful trees. This 
appears to be an extreme case, and yet it may rightfully be classed 
as a type of morphological sterility. GARTNER (7), in his well 
known essay, also refers to pollen sterility as a criterion for hybridi- 
zation. Again, according to DucHARTE (8), NAUDIN observed that 
the degree of fertility of hybrids was en rapport with the number of 
normal pollen grains formed in the hybrid. 
Since the discovery of MENDEL’s work (9) a great impetus has 
been given to plant breeding, and we find many scientists entering 
into the problems opened up thereby. Most of the work done has 
been in regard to the inheritance of factors and to the working out of 
Mendelian laws of heredity. However, several observers have given 
their attention to the question of the pollen condition. RosEN- 
BERG (ro) discovered that in an apparently sterile hybrid sundew 
(Drosera longifolia obovata) the young pollen grains mostly lost 
their contents, while the embryo sacs usually did not develop 
beyond the binucleate stage. DeTLEFSEN (11), working upon the 
animal side, has shown that a partial abortion of gametes occurs 
in the male progeny of a Cavia species cross. 
