Nov., 1923] 
EAST — SELF- AND CROSS-STERILITY 
473 
probable that each population had been so selected as to reduce the number 
of intra-sterile groups materially. And so it proved. There were only 
three or four such classes in each species; and the interesting thing is that 
no single class of one species was found in the other species or in the classes 
observed in the descendants of the original hybrids between them which 
had been made between 1910 and 1912. We can not say therefore whether 
N. alata considered as a species contains any factors governing the behavior 
of self-sterile plants which are the same as those possessed by N. Forgetiana , 
or not; but from the results obtained we are justified in believing that each 
species does contain a relatively large number of such factors. They 
contain such a large number of these factors that the practical result of 
making random crosses in an unselected population is to obtain such a high 
percentage of success as to make one believe, with Darwin, that each plant 
is fertile with the pollen of every other plant. 
In conclusion I will say only this: though the study of self-sterility in 
detail has opened up various fruitful lines of research of which no one can 
see the end, and though an adequate physiological interpretation of the 
behavior of incompatible pollen tubes as compared with compatible pollen 
tubes has not been forthcoming, the genetical problem in its narrow sense, 
that is to say the problem of the mechanism of its heredity, may be said to 
be solved. The corroborations of our results coming in from Europe on 
other species are too exact in detail for one to feel that there is much weight 
in the criticism that these results refer only to four species of Nicotiana. 
One can only hope that they will be helpful not only to genetic theory, but 
to the practical problems of the orchardist who has to deal with self-sterile 
fruits. They do indeed show why whole varieties of asexually propagated 
fruits are self-sterile. And, further, the work on the causes and control of 
pseudo-fertility points the way to a practical method of orchard procedure. 
Bussey Institution, 
Harvard University 
