EAST: INTERCROSSES BETWEEN SELF-STERILE PLANTS 147 
as if each carried the composition of the mother plant from which it 
came. In other words, as far as its action in fertilization is concerned, 
a pollen grain partakes of the character of its mother plant and is 
like its sisters; as far as the hereditary characters carried on to the 
next generation are concerned, sister pollen grains may differ both 
from their mother and from each other. 
A part of our evidence on these points we shall present. For 
further details the reader isreferred to a forthcoming paper in Genetics.’ 
The first experiment to which attention is called is an inbreeding 
experiment performed on a cross between Nicotiana forgetiana and 
Nicotiana alata. If sister plants are mated in successive generations 
after an original mating Aa & Aa, by Mendelian recombination there 
results a gradual approach to 1/2 AA, 1/2aa ando Aa. Expectation 
of homozygosis in successive matings is 1/2, 5/8, 11/16, 24/32 --- I 
(Jennings, 1916). If, therefore, plants of like constitution as far as 
effective factors are concerned are cross-sterile with each other, 
cross-sterility should become more and more apparent in generations 
succeeding F.. To test this possibility, a comparatively small number 
of cross-matings was made on the Fy», F3, Fy and F; generations. In 
the F, generation, out of 131 intercrosses on 20 plants only 4 were 
unsuccessful. The percentage of unsuccessful matings increased from 
this time on, until in the F; generation about 21 percent of the cross- 
matings tried on 20 plants were impossible to make. 
In this experiment as well as in all others, results showed that 
reciprocal crosses were alike in their compatibility. If two plants 
were fertile together, they were fertile reciprocally; if two plants 
were incompatible, they were incompatible reciprocally. This is 
proof of the sporophytic behavior of the factors affecting the behavior 
of self-sterile plants. 
The two crosses to be described next are reciprocals made with the 
same two individuals. Made with Nicotiana alata and Nicotiana 
forgetiana as parents, they are in a sense repetitions of the cross just 
described, but it is hardly probable that they duplicate it. Both of 
these species must consist of plants which differ among themselves in 
the factors which affect self-sterility, hence any crosses in which 
different individuals are used may show different results. 
All of the individuals resulting from this cross were grown in a 
greenhouse as potted plants. The F; generation came into blossom 
during the latter part of the winter. Conditions were extraordinarily 
favorable for growth and the pollinations were all made while the 
plants were vigorous, hence scarcely any trouble arose over classi- 
fication of the results through end-season pseudo-fertility. 
’This paper has since appeared. See ‘‘Studies on Self-sterility I. The 
Behavior of Self-sterile Plants.’’ Genetics 2: 505-609. 1917. 
