10 
BULLETIN 1489, U. S. DEPARTMENT OF AGRICULTURE 
true, whereas the heterozygous plants will continue to segregate as 
did the F x . If all of the plants in each of several successive genera- 
tions are selfed, there evidently will be a much larger proportion of 
homozygous than of heterozygous plants in the later generations. 
This follows from the fact that the homozygous plants produce 
nothing but homozygous plants, whereas the heterozygous ones con- 
tinue to produce both heterozygous and homozygous plants. The 
number of volants that are homozygous or heterozygous with respect 
to the japonica factors in six successive generations of self-fertiliza- 
tion, assuming equal reproduction of all classes, is shown in FigTire 5. 
The percentage of plants homozygous for either japonica or green 
is shown to rise from 50 per cent in the F 2 to more than 98 per cent 
in the F 7 generation. This will be considered further in connection 
with the discussion of hybrid vigor. 
GENERATIONS 
KIND OF Fi 
PLANTS 
J J 
J J 
J J 
HOMO- 
ZYGOUS 
240 
93. 75 
992 
96,875 
Pig. 5. — Diagram showing the proportions of homozygous and heterozygous indi- 
viduals in six successive generations of self-fertilization, when the original plant 
was heterozygous for a single-factor pair and all classes are assumed to be propa- 
gated equally. The homozygous individuals produce only homozygous progeny, 
and the heterozygous individuals continue to segregate into heterozygous and 
homozygous progeny 
XENIA 
It is a matter of common observation that when white corn is 
grown near yellow corn the resulting ears may contain a sprinkling of 
yellow kernels. This is the result of partial pollination by the ad- 
jacent yellow corn. The reason for this immediate effect of pollen, or 
xenia as it has been called, was not understood until it was determined 
that the endosperm of the kernel (fig. 2) resulted from the fertiliza- 
tion of the polar nuclei by a sperm nucleus as already described. 
With this discovery it was evident that the male parent played its 
part in the formation of the endosperm as well as in that of the 
embryo, and with a further knowledge of Mendelian inheritance the 
behavior of kernel characters in transmission became increasingly 
clear (18, 77). 
After the reduction division, one of the cells with the reduced 
number of chromosomes divides equally to give rise to the polar 
nuclei and the egg, and these consequently are identical in so far as 
their chromosome content goes. The two sperm nuclei likewise are 
identical. It follows that the embryo and the endosperm always are 
