248 
GENETICS: D. F. JONES 
largest pistillate inflorescences and are most productive of grain produce a 
very small quantity of pollen. Doubtless many plants are produced which 
completely lack one or the other or both functions but such types, of course, 
can not be maintained in self-fertilization. There is, therefore, a decided 
tendency for inbreeding to change a monoecious plant into a functionally 
dioecious plant. 
All of the inbred strains obtained show an immediate return to the vigorous 
condition of the non-inbred parents when crossed. The return of vigor is 
greater in some combinations than in others. In general there seems to be a 
correlation between the productiveness of the female parent strain and the 
productiveness of the first generation hybrid. This is in part due to the fact 
that hybrid plants start from better developed seeds when the female parent 
is the more vigorous. It is also shown in a comparison of the F 2 generation, 
starting from large seeds grown on vigorous hybrid plants, with the Fi gen- 
eration starting from seeds produced on the reduced inbred plants. The first 
generation overcomes this handicap and in every case may be expected to 
surpass the second or subsequent generations as shown by the curves in 
figure 2. 
The manifestation of heterosis is greater in some characters than in others. 
In the many crosses between inbred strains observed yield of grain is in- 
creased 180%, height of plant 27%, length of ear 29%, number of nodes 6% 
and the number of rows of grain on the ear 5%. Abundant evidence has been 
obtained from these inbred strains to show that the endosperm also is imme- 
diately affected by crossing. Differences in weight of reciprocally crossed 
seeds as compared to selfed seeds developed in the same inflorescences have 
averaged from 15 to 20% in favor of the cross. Furthermore in spite of this 
increase in weight the hybrid seeds mature faster as shown by their lower 
water content. Due in part to this latter fact there is a noticeable difference 
in viability as shown by the per cent of germination between the selfed and 
crossed seeds produced by the same plants. The crossed seedlings also appear 
from one to two days earlier than the selfed seedlings. 
In view of the notable advantages to be gained from cross-fertilization it 
was thought that when mixtures of a plant's own pollen with that of a dis- 
tinct sort were applied there might be a selective fertilization favoring the 
foreign pollen. By taking advantage of xenia it was possible to obtain and 
to distinguish selfed and reciprocally crossed seeds from plants of two dif- 
ferent inbred strains. Since the same mixture of pollen was applied to plants 
of both strains, the number of seeds produced by one kind of pollen should 
be in the same ratio to the number of seeds produced by the other kind of 
pollen, on both types of plants to which the mixture was applied, irrespec- 
tive of the amounts or viability of the two kinds of pollen used. Any devia- 
tion from a perfect proportion in the form of an excess of crossed seeds or an 
excess of selfed seeds would indicate selective fertilization in one or the other 
