68 
GENETICS: D. F. JONES 
pROc. N. A. vS. 
numbers of the two kinds of fertilizing elements present in a mixture. 
Even if it were possible to obtain equivalent amounts of pollen or seminal 
fluids there would be no way of knowing the ratio of the two kinds of 
effective germ cells, since in many species such cells rapidly lose their 
viability. 
In many experiments performed with maize, pollen was collected from 
a number of plants of two distinct but uniform types of plants. Ap- 
proximately equal quantities of each kind of pollen were taken, thoroughly 
mixed by shaking in a bag, and applied to a number of plants of each of 
the two kinds supplying the pollen. The aim was to have from 1000 to 
2000 seeds in each of the two parts of the proportion. Twenty such mixed 
pollinations were made, using many different types of maize, and alto- 
gether there resulted 63,694 seeds. These have been classified, counted, 
and a sample of each lot grown to test the accuracy of separation, and 
the proportions have been calculated as percentages. See table 1. Seven- 
teen of the twenty show a deviation in favor of the plant's own kind of 
pollen, while only three show the reverse effect. Of the 17 experiments 
which indicate a prepotency of pollen on the stigmas of the plants by 
which it was produced, 2 are not significant when compared with the devia- 
tions expected in sampling. In the remaining 15 the deviations are so 
large that there can be no question that in maize there is a pronounced 
preference of the plant for its own kind of pollen, even though the foreign 
pollen is perfectly capable of accomplishing fertilization when not in 
competition. 
Similar experiments have been carried out with another plant, the 
tomato. Two pollen mixttires were made, utilizing characters by which 
the seedlings could be separated. As with maize, the results give a devia- 
tion favoring the familiar pollen. However, the number of plants ob- 
tained was not large and classification was not so sure as in the experi- 
ments with maize. 
Taking the results from maize alone, magnifying the experimental error 
to its fullest extent, giving due allowance to differential viability, and 
taking into consideration the differences arising from random sampling, 
the conclusion is inevasible that in this species at least there is a definite 
receptiveness of the plant to its own kind of pollen. This is notable in 
view of the great advantages which hybrid vigor gives immediately to the 
cross-fertilized seeds and the plants grown from them. The weight of 
cross-pollinated seed is increased as much as 50% in some cases. Both 
the embryo and endosperm are larger, the seeds have a higher specific 
gravity, and they mature faster, as shown by their lower water content 
at the end of the growing period. The increase in weight, expressed as 
per cent, permits a comparative estimation of the amount of heterosis 
shown by the various combinations involving different materials. There 
is a significant correlation between the amount of heterosis and the prefer- 
