Nov., 1923] 
DORSEY — STERILITY AND HORTICULTURE 
479 
the unfruitfulness of the pistillate varieties has been expensive in many 
instances, and this mistake is still frequently made in new plantings. 
Self- and cross-sterility as found in the apple, cherry, plum, and other 
fruits is the type resulting from slow pollen-tube growth. It has not been 
demonstrated as yet whether the gametes would unite if brought together. 
Incompatibility does not appear to be influenced greatly by nutrition. In 
such extreme cases as the native plums, self-sterility has not been changed 
to self-fertility under the conditions of any of the tests, and in controlled 
crosses self- and cross-sterility are encountered on the same tree with cross- 
fertile combinations. Pollen germination takes place in both the self- 
sterile and cross-sterile relationship, although tube growth is slow and 
variable in length from different grains. The effect of incompatibility on 
the crop appears at the second drop and may be as detrimental as that of 
dioeciousness. 
Pollen abortion is extensive in the hybrid varieties. Aborted grains 
become turgid in contact with a receptive stigma but do not send out tubes. 
Pistil abortion, however, does not appear to be so extensive. In some of 
the hybrid plums, all the pistils develop normally while more than half of 
the pollen may be aborted. If the chromosome combinations condition 
development in the pollen, why not also to the same extent in the egg? 
The probable explanation for this difference lies in the tetrad of megaspore 
nuclei where a replacement could take place if any one of the four cells 
could form the embryo sac in case the others were aborted. This adjust¬ 
ment would theoretically allow for only one fourth as much abortion in the 
embryo sac as in the pollen grain. It is interesting to note in this connection 
that deficient nutrition causes more extensive pistil abortion in the plum 
than pollen abortion. Pollen abortion has but little influence upon fruit¬ 
setting, except in those few cases where it is complete, because of the 
quantity of normal pollen produced by most fruits. 
Embryo development may also be conditioned by genetic factors. The 
June or third drop is composed for the most part of young fruits in which 
the embryo ceases growth, and consequently is a measure, at least in some 
instances, of genetic influences although it is also greatly affected by nutri¬ 
tion. To illustrate, in the cross Compass X Yellow Egg 652 flowers out of 
1,327 set, but only 8 matured, as contrasted with the cross Compass X Bur¬ 
bank in which 116 fruits set out of 175, and 114 matured. The influence of 
embryo abortion is much more sharply defined in the fruits with one seed 
than in those with more than one, and this phase of the varietal relationship 
must be considered along with many others in finally deciding upon 
pollenizers. 
It will be seen, then, that fruit-setting is influenced by genetic considera¬ 
tions which are independent of the influence of weather or of nutrition. 
These may affect the first drop through abortion in the embryo sac, the 
second drop through slow pollen-tube growth as a result of incompatibility, 
