478 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
winter or early spring. Freezes and frosts are destructive to flowers and 
young fruits, but periods of low temperature during bloom may be just as 
damaging to the crop. Storms of short duration do not necessarily interfere 
seriously with the set. Strong winds, rains, cloudy weather, and low- 
temperature periods influence pollen dissemination directly, and indirectly 
through hindrances to insect activity. 
In addition to the factors mentioned above, weather may have a detri¬ 
mental influence upon the processes taking place at bloom, especially by 
delaying them. The length of time that pollen is available after dehiscence 
varies from a few hours to a day or so according to the weather and to the 
succession in the opening of the anthers. In most seasons there appears to 
be sufficient dissemination of pollen if it is available. 
Stigmas remain receptive under orchard conditions from two to six 
days. This time may also be considered as the length of life of the stigma. 
Delays in pollination subtract just so much from the time left before the 
abscission of the style or disintegration in the egg, which in the apple begins 
as early as 120 hours after bloom (Knight, 1917), while in the plum the 
egg appears normal two weeks after the bloom and can still be recognized 33 
days afterward (Dorsey, 1919 b). The rate of pollen-tube growth becomes 
of vital importance in relation to fruit-setting when pollination is delayed, 
when the length of life of the egg is short, or when the growth rate of the 
pollen tube is retarded by low temperatures during bloom. 
On account of the time limit set by the period of activity of the processes 
taking place at bloom, adverse weather affects fertilization chiefly by causing 
delay. This appears to be more of a hindrance than direct injury from 
rain or wind. Aside from killing by frosts and freezes, the effect of weather 
is indicated at the second or the non-fertilized drop by the loss of flowers 
in the apple, plum, peach, or cherry, by imperfect clusters in the grape or 
currant, and by imperfect fruits in the strawberry and raspberry. 
Genetic Considerations. The genetic factors inherent in horticultural 
material which affect the self- and cross-relationship are encountered in 
dioeciousness, in self- and cross-sterility, and in aborted sex structures. 
It was shown in the discussion on the status of sterility in the different 
fruits that these considerations present a serious problem to the grower in 
a large number of the most important varieties. 
The dioecious condition is encountered in the grape and the strawberry. 
Since incompatibility is practically absent in both, the flower type may be 
taken as a guide to both the self and intervarietal relationship. Inter¬ 
mediate stages in stamen development which produce defective pollen and 
apparently normal pistils which, however, do not function (Valleau, 1918) 
have been confusing to growers. Likewise, in the grape the pollen borne 
by the reflexed stamens of the pistillate varieties, while forming no germ 
pore but having a defective generative nucleus (Dorsey, 1914), yet has much 
the same external appearance as other grape pollen. The failure to recognize 
