250 The Commercial Apple Industry 



influence self-sterility or self-fertility. Poorly nourished 

 trees are more likely to be self-sterile than well nourished 

 ones. 



Imperfect pollination is more often due to other causes 

 than the absence of suitable varieties for cross-pollination. 

 Furthermore, it is not always necessary to have a heavy set 

 of bloom to secure a heavy yield of fruit. It is well known 

 that under normal conditions if 4 to 7 per cent of the blos- 

 soms set fruit in a good bloom year, a large crop of fruit is 

 insured. If 10 per cent of the blossoms set fruit when the 

 bloom is heavy, it is an indication of a very large crop. 

 These facts do not minimize the importance of pollination, 

 however, and it is highly important that every care should 

 be taken to insure proper fertilization of the flowers. 

 While the benefits of cross-fertilization are recognized as 

 important in all plant-breeding work, the experimental 

 data on the secondary effect of pollination are so contra- 

 dictory as not to offer a field for definite discussion. The 

 results of experiments to determine the effect of cross- 

 pollination on color, form and flavor of the fruit have been 

 largely negative. For that reason, this discussion will be 

 largely confined to the primary effect of pollination. 



CAUSES FOE THE FATLUBE TO SET FBTJIT 



Failure of blossoms to set fruit properly is not by any 

 means due in all cases to imperfect pollination. Many 

 factors other than this affect the set of fruit and are more 

 often the cause of light crops. Among the factors causing 

 a light set of fruit are : 



1. Fruit-spurs and trees may be weak on account of a 

 lack of proper nourishment. When trees are starved for 

 proper plant-food or when it is not made available by good 



