508 FUNDAMENTALS OF FRUIT PRODUCTION 



particularly of grapes, produce large numbers of impotent pollen grains 

 and they have all the appearance of perfect-flowering sorts, though 

 in reality they are pseudo-hermaphrodites. If the embryo sacs degen- 

 erate and fruit still forms, seedless specimens are produced. The self 

 sterility of many varieties is associated with the hybrid condition of 

 the plant. Hybrids between rather distantly related forms are likely 

 to be self sterile and often self unfruitful as well. On the other hand, 

 there is some evidence that very narrowly bred varieties or strains are 

 rather inclined to sterility. When sterility is due to hybridity it is 

 likely to be associated with pollen or embryo sac degeneration. Incom- 

 patibility is another cause of much self unfruitfulness. This is par- 

 ticularly important in the apple, pear, plum and cherry. Not only are 

 some varieties self unfruitful but incompatibility exists between them and 

 certain other varieties. This characteristic has immediate importance 

 in the sweet cherry and almond. In some cases failure to set fruit 

 properly is due to premature or delayed pollination or to a slow growth of 

 the pollen tube. Unfavorable nutritive conditions within the plant are 

 responsible for much failure in fruit setting. Trees that have been 

 weakened by overbearing or other causes are very likely to produce 

 pistils which are defective or pollen that is low in vitality. There is often 

 considerable difference between flowers borne in various positions, or 

 between those borne on strong and weak limbs, in their abilities to set 

 fruit. 



