June, 1933] Pollination and Fruit Setting in the Apple 



13 



As the nuclei move farther and farther down the style, the older part of 

 the tube becomes plugged with callose, which separates the growing part 

 forever from the original pollen grain. 



The pollen tube finally reaches the ovary, enters an ovule and discharges 

 the male nuclei or sperms. One of these fuses with the egg nucleus and 

 an embryo is formed. The ovule then develops into a structure known as 

 the seed. Without fertilization the seed would not develop. If reports of 

 their occurrence can be considered authentic, cases of seed development 

 without fertilization of the ovules rarely occur in the apple. 



It is the development of viable seed that is so important to the set and 

 further development of the fruit. A tree soon sheds flowers in which a 

 sufficient number of ovules are not fertilized. Other flowers in which ovules 

 become aborted are also generally dropped. Flowers may set fruit heavily, 

 apparently with plenty of seeds, yet the June drop may be heavy because of 

 the competition for food and water. To insure the sticking of fruit that has 

 been fertilized, it is important, therefore, that the tree be properly supplied 

 with moisture and nitrogen. 



__b 



Fig. 11. a. Pollen grain after alighting on stigmatic surface b. 



IV. Pollination of the Apple Under 

 Neyv Hampshire Conditions 



The Need fcr Po'Jenizers: 



Self-unfruitfulness. A large majority of apple varieties and nearly all 

 of the important commercial sorts are unable to develop seed or set a crop 

 of fruit when pollenized with their own pollen. These are considered self- 

 unfruitful, because of the general inability of the pollen to efifect self- 

 fertilization. 



In orchard observations the self-unfruitfulness of certain varieties has 

 often been strongly suspected, but carefully controlled hand-pollination 

 experiments have been necessary to prove the fact. The supposition has 

 also been advanced that certain varieties were especially good poUenizers 

 for these self-unfruitful sorts. This has often been indicated in orchards, 

 one section of which has a given variety in the vicinity of certain other 



