Self Sterility and Cross Sterility in the Apple. 81 



planted. In any case not more than 4 or 5 rows should separate 

 the pollenizer trees from those to be pollinated. 



For orchards already planted, regrafting a desirable pol- 

 lenizer in the above mentioned proposition may be practiced. 

 While waiting for these pollenizers to grow to bearing age a 

 practical relief may be had by cutting large branches of other 

 good pollinating varieties and placing them in water pails hung 

 from the tree limbs. 



Experiment has shown that little pollen fertilization is 

 brought about by wind. Insects, wild and cultivated are the 

 best agents to transport pollen from one variety to another. It 

 is therefore commercially profitable to keep bees in the orchard 

 for this purpose even though no honey is produced. 



Causes of Self Sterility and Cross Sterility. 



Sterility within the different species of plants appears to 

 be due to several causal agents. These agents may be external 

 or they may be internal. The external agents include such things 

 as disease affecting the vitality of the tree or its blossoms such 

 as scab, fire-blight, insect infections, spray injury before, during 

 or after flowering. Low temperature and cold continued rains 

 at flowering time, may be other factors determining the amount 

 of fruit set and consequently its yield. These factors are more 

 or less under the control of the apple grower and should receive 

 careful attention. They need not be discussed here, for the 

 remedial agents are well known. 



The internal causes for sterility include degenerate pollen; 

 pollen which is not able to cooperate properly with the style to 

 facilitate the growth of the pollen tube at a sufficient rate of 

 growth to reach the ovule and cause fertilization; and lack of 

 proper development of ovule. 



Within the apple the phenomena of self sterility is appar- 

 ently quite universal. The crosses of the varieties which are 

 self sterile with pollen which is crossed fertile with them show 

 that the ovules are capable of fertilization and are therefore not 

 responsible for the sterility resulting from the self fertilization. 

 Similarly the argument could be made that since the pollen from 

 a self sterile variety is capable of fertilizing other varieties the 



