82 Maine Agricultural Experiment Station. 1920. 



pollen as such is not responsible in self sterile varieties of apples 

 for the fruits not setting. 



Investigation shows that the problem is one of the interre- 

 lation between the pistil and the pollen and .the pollen tube. It 

 has been shown that in the self sterile varieties self fertilized 

 the pollen tube grows much more slowly than does the pollen 

 tube of other varieties of pollen when used on the same pistils.* 

 Thus in the self fertilized flower the rate of growth of the pollen 

 tube is so slow that it cannot traverse the length of the style 

 and fertilize the ovule before the ovule withers and dies. With 

 the cross pollinated flowers the pollen tube grows much more 

 rapidly and easily reaches the ovule in time for fertilization to 

 take place. The physical basis of one form of this sterility is 

 consequently due to some factors which inhibit the growth of 

 the pollen tube in the style of the same variety. What this dif- 

 ference is, is a matter now under further investigation. 



Literature List. 



1. Waugh, F. A. 



1900. Report of the Horticulturist. In Thirteenth Annual Report 

 of Vermont Agricultural Experiment Station. 1899-1900. p. 364. 



2. Powell, G. Harold. 



1901. Report of the Horticulturist. In Thirteenth Annual Report 

 of the Delaware Agricultural Experiment Station, 1901, p. 114. 



1900. Report of the Horticulturist. In Twelfth Annual Report of 

 the Delaware Agricultural Experiment Station, 1900. p. 134-139. 



4. Lewis, C. I. and Vincent, C. C. 



1909. Pollination of the Apple. In Bui. 104 Oregon Agricultural 

 Experiment Station, pp. 19-20. 



5. See reference 3. 



6. Hedrick, W. P. and Wellington, Richard. 



1912. An Experiment in Breeding Apples. In Thirty-First Annual 

 Report of the New York Experiment Station, p. 457. 



7. Ballard, W. R. 



1916. Methods and Problems in Pear and Apple Breeding. In 

 Bui. 196. Maryland Agricultural Experiment Station, p. 88. 



8. See reference 7. 



9. See reference 4. 



♦Knight, L. I., 1917. Physiological Aspects of Self-Sterility of the 

 Apple. In Proc. Amer. Soc. Hort ScL p. 101-105. 



