300 POMOLOGY 



modifications of perfect flowers which prevent self-pollina- 

 tion; and (7) the possible case of the inability of the pollen- 

 tube to grow sufficiently long to reach the ovaiy. 



Among the physiological causes the following may be 

 included: (1) possible lack of nourishment of the pollen- 

 tube in the case of some pistils; (2) negative chemotactic ac- 

 tion, although this is not known to occur with fruits; (3) pos- 

 sible toxic effect of the stigmatic fluid on the pollen or 

 vice-versa (investigations tend to disprove this with the 

 fruits) ; lack of fertilization; lack of development of the embryo 

 after fertilization may have taken place. It might also be 

 added that hybridity frequently results in entire sterility 

 (heterosis). In studying the causes of self-sterility, it has 

 been observed that there is a slow growth of the pollen- 

 tube which results in a lack of fertilization, as with the Rome 

 Beauty apple ^ and also with the tobacco.- In the latter 

 case, it was noted that the pollen germinates as freely on 

 the stigmas of flowers of the same plant as those of other 

 kinds with which they are compatible. After germination, 

 however, the pollen-tubes on selfed flowers grow so slowly 

 that decay of the flower occurs before fertilization can be 

 effected. 



268. The cherry. — One of the most interesting cases of 

 sterility that has developed in American pomology is that 

 of the cheriy. It had been known for some time that vari- 

 eties of the sweet cherry in particular were inclined to be 

 self-sterile and were not parthenocarpic, but it later devel- 

 oped not only that the sweet cherry was practically always 

 self-sterile as grown in Oregon, but also that several of the 

 standard varieties were inter-sterile. 



This appears to be a clear case of "lack of affinity" be- 

 tween certain varieties and is not due to any lack of germin- 



iRnight, L. I. Proc. Amer. Soc. Hort. Sci. 1917. pp. 101-105. 

 2 East, E. M., and J. B. Park. Genetics, 3: 353-366. 1918. 



