UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 503 



to that of the stigma (legitimate polHnation) placed on stigmas 24 hours 

 after pollination from stamens of anothcn- height (illegitimate pollination) 

 was found to effect fertilization, the earlier applied pollen still remaining 

 ungerminated. Plants raised from the few seeds obtained from illegiti- 

 mate pollinations showed many of the characteristics of hybrids between 

 species, being few flowered, weak or perhaps profuse flowered and partly 

 sterile. Practically the same has been found in the heterostyled flowers 

 of buckwheat. ^^^ In the legitimate pollinations less than 18 hours 

 was required for the growth of the pollen tube and the fusion of its 

 generative cell with the egg cell of the embryo sac. In the illegitimate 

 pollinations more than 72 hours were necessary for the same series of 

 events. Discussing the cause of self-sterility in Nicotiana East and Parks 

 say:^^ "... The immediate difference between a fertile and a sterile 

 combination is in the rate of pollen tube growth. If at the height of the 

 season a series of self pollinations and a series of cross pollinations are made 

 on a single plant and the pistils fixed, sectioned and stained at intervals of 

 12 hours, it is found by plotting the average length of the pollen tubes 

 in each pistil against time in 12-hour periods that the growth curve of 

 selfed pollen tubes is a straight line which reaches less than half the 

 distance to the ovary during the life of the flower, while the curve of 

 crossed pollen tubes resembles that of an autocatalysis and reaches the 

 ovary in less than 96 hours." Similar differences have been found in the 

 rate of pollen tube growth in selfed and crossed apples.*^ 



Obviously, slow pollen tube growth alone cannot be responsible for 

 a failure of the fruit to set, for eventually the tubes would reach the 

 ovules. However, flowers do not remain attached to the flower cluster 

 or to the stem indefinitely when fertilization does not occur. Unless it 

 occurs within a fairly short time, varying with species, variety and 

 environmental conditions, abscission takes place at the base of the style, 

 ovary, pedicel or peduncle and fruit setting is prevented. 



The failure of the flowers to set fruit through the retarding of pollen 

 tube growth by low temperature is discussed in another connection. 



Premature or Delayed Pollination. — Hartley^^ has found that the 

 flowers of tobacco are very susceptible to injury from premature pollina- 

 tion. When mature pollen grains are applied to immature pistils they 

 germinate, penetrate the styles and enter the ovules and if the ovules are 

 not ready for fertilization the flowers soon fall. In cases of this kind 

 "the separation of the flower from the plant was rapid and complete and 

 not accompanied by any previous wilting of the flower, but invariably 

 occurred at a j oint situated at the base of the peduncle. ' ' This is somewhat 

 different from the falling of flowers from other causes. Table 2 shows the 

 results of one series of pollinations at various stages of pistil maturity. 

 Hartley did not find any injurious results from pollinating orange blossoms 

 nine days before opening and but little injury from premature polhnation 



