UNFRUITFULNESS ASSOCIATED WITH EXTERNAL FACTORS 513 



diate cause of the normal self sterility was a slow growth of the pollen 

 tubes, presumably a result of chemotropic influences; the appearance 

 of the self fertile condition followed an acceleration in pollen growth. 

 These investigators remark: "Since we have reason to believe that the 

 difference between a sterile and a fertile combination in these plants is the 

 ability of the pollen grain through something inherent in its constitution 

 to call forth in the tissue of the style in the former and not 

 in the latter case a secretion which accelerates pollen-tube growth, it 

 follows that in weakened style tissue some change has occurred that renders 

 this secretion more easily produced." They report that self sterility can 

 be restored in these weakened plants by allowing them to go through a 

 period of rest and then forcing them into vigorous growth. Their sugges- 

 tion that "truly self fertile plants cannot be forced into self sterility by 

 any treatment" obviously holds if self fertility is defined to agree with that 

 concept. However, if that is to be the concept of self fertility it may be 

 questioned whether any of our cultivated fruits be self fertile. In the 

 fruit plantation there are fruit setting, fruitf ulness and fecundity conditions 

 which vary with environment. 



Contrasting sharply with the end-season fertility that has just been 

 mentioned as sometimes occurring in the grape, mango and tobacco is an 

 end-season sterility found by Valleau^^'^ to be quite common in the 

 strawberry. 



A striking example of seasonal influence on fruit setting and fruitful- 

 ness occurs in figs of the San Pedro class. -^ In varieties of this group the 

 early crop, or brebas, set freely without pollination, developing seedless 

 fruits. The later main or summer crop will not set and mature without 

 caprification. This, like the strawberry, is particularly interesting both 

 because it is an instance of early season rather than late season fruitful- 

 ness and because it is a constant characteristic of these varieties. 



Change of Sex with Season. — Related to the influences of season on 

 fruit setting, fruitfulness and fertihty, or, more accurately, to be mentioned 

 as the immediate explanation of some of those influences, are the occa- 

 sional effects of season upon the complete suppression of one or the other 

 of the two sex organs, its effect upon their development when normally 

 they are undeveloped or non-functional and its effect upon change of 

 sex. The sweet gale or bog myrtle {Myrica gale) is a small shrub which 

 grows abundantly in the swamps of Europe, Asia and North America. 

 It is described by many authorities as strictly dioecious. However, it has 

 been found that intersexes or mixed plants of many gradations are present 

 everywhere in the peat moors of England. ^^ Furthermore, a .study of 

 individual plants for a series of years showed that changes of sex occurred 

 from year to year. Plants entirely female in 1913 were entirely male in 

 1914. Plants female in 1913 were mixed in 1914, entirely male or nearly 

 all male in 1915 and again female in 1916. There is a record of a hybrid 



