270 BREEDING CROP PLANT* 



very noticeable. Increases in size of fruit are also of much 

 importance. 



For the commercial grower or the fruit breeder, it is essential to 

 know which varieties are self-sterile. In order to illustrate the 

 conditions generally found regarding sterility, a compilation of 

 some results is presented in Table LXXII. Citations to literature 

 are given so that the reader may go to the original sources when 

 he desires to know what category any particular variety belongs 

 to. 



The causes of sterility have been determined in some cases. 

 In the strawberry it is due to at least two causes (Valleau, 1918) : 



1. The dioecious condition. 



2. The production of aborted pollen grains or microspores in 

 otherwise normal anthers. 



In the grape, Dorsey (1914) has found sterility to be asso- 

 ciated with both hybridity and the dioecious condition. The 

 varieties which produce reflexed stamens seldom produce fertile 

 pollen. Dorsey states that : 



" Sterility has been found to be due to the pollen rather than in the 

 pistil. Sterile pollen in the grape results from degeneration processes 

 in the generative nucleus or arrested development previous to mitosis 

 in the microspore nucleus." 



Pollen abortion occurs both in pure and hybrid forms but is not 

 considered a cause of lack of fertility as abundant pollen is pro- 

 duced in the grape. 



In the plum, pollen abortion is not as a rule the cause of self- 

 sterility. The outstanding features as given by Dorsey (1919) 

 are: 



"(a) A constancy of expression of self-sterility even in P. domestica 

 in which about one-half of the varieties are self-f ertile ; (b) the occur- 

 rence of cross-sterility; and (c) the slow growth of pollen tubes under 

 the condition of self- and cross-sterility." 



This type of sterility is comparable with that in the tobacco 

 crosses previously discussed, where sterility resulted from slow 

 pollen tube growth. In this case the pollen tube growing 

 from the pollen grain into the tissues of the style never reaches the 

 embryo sac. The self-sterile condition is believed by Dorsey to 

 be a dominant character in the plum and to be inherited, segrega- 

 tion into sterile and fertile forms occurring at reduction division. 



