498 FUNDAMENTALS OF FRUIT PRODUCTION 



and a frequent cause of unfruitfulness. Osawa^"^ reports irregular 

 development of the pollen mother cells and much defective pollen in 

 Daphne odora. Two to 10 per cent of the pollen of the mango is regu- 

 larly defective, i"*^ Dorsey^^ finds pollen abortion common in the plum, 

 noting that in that fruit the disintegration processes usually occur after 

 the liberation of the tetrad from the pollen mother cell. If distinction 

 is to be made between pollen sterility and pollen abortion, in this case 

 as in the grape, the defective pollen of the plum is sterile rather than 

 aborted. In neither the plum nor the mango, however, is the percentage 

 of defective pollen high enough to interfere seriously with the setting of the 

 fruit. Pollen abortion has been reported as a practically constant char- 

 acteristic of blackberries in New England. ^^ Furthermore it has been 

 found to vary greatly with the variety and species. For instance Ruhus 

 allegheniefisis was found to have about 96 per cent, while R. hispidus 

 had less than 10 per cent, morphologically perfect pollen. Between these 

 extremes were all gradations. The higher percentages of defectiveness 

 were enough to reduce very materially the set of fruit. A similar condi- 

 tion is reported in the strawberry. ^^^ 



Degeneration occurs in nearly all the pollen mother cells of the 

 Washington Navel orange. ^^^ ^<"' Consequently practically no mature 

 and perfect pollen grains are formed. In the Unshu variety ^°° degen- 

 eration is not so general; nevertheless it affects a large number of the 

 pollen mother cells. In these two varieties, as in certain others, pollen 

 abortion is not accompanied by unfruitfulness because the fruits are 

 capable of parthenocarpic development, but it is responsible for partial or 

 complete suppression of their seeds. 



DUE PRINCIPALLY TO GENETIC INFLUENCES 

 The forms of self sterility and self unfruitfulness discussed up to this 

 point are due plainly to factors associated with the fundamental constitu- 

 tion of the protoplasm. It is also clear that sterility due to these factors 

 is inherited, though the underlying causal agents are evolutionary 

 tendencies within the species. Self sterility and self unfruitfulness that 

 are to be attributed more directly to genetic factors, to the inheritance 

 received, are here discussed under the headings of hybridity and incom- 

 patibility. However, it is impossible to differentiate sharply between 

 these two types of sterility. 



East and Park^^ remark: "Self-sterility is a condition determined 

 by the inheritance received, but can develop to its full perfection only 

 under a favorable environment." In his study of fertility in chicory 

 Stout^^^ found that out of a total of 101 plants in one crop which came 

 from three generations of known self sterile ancestry 11 were self fertile 

 and 90 were self sterile. From his data he was able to conclude not only 

 that self sterility is inherited but that in this species narrow breeding 



