500 FUNDAMENTALS OF FRUIT PRODUCTION 



aborted pollen and, furthermore, many varieties of known hybrid origin 

 are highly self fertile. In discussing this matter Dorsey^** says: "Since 

 both fertile and sterile hybrids occur among the cultivated varieties of 

 American grapes, hybridity is not necessarily a cause of sterility. The 

 relation of the sterile pollen to the absence of the germ pore, the reflexed 

 type of stamen, and the tendency toward diceciousness, suggest that 

 pollen sterility in the grape is only a step toward functional dicliny." 

 The same investigator^^ reports somewhat more aborted pollen in some 

 of the hybrid plum varieties than in some of those of pure species and also 

 a tendency for the degeneration processes to start earlier in the hybrids. 



All the available evidence warrants the conclusion that the highest 

 fertility is correlated with neither the narrowest nor the broadest breeding 

 possible. 



Incompatibility. — One of the most common causes of self unfruit- 

 fulness and self sterility is incompatibility between the pollen and the 

 ovules of the same plant or of the same variety. That is, both the ovules 

 and the pollen of the plant are fertile in themselves, but they fail to 

 effect conjugation. Miiller found self incompatibility in Oncidium flexuo- 

 sum and a number of other species of orchids. ^^ In some instances not 

 only did the pollen fail to impregnate the ovule but its action was injurious 

 or poisonous to the stigmas, causing them to turn brown and to decay 

 prematurely. At the same time unpollinated stigmas remained fresh. 

 Those that were pollinated with pollen from other plants showed no signs 

 of injury; fertilization took place and fruit set; the pollen that acted so 

 injuriously upon the stigmas of its own flowers functioned perfectly on 

 other plants. The same condition has been reported in Lobelia^^ and as 

 not uncommon in Cichorium intybus.'^^'^ 



The self sterility or self unfruitfulness that has been reported in the 

 apple, 88, 107 in pears,^^; "^ [^ ^^^ sweet cherry,^"; ^^s j^ the plum,^^; i^" 

 in dewberries and blackberries^^ and in the almond^^^ is probably in large 

 part attributable to incompatibility. In practically all of the instances 

 cited the varieties set fruit properly when cross pollinated, showing that 

 the pistils were perfectly developed and functional. Furthermore the 

 pollen from these same varieties proved viable and capable of taking part 

 in the fertilization process and in yielding mature fruits and seeds when 

 it was applied to other varieties of the same species. Nevertheless, 

 barrenness followed self pollination. However, in most cases data 

 are lacking to show whether or not pollination was followed by fertiliza- 

 tion. It is possible that in many instances fecundation took place and the 

 immediate cause of the failure of the fruit to set or mature was embryo 

 abortion at a later stage. This has been mentioned as a distinct cause 

 of fruit dropping. It is, however, in most cases very closely related to, 

 if it is not actually one aspect of, incompatibility. Therefore the self 

 sterility and self unfruitfulness of these common fruits may be considered 



