Kraus: The Self- Sterility Problem 



555 



aphids are present at blooming time, 

 but it is actually possible to produce 

 embryo-containing seeds, none of which 

 have we ever succeeded in germinating. 

 Accidentally and intentionally this re- 

 sult has been obtained many times. Of 

 course, such fruits are usually deformed: 

 the longer and more severe the infesta- 

 tion, the greater the deformity. Gardner 

 has found in cherries a similar condition, 

 produced by the black cherry aphis. 

 None of the pits, however, matured 

 embryos. There is a radical change in 

 the vascular system of fruits badly 

 deformed through aphis infestation and 

 such fruits may remain hanging on the 

 trees for several years. The greatest 

 change, however, is that the embryo 

 resulting from self-fertilization at times 

 will complete its development under 

 such circumstances and absolutely fails 

 under normal conditions. It would 

 seem that this peculiar behavior of 

 aphis infested fruits has a fundamental 

 significance, especially when considered 

 in connection with a study of vegetative 

 parts. 



It was soon realized that to arrive 

 anywhere in a study of this problem it 

 would be necessary to begin such a 

 study very early in the history of the 

 flower. In our own work on the apple 

 and pear, we have not only examined 

 the fruit bud in its very beginning, but 

 also the vascular anatomy of the spur or 

 branch from which it arose. Why this 

 latter seemed important will become 

 evident directly. It has been demon- 

 strated that morphologically defective 

 pollen is present in many varieties of 

 orchard fruits at the time the anthers 

 open. At once the question arises, at 

 what stage of development of the 

 blossom does the abnormality of devel- 

 opment occur, and what is the nature of 

 such abnormality? Recent investiga- 

 tions have given one answer to the 

 problem. The same question may well 

 be asked regarding any abnormality 

 of ovules. When there appears to be no 

 particular abnormality of either pollen 

 or ovules, one naturally seeks to discover 

 other causes. 



To observe that apparently there is a 

 normal union of sperm and egg cells and 

 nuclei, and then a varying development 



of endosperm and embryo from little 

 more than none to complete, demands 

 further search for the causes underlying 

 such behavior. Anyone who has carried 

 on polHnation experiments knows full 

 well that the application of so-called 

 acceptable pollen does not necessarily 

 insure fertility. To a lesser degree, the 

 phenomena above enumerated under 

 self-fertilization take place after cross- 

 fertilization, and this is true even though 

 pollen of the highest germinability be 

 used. 



Such a result leads one to look beyond 

 the pollen and egg cells, important 

 though they may be, and he is naturally 

 led to question whether there is not 

 something to be gained from a study of 

 the nutritive processes of structures 

 concerned in seed formation and de- 

 velopment. Are there abnormalities of 

 somatic tissue which actually preclude 

 seed and fruit formation even though 

 the eggs and sperms immediately con- 

 cerned are not lacking in their essential 

 qualities ? Often the high percentages 

 of apparent set from self-pollination or 

 cross-pollination shrink to a discouraging 

 zero before the maturity of the fruit. 

 How often, too, a fair sized fruit is 

 found to be without "plump" seeds. 

 The careful observer has noticed that 

 this latter occurrence is comparatively 

 frequent among certam varieties and 

 extremely rare among others. Is there 

 a reason for this ? What is the cause 

 for certain varieties being able to 

 produce seedless fruits when self-pol- 

 linated, while others absolutely fail to 

 mature fruit unless good seeds are 

 present ? Or why will many fruits 

 develop parthenocarpically ? As more 

 cases are investigated, the evidence of 

 differences in texture and even chemical 

 composition of seed bearing and seedless 

 fruits, is increasing rapidly. Perhaps 

 all are familiar with the differences in 

 flavor and texture of seedless and seed 

 bearing grapes in the same cluster. 

 Hume has pointed out the differences in 

 persimmons. English cucumbers are 

 "often rendered unfit for use when they 

 bear seeds." The quality and texture 

 of apples and pears are distinctly dif- 

 ferent when the fruits have developed 

 parthenocarpically. To me all this 



