Developmental Effects of Auxins 109 



normal process of fruit-set with pollination involves a release of large 

 amounts of endogenous auxins. Since Gustafson's discovery, auxins 

 have come to be widely used in commercial practice for bringing about 

 fruit-set in some crops when natural set does not take place readily. 

 The researches of Muir (19,51) have indicated that tobacco fruit, 

 set either by pollination or by auxin sprays, builds up high concen- 

 trations of auxins, much too high to be accounted for in terms of the 

 amount of auxin in the spray or in the pollen. This flush of auxin ap- 

 pears to be directly involved with the phenomenon of fruit-set and 

 subsequent commencement of fruit growth. A flush of auxin from an 

 auxin spray leads to a parthenocarpic fruit in some species. 



TABLE 6 



The Auxin Contents of Ovaries of Seeded Varieties of Some Fruits as Com- 

 pared WITH Seedless or Parthenocarpic Varieties. 

 (Gustafson, 1939) 



Since the artificial application of auxins to some kinds of flowers 

 can cause parthenocarpic fruit-set, one might expect that the presence 

 of natural auxins in the ovary might be related to natural partheno- 

 carpy. In fact it has been found that species which produce seedless 

 fruits have a rich supply of auxin in the ovaries (Gustafson, 1939). 

 Some sample data can be seen in table 6, in which the auxin contents 

 of seedless varieties of orange, lemon and grape are compared with 

 seeded varieties. The seedless varieties in each instance contain at 

 least 75 per cent more auxin than the seeded varieties. 



The actual means by which auxin brings about fruit-set is not 

 clear, but there appear to be at least two roles for auxin. One of these 

 is the incitement of growth of the young fruit and the other is the 

 prevention of abscission of the flower. The separation of these two 

 functions has been indicated by Leopold and Scott (1952), who found 

 that fruit-set could be obtained after abscission, and conversely that 

 abscission could be prevented without fruit-set in tomato flowers. 



