216 Auxins in Agriculture 



stimulate overall plant growth has been demonstrated by Wittwer 

 (1943) in several species of plants. 



Natural parthenocarpy takes place in ovaries which have naturally 

 high auxin contents. This was found by Gustafson (1939), who showed 

 that varieties of several fruits which set fruit parthenocarpically all 

 contain much more auxin than closely related varieties which are not 

 naturally parthenocarpic. 



Since large amounts of auxin are released at the time of pollina- 

 tion and large amounts of auxin are present in ovaries that do not 

 require pollination, it is hardly surprising that the application of 

 auxins to flower ovaries can often artificially induce parthenocarpic 

 fruit-set. 



The change of auxin level in the ovary at the time of pollinated 

 fruit-set is too great an increase to be accounted for simply on the 

 basis of the auxin supplied by the pollen itself. Muir (1947) has shown 

 that as much as one hundred times more auxin is formed in the 

 ovary than is presented to the ovary by the pollen. Subsequent evi- 



> 



< 



3d- 



20 



10 



UJ 

 01 



Z) 



h- 



< 

 > 



tr 



< 



oy/\Rr 



Pollinated 



Unpollinated 



• o 



PEDICEL 



Pollinated 



Unpollinated 



84 HRS. 



TIME 



Fig. 91. Changes in diffusible auxin content of pollinated and unpollinated tobacco 

 ovaries and of the pedicel below the ovary (data of Muir, 1942). 



