CHAPTER XI 



Parthenocarpy 



The capacity of some species of plants to set fruit parthenocarpically, 

 i.e. without pollination, has been recognized for a very long time. 

 The first indication that specific substances might be involved in this 

 step came from the observation of Fitting (1909) that water extracts 

 of pollen were able to prevent floral abscission in orchids and to pro- 

 duce ovary swelling which resembled fruit-set. Some years later Yasuda 

 (1934) succeeded in producing parthenocarpic fruits by applying ex- 

 tracts of pollen to cucumber flowers. The fact that pollen extracts 

 could exert such interesting effects led Thimann in 1934 into a study 

 to identify the nature of the materials in such an extract. He found 

 that there were auxins present. This information led Gustafson (1936) 

 to attempt to induce fruit-set parthenocarpically by the application of 

 synthetic auxins directly to the pistil of various flowers. He established 

 that auxins could induce parthenocarpy in many species of plants, and 

 his subsequent research has shown that parthenocarpy and fruit-set 

 itself are brought about in nature by the growth hormone. 



THE PHYSIOLOGICAL BASIS FOR PARTHENOCARPY 



The Role of Auxins 



If auxins play a crucial role in fruit-set in general, one would 

 certainly expect that they should be present in large quantities when 

 fruit-set occurs. This is the case, for Muir (1942) has shown that 

 promptly after pollination a great increase in auxin content of tobacco 

 ovaries occurs (figure 91). Without pollination such an increase is 

 entirely lacking. The implication is that a large amount of auxin is 

 released in the ovary by the consummation of pollination, and this 

 auxin is the immediate cause of fruit-set. The upsurge of auxin con- 

 tent in the ovary is followed by the appearance of large amounts of 

 auxin in the pedicel (figure 91), indicating that the auxin which is 

 formed at pollination moves out of the ovary and into the rest of 

 the plant. That this new supply of auxin moving into the plant may 

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