The Growth Hormone Mechanism 

 in Fruit Development 



ROBERT M. MUIR 



THE studies of Gustafson (5,6) and others on parthenocarpy indicate 

 that growth hormones control the development of fruit. The 

 mechanism whereby this control is implemented in the normal process of 

 pollination and fertiUzation is incompletely known, however. Relatively 

 few investigations of this aspect of auxin physiology have been made and 

 their results are not in complete agreement. Undoubtedly some variation 

 exists among plant species as is indicated by the differences in degree 

 of natural parthenocarpy; yet, iDcfore ascribing different mechanisms to 

 the species investigated, the differences which have been reported must 

 be evaluated on the basis of the techniques used in the assay of auxin. 

 Since the normal stimulus for fruit development is fertilization subse- 

 quent to pollination, it follows that the pollen may furnish directly 

 the growth hormones which cause the enlargement of the ovary, or it 

 may furnish a part of a system responsible for the production of the 

 hormone in the ovary. Laibach (8) first identified auxin in extracts of 

 pollen of several orchids and Hibiscus and later (9) reported that extracts 

 of Cucurbita pollen were active in the Avena test but those of a number 

 of other species were not. Extracts of pollen of Sequoia (18), Zea 

 (10,12,13), and Helianthus (10) have been shown to contain auxin. 

 Laibach and Meyer (10) report relatively large amounts of auxin in 

 both unripe anthers and ripe pollen of corn, and Wittwer (21) found that 

 ether extracts of mature corn pollen were five times as active as extracts 

 of immature anthers. This sequence of ontogenetic changes in the hor- 

 mone content of pollen is different from that reported by Hatcher (7) 

 for rye in which water, phosphate buffer (/?H 10), and N/50 NaOH 



