342 PLANT GROWTH SUBSTANCES 



in relatively weak positions as regards food supply may also be more 

 sensitive in this respect (39). 



When still higher concentrations of NAA are employed during ad- 

 vanced development of the apple fruit (20 to 50 days after bloom) the 

 seed collapsed, fruit growth either ceased completely or was greatly 

 retarded, but abscission did not always follow. As yet we have not been 

 able to conduct a detailed study of the histological changes in fruit as a 

 result of NAA appHcation. Whatever the mechanism responsible for 

 thinning may turn out to be, this synthetic growth regulator (NAA) has 

 given us a powerful tool to control the size of the crop of certain fruit. 



Control of Preharvest Drop of Fruit 



Mention should be made, briefly at least, of the well-established 

 practice of using a weak solution of NAA to retard the preharvest drop 

 of apples, pears, and a few other fruits (28). The effect evidently 

 expressed itself through a delay of the natural changes in the abscission 

 layer between the fruit and its pedicel coincident with ripening, which 

 lasts, from a NAA spray or dust, for about 10 days. 



Batjer and Thompson (2) have demonstrated that 2,4-D is even more 

 active in its lasting effects for apple drop prevention, but for some 

 unknown reason it seems to work mainly on the Winesap group of apples. 

 Stewart et al. (81) have discovered that 2,4-D can prevent also the 

 abscission of citrus fruit. 



Our tests (63) point to the fact that 2-methyl-4-chlorophenoxyacetic 

 acid is almost as potent as 2,4-D for length of delay of the preharvest 

 drop of apples and that 4-chlorophenoxyacetic acid stands in a position 

 between the above two chemicals and naphthaleneacetic acid. 



Conclusion 



This somewhat sketchy paper should serve as a brief summary of the 

 present status of studies on the function of hormones in sexual reproduc- 

 tion of some crop plants. Special emphasis has been placed in this 

 discussion on the use of synthetic growth regulators in connection with 

 certain cultural practices of horticultural plants. Objections have been 

 raised to the widespread employment of the term auxin in designating 

 both naturally occurring hormones and synthetic substances used both 

 experimentally and in crop production as growth regulators. 



From flower initiation to delay of fruit abscission during the preharvest 



