234 



PLANT GROWTH SUBSTANCES 



elimination by treatment with a suitable auxin, it would seem justified 

 to conclude that a low auxin level is the causal agent (i6). 



Increased fruit ripening in the banana. — Speeding up of fruit ripening 

 by auxins has also been observed in bananas after they have been removed 

 from the plant; a thorough spraying with .02 to .05 per cent of 2,4- 

 dichlorophenoxyacetic acid replaces the customary ethylene-induced 



Figure 4. Experiment showing acceleration of fruit ripening of the Cali- 

 myrna fig by treatment with an aqueous spray containing 25 ppm. of 2,4,5- 

 trichlorophenoxyacetic acid (2,4,5-T). Normal fruit growth is shown by the 

 solid curve. The first part of this curve is due to growth of the fruit (syconium) 

 prior to pollination, which took place about June 30. Without pollination 

 or suitable auxin treatment fruits drop off shortly after this time. Treatment 

 at this time of the nonpollinated fruit causes growth represented by curve A. 

 Pollinated fruit treated on July 31 immediately came out of the rest period 

 and followed curve B. (3). 



ripening (30). The effect may be attributed to the known stimulatory 

 effect of auxins on the amylolytic enzyme system in plants (29). 



Uses of auxins on citrus fruit. — Preharvest drop of citrus fruit is a 

 serious problem, and when it was shown (20) that NAA can control 

 a similar drop in apples, attempts were made to reduce drop in citrus 

 by similar means. These attempts all met with failure until 2,4-D was 



