J. VAN OVERBEEK 233 



fig varieties are parthenocarpic, the Calimyrna fig requires pollination 

 by a specific wasp. It has been shown that when the mature pollen- 

 receptive inflorescences are sprayed with suitable auxin preparations, 

 the Calimyrna fig also can be forced into producing parthenocarpic 

 fruit (2). This in itself might not be of great physiological interest were 

 it not for the fact that some of the auxins reduce the normal ripening 

 period of the fruit so drastically that the average 120 day period is cut 

 in half. This was the first example where auxins cause a drastic reduction 

 in ripening time of fruit; recently it has also been shown in apples and 

 peaches. 



The case assumes further interest if the active compounds are examined 

 (3,16). 4-Chlorophenoxyacetic acid will cause ripening of the fruit with- 

 out materially speeding up the ripening period. Except for the absence 

 of seeds parthenocarpic fruit produced with this chemical resembles that 

 produced by normal pollination. It is entirely possible that this treat- 

 ment will replace the cumbersome caprification by the wasp. Quantities 

 of approximately 100 grams of 4-chlorophenoxyacetic acid per acre, 

 at 40 to 60 ppm., are required. 



When an extra chlorine atom is added to the molecule so that 2,4- 

 dichlorophenoxyacetic acid is formed, a compound of considerably lower 

 activity than the original 4-chlorophenoxyacetic acid is obtained. The 

 activity of this dichloro compound is so low that it was originally thought 

 to be inactive. Recently it was found that it can bring about partheno- 

 carpy in the fig in concentrations in excess of 250 ppm. (40). When still 

 another chlorine atom is added to form 2,4,5-trichlorophenoxyacetic 

 acid, a compound of unusual physiological activity is obtained. Not 

 only does it cause parthenocarpic fruit setting at 10 ppm., but in addition 

 it hastens the ripening period. As is shown in Figure 4, curve A, it does 

 so by omitting the rest period between the initial and the final stage 

 of rapid fruit growth. The trichloro compound also will accelerate 

 ripening when appHed to pollinated Calimyrna fruit (Fig. 4, curve B), 

 and to naturally parthenocarpic fruit of varieties such as the Black 

 Mission. Why such closely related chemicals have such widely different 

 effects, which, however, are more quantitative than qualitative, is not 

 known at present. 



The double sigmoid growth curve C of Figure 4 reflects two periods 

 of rapid fruit growth separated by a rest period. Many explanations have 

 been offered for the occurrence of this rest period. In view of its successful 



