Parthenocarpy 219 



hundred times more effective in setting fruit tfian the free acid of 

 indoleacetic acid. However, in view of the relatively weak action of 

 indoleacetic acid, this does not make the methyl ester very high in 

 activity. It is interesting to note that esterified auxins do exist in pollen 

 and that they are quite effective in inducing parthenocarpy. 



Another derivative of indoleacetic acid which is more active in 

 fruit-set than the acid itself is indoleacetonitrile (Bentley and Bickle, 

 1952). The response to the nitrile is delayed for three weeks after 

 application, which suggests that the material is inactive itself but is 

 converted into an active substance in the plant. The conversion of the 

 nitrile into the acid is discussed in chapter III. 



A tentative report has been made (Wood and Fontaine, 1952) 

 indicating that amide derivatives of 2,4-D with several D-amino acids 

 are capable of setting fruit without producing epinastic deformations. 

 Amide derivatives with L-amino acids on the other hand are strong 

 epinastic agents like the 2,4-D acid itself. 



COOH 



COOH 



^3 



2,4-D-niethioiune amide N-meta-tolylphthalamic Benzothiazole-2-oxyacetic 



Acid Acid 



Epinastic agents which are not auxins are effective in fruit-set as 

 well as auxins. Derivatives of phthalamic acid have been shown to 

 induce fruit-set (Hoffman and Smith, 1949), as also have various ben- 

 zoic acid derivatives (Zimmerman and Hitchcock, 1942). N-meta-tolyl- 

 phthalamic acid has shown promising results as an overall plant spray 

 for setting tomatoes (U. S. Patent No. 2,556,665). This is a highly de- 

 sirable characteristic, for the application of sprays to the flower clusters 

 only is costly in labor. N-meta-tolylphthalamic acid is not an auxin, 

 but has synergistic activity with auxin in the pea straight-growth test 

 (Leopold, unpublished). 



The concentration of auxin used has a quantitative effect on 

 fruit-set. Increasing concentrations of auxin give a comparable in- 

 crease in percentage fruit-set obtained (figure 93) as well as in initial 

 growth rate (figure 23). Although the optimum concentration for 

 fruit-set varies considerably from one experiment to another, still the 

 greatest percentage of fruit-set is generally obtained at concentrations 

 between 30 mg./l. and 250 mg./l. of PCA or BNOA. The quantitative 



