J. VAN OVERBEEK 235 



tried (41). This compound persists longer in the plant than NAA which 

 may account for its effectiveness in citrus. Concentrations of about 

 8 ppm. of 2,4-D applied as a drenching aqueous spray to the trees have 

 reduced preharvest drop in Valencia and Navel oranges (41), as well as 

 in grapefruit (42). 



Further beneficial effects of 2,4-D were discovered. Thus, it turned 

 out that treated fruit after storage kept better than the nontreated con- 

 trols because less black button developed. In addition the fruit stems of 

 treated plants showed less die-back (43). The storage life of lemons also 

 could be increased by 2,4-D treatments of the fruit before storage. 



One of the difficulties often associated with insecticidal oil emulsion 

 sprays on citrus has been an increased drop of leaves and immature 

 fruit. It was now found that the addition of 2,4-D to these emulsions 

 could check this drop. 2,4-D is now added to oil emulsions for a dual 

 control program (43) ; not only does it check the fruit and leaf drop, 

 but it reduces fruit stem die-back, and during storage cuts losses due to 

 black button. An ester form of 2,4-D sprayed on at 4 ppm. in terms of the 

 final emulsion is most efficient. Related chlorinated phenoxyacetic acids 

 seem to have effects comparable to 2,4-D; 2,4, 5-T may even be more 

 effective (39). 



Rooting 



After the discovery that auxins promote root formation on cuttings 

 (45) general use has been made of this principle. One might say that 

 auxins have been tried on almost every plant for which vegetative 

 propagation might offer advantages. The results of all these trials, which 

 include many tropical species, have been summarized in extensive tabular 

 form (1,30,31,44). It is intended to discuss here first an example of the 

 use of growth substances in the vegetative propagation of a tropical 

 crop, to be followed by some fundamental aspects of the process of 

 root formation as worked out on a tropical plant in the tropics. 



Root formation on cacao cuttings. — The use of synthetic plant hormones 

 for the promotion of root formation on cuttings as applied in the tropics 

 can perhaps be illustrated best by its use in the vegetative propagation 

 of cacao in Costa Rica. In order to appreciate the importance of cacao 

 in the economy of Central American agriculture a few words must be 

 said about the banana culture with which the cacao culture is closely 

 allied. When the banana industry came into being in this area at the 



