404 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



The following problems need further study in this connection: 

 (1) selection of the most effective chemical substance for inducing 

 fruit set; (2) method and time of applying the substance; and (3) 

 prevention of some undesirable secondary effects such as blossom- 

 end rot and malformations in the fruit or the plant. 



According to recent tests (Howlett, 1946) a combination of in- 

 dolebutyric acid and /3-naphthoxyacetic acid is superior to indole- 

 butyric acid used alone, with regard to both fruit set and the filling 

 of the loculi. It was also found that an aqueous solution of these 

 chemicals is as effective as an emulsion. 



Regarding the method of application, the usual procedure is to 

 spray the inflorescences with an atomizer, but since this is laborious 

 in large-scale projects some workers have suggested the so-called 

 vapor method. The original procedure was to place the potted 

 plants under bell jars and expose them for various lengths of time 

 to vapors of the desired growth substances. Zimmerman and 

 Hitchcock (1939) found this to be entirely successful with Ilex 

 opaca. More recently another method has been tried, based on 

 the aerosol principle used by entomologists for killing insects (see 

 Zimmerman and Hitchcock, 1944; Hamner, Schomer, and Marth, 

 1944; Howlett, Freeman, and Marth, 1946). Briefly, the hormone 

 is first dissolved in some liquid in which it is readily soluble and then 

 added to a highly volatile liquefied gas; or, if the hormone is soluble 

 in the liquefied gas itself, it is added directly to the latter. The 

 mixture is then held under pressure in a container from which it can 

 be released as a very fine mist. The gas soon volatilizes, leaving 

 the hormone as a finely divided liquid or solid. 



While the effectiveness of the aerosol method has been amply 

 demonstrated and there is no doubt as to its rapidity and sim- 

 plicity, the chief drawback is that it exposes not only the flowers 



acetic acid, it has been possible to induce the flowering and fruiting of pineapples 

 at any time of the year, even in varieties which are normally difficult to get into 

 bearing. The procedure is simple. When the plants have produced sufficient 

 leaves to support and mature a good-sized fruit, a few drops of an aqueous solution 

 of one of the two chemicals are placed into the tip of each plant. One ounce of 

 the dry chemical is sufficient for treating 113,000 plants. 



"Stewart and Condit (1949) report that by spraying aqueous solutions of 

 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid they were 

 able to obtain seedless figs of a size and sugar concent comparable to that of caprified 

 fruits. 



