p. W. ZIMMERMAN 83 



their derivatives; substituted phenoxyaliphatic acids especially 2,4-di- 

 chlorophenoxyacetic acid (2,4-D) and higher homologs, a-(2-chloro- 

 phenoxy) propionic acid, 2-methyl-4-chlorophenoxyacetic acid, 2,4,5- 

 trichlorophenoxyacetic acid and higher homologs; and unsaturated hy- 

 drocarbons, especially ethylene and acetylene. While there is some varia- 

 tion in uses and responses induced with acids, salts, esters, and amides, 

 for our purpose they may be thought of as similar in bringing about 

 desired results. There are many natural hormones which have not been 

 isolated and definitely identified but nevertheless are known to exist. 

 Indoleacetic acid and ethylene are two compounds manufactured by 

 plants and definitely identified by chemical methods. 



Today there are many lines of research as indicated by the titles of 

 publications just read. From the fundamental research angle workers 

 are trying to find how hormones work and how they are synthesized. 

 Much emphasis is being placed on enzymatic activities, precursors, and 

 reactions ultimately leading to active substances. Another group is trying 

 to harness plant hormones and put them to work. Physiological activity 

 is found with simple structures like ethylene and with the more complex 

 structures hke naphthaleneacetic acid and substituted phenoxyaUphatic 

 acids. New compounds are being discovered almost daily, and it is 

 likely that the most effective ones are yet to come. 



Methods are adapted to meet the needs for each particular kind of 

 work to be accomplished. New and better methods are constantly being 

 developed. Among others the following apphcations resulting from plant 

 hormone research have been established : 



Propagation of plants through hormone treatment of cuttings 

 and scions. 



Prevention of pre-harvest drop of fruit, especially apples and 

 oranges, with naphthaleneacetic acid or 2,4-D. 

 Increasing fruit set and induction of seedless tomatoes with 

 numerous hormone-like substances. 



Inhibition of buds to prevent potatoes from sprouting by 

 the use of vapors from esters of naphthaleneacetic acid. 

 Inhibition of fruit tree buds or prolonging dormancy to pre- 

 vent loss through late frost though the methods are as yet not 

 well perfected. 



Inhibition of buds of nursery stock to prevent growth until 

 the young plants are established. 



