84 PLANT GROWTH SUBSTANCES 



Regulation of flowering of pineapples by the use of ethylene, 



acetylene, naphthaleneacetic acid, naphthoxyacetic acid, and 



substituted phenoxy compounds. In this way a crop can be 



staggered to facilitate harvest and canning operations and 



also increase size of fruit of certain varieties. 



Defoliation of plants with ethylene, acetylene, propylene 



gases, and other chemical means. 



Prevention of leaf fall in contrast with defoliation. 



Thinning of fruit, especially apples, with hormone sprays 



bringing about better size and quality of fruit and causing 



biennially bearing trees to bear annually. 



The use of hormone-like chemicals as selective weed killers. 



Defoliation and inhibition of growth were given great prominence 

 during the war. It is conceivable that chemicals may be developed which 

 can be sprayed on the forest to defoliate the plants and thereby expose 

 the enemy. Crops may be killed over large areas by the use of chemicals 

 dusted or sprayed by means of an airplane or allowed to float over the 

 fields through artificial fogs or smudges. 



Extensive use of plant-hormone-like substances became evident during 

 the past year when the sale and use for 2,4-D practically equaled that 

 of DDT which has been leading the list of all other organic agricultural 

 chemicals. a-Naphthaleneacetic acid was also high on the list since large 

 orchards were sprayed to prevent preharvest fruit drop. 



One of the best illustrations of growth regulation of plants is the 

 modification of the pattern of new organs growing under the influence 

 of applied hormone-like chemicals. Substituted phenoxy, benzoic, and 

 naphthoxy acids are particularly effective for this purpose. Mature 

 organs do not change their shape, but all new organs which grow under 

 the new chemical influence are modified. This leads to the assumption 

 that normal leaf patterns are determined by natural chemical influences 

 within the plant. Under new and stronger chemical influences the leaves 

 and other organs assume a new pattern. So far no practical applications 

 for this odd response have been found. However, we should not be 

 surprised if one crops up since many other curiosities in the plant- 

 hormone field have eventually led to practical applications. 



To date no hormone-like chemicals or any chemical alone have been 

 found to stimulate growth of the entire plant in the same sense as is 



