240 PLANT GROWTH SUBSTANCES 



added precaution, when there are no susceptible crops near by. From 

 the point of view of drift damage to susceptible crops dust formulations 

 of 2,4-D are the most dangerous, and within the continental United 

 States the Civil Aeronautics Commission will not grant licenses for 

 2,4-D dust applications. In the tropics dusts are still used. In areas with 

 dusty soils there is considerable danger of 2,4-D drifting to adjacent 

 areas even though the substance may be applied in an aqueous form. 

 The finely pulverized soil serves as a vehicle which carries the absorbed 

 2,4-D to localities where it is not desired. 



The exclusive use of 2,4-D as a selective herbicide brings with it the 

 danger that the weed population which is effectively eradicated by it 

 will be replaced by a group of plant species resistant to it (53). Thus it 

 was observed in sugar-cane fields in Puerto Rico that the easy-to-kill 

 Comtnelina was being replaced by the more resistant Poinsettia hetero- 

 phylla and highly resistant grasses such as Digitaria. 



2,4-D is used also as a so-called pre-emergence spray. In sugar-cane 

 technology this term is used with reference to the emergence of the 

 weeds, while in the middle latitudes it usually refers to the crop emer- 

 gence. For pre-emergence purposes the 2,4-D is applied in such large 

 dosages that it prevents the germination of weed seeds, including those 

 of grasses. Amounts up to 5 pounds of 2,4-D per acre are applied. 

 Generally speaking no damage is done to the sugar cane if this amount 

 is not exceeded. Whether or not damage is done to the crop by pre- 

 emergence applications of 2,4-D depends greatly upon a combination of 

 soil properties and rainfall. These conditions are quite well understood 

 at present due to the work done at the Hawaiian Sugar Planters Experi- 

 ment Station (7). 



The principle upon which pre-emergence weed control in sugar cane 

 depends is that the 2,4-D stays in the surface layers of the soil. When this 

 condition is realized the 2,4-D will prevent weed seeds from developing, 

 while it will not interfere with the cane roots which are growing below 

 the zone in which 2,4-D is present. The shoot of the cane which is 

 relatively Insensitive to 2,4-D will penetrate through the 2,4-D layer 

 unharmed. 



The Hawaiian investigators showed (7) that under the influence of 

 rain 2,4-D moves downward through the soil as a concentration front 

 comparable to those seen in chromatographic columns. In some soils 

 this 2,4-D front moves very slightly even after much rain, while there 



