414 CRAFTS 



States, and by the turn of the century iron sulfate, copper sulfate, and sul- 

 furic acid were being used in considerable quantities for this purpose. Selec- 

 tivity of this type depends upon differential wetting of the leaves and upon 

 the growth habit of the cereal plants. In the latter the meristems responsible 

 for new shoots and roots are protected by the bases of the older leaves and 

 also by their location at or below the soil line ; buds of the mustard plant are 

 exposed. 



Sodium chlorate received some attention as a selective spray on grains dur- 

 ing the 1930's, but it remained for the dinitro-substituted phenols to usher in 

 the new era in chemical weed control. Sinox (sodium dinitro-o-cresylate) was 

 discovered in France in 1933. Tested in California in 1938 and 1939, it proved 

 by far the most toxic herbicide available — so toxic in fact that it could be 

 applied by airplane (Westgate and Raynor, 1940). In the early tests it was 

 used at 4 to 6 pounds of active chemical per acre; when its activation by 

 acid salts was discovered, this dosage was cut in half. And the chemical proved 

 useful not only on cereal crops but on flax, peas, onions, and alfalfa as well. 

 During the early 1940's use of this material approached 100,000 acres per 

 year in California, 



Use of the airplane placed herbicides into a class with insecticides as 

 capable of application to large areas, making large-scale operations feasible 

 for the first time. Operators were no longer hindered by fence lines and wet 

 or uneven soils. In the case of 2,4-D, use of aliphatic esters in oil relieved 

 the operator of the water requirement and enabled him to cover many hundred 

 acres in a day. For the first time chemical weed control could actually meet 

 the farmer's needs. 



The next advance in the use of selective sprays resulted from the discovery 

 that carrots tolerate light fuel oils, whereas grasses and many common broad- 

 leafed weeds succumb. Stove oil was first used; then Stoddard solvent was 

 found less persistent on the crops. At present most carrot crops are sprayed 

 one or more times with a light-oil fraction to kill weeds. The method has 

 also been extended to celery in the seedbeds, to parsnips and certain herb crops, 

 and to onions, flax, and forest-tree nurseries. 



Oil selectivity is not a matter of wetting, since all plants are thoroughly 

 wetted by the spray. Currier and Peoples (1954) found that the toxicity of 

 an oil is related to the degree to which it saturates the lipoid phase of the 

 plant cells; a highly toxic oil saturates this phase at a lower oil content than 

 does an oil of low toxicity. And selectivity apparently relates to the inherent 

 saturability of the lipoid phase of the cells of different plants; susceptibility 

 results from a low saturation requirement. Apparently all plants of the carrot 

 family require relatively high oil contents for saturation; hence they tolerate 

 oil sprays. 



The announcement of 2,4-D in 1944 initiated a new era in the use of 

 herbicides. This chemical is unique in being highly toxic, readily absorbed. 



