144 PLANT GROWTH SUBSTANCES 



per cent concentration was sufficient to kill morning glory plants (28). 

 With respect to the mode of action of Car bo wax 1500, its hygroscopic 

 properties are probably not of prime importance, since glycerine and 

 Carbowax both increased the effectiveness of 2,4-D in a dust carrier, 

 while calcium chloride had no effect in this respect. Dispersing agents, 

 such as Tween 20 and Emulfors, and surface-active substances, such as 

 soapless washing powders which contain lauryl sulphates, are effective 

 in increasing the activity of the acid and salt forms of 2,4-D (30,54). 



The nature of the action of surface-active substances is not understood 

 fully. Their effectiveness may be due in part to the fact that they serve 

 to spread the growth regulating chemical and make it adhere closely 

 to the surface of the plant. 



Staniforth and Loomis (54) concluded from experiments with corn, 

 flax, and soybeans that the power of these detergents to reduce surface 

 tension is not directly responsible for their effect on the activity of 

 aqueous 2,4-D mixtures. Five hundredths of one per cent of a detergent 

 gave a maximum reduction in surface tension, but the detergent in- 

 creased the effectiveness of 2,4-D up to a concentration of 2 per cent of 

 the surface-active substance. It is of further interest that these detergents 

 may increase the effectiveness of water mixtures containing salt forms of 

 2,4-D by five or more times, but they had little effect on a water mixture 

 of the ester form of 2,4-D. 



Staniforth and Loomis believe that a surface-active agent fike lauryl 

 sulphate merely accelerates the initial responses to 2,4-D, such as 

 epinasty and stem curvature, and that in the end these detergents do not 

 really increase the herbicidal properties of 2,4-D. This effect may be due 

 to more rapid absorption and translocation of 2,4-D when surface active 

 agents are used than when the growth regulator is applied alone. 



Crafts (11) has proposed that herbicides might be divided into 2 

 general classes, those that are polar and those that are nonpolar. This is 

 of interest here because the growth regulator 2,4-D is now an important 

 herbicide. Crafts concludes on the basis of general results with a variety 

 of herbicides, that polar compounds are most effective when applied to 

 the roots of plants and that nonpolar ones are most effective when 

 applied to the above-ground parts of plants. He suggests that this 

 difference may be due to a difference in the rate of absorption. Since 

 radioactive polar and nonpolar forms of the 2,4-dichlorophenoxy com- 



