JOHN W. MITCHELL 143 



and oat colcoptlles (22,35,61). The rate of absorption of growth-regulat- 

 ing substances is rapid, on the basis of these data, but the absorption 

 of these chemicals is influenced to some extent by environmental factors, 

 such as light, temperature, and the presence of a wetting agent or of 

 surface-active substances. That light influences the rate of absorption 

 of 2,4-D was shown by Rice (48). Absorption of the ammonium salt of 

 2,4-D by bean leaves was greatest immediately after the chemical was 

 applied. More of the salt was absorbed by illuminated leaves than by 

 others kept in darkness. Leaves do, however, absorb 2,4-D at a slower 

 rate during periods of darkness. Rice reported that increasing the light 

 intensity from 100 to 900 foot-candles had no appreciable effect on the 

 rate of absorption of the ammonium salt of 2,4-D. Temperature, on 

 the other hand, affected absorption of the salt since the amount taken 

 up by a plant at 46° to 58°F, was less than at a temperature of 78° to 

 8o°F. Injurious effects of 2,4-D are more pronounced in plants grown 

 at relatively high temperatures (7o°-85°F.) than in plants grown at 

 relatively low temperatures (5°-i5°F.) (23,27). 



Wetting agents and some hygroscopic substances that dissolve growth- 

 regulating chemicals have a marked effect upon the activity of the 

 growth regulator. This is apparently so because they make it possible 

 for the plant to absorb the growth-regulating substances more readily 

 than when the wetting agents are not added to the aqueous mixture. 

 Polyethylene glycols are among these activating substances (64). Growth- 

 regulating chemicals are generally soluble in polyethylene glycols. Some 

 of the glycols are hygroscopic. They serve as solvents and wetting agents 

 and tend to keep the growth-modifying substance in close contact 

 with the surface of the plant (12,37). I^ ^^ reasoned that growth regulators 

 applied as aqueous mixtures of the salts, such as the sodium or ammonium 

 salt of 2,4-D, tend to crystallize out as the water carrier evaporates. 

 Absorption of the growth regulator is thus reduced since upon crystal- 

 lization the chemical is no longer in close contact with the surface of 

 the plant. By actual measurement the rate of absorption of ammonium 

 salt of 2,4-D in water by bean leaves decreased after the first 4 hours. 

 When Carbowax was added to the mixture of the salt and water, a 

 relatively rapid rate of absorption prevailed for an extended length 

 of time (11). 



In a similar way 2,4-D in a dust carrier was relatively ineffective as an 

 herbicide, but when 3 per cent of Carbowax 1500 was added, a 0.05 



