Factors Altering Effectiveness 1 39 



Another factor in absorption is light. Rice (1948) found that in 

 the dark there is a substantially greater absorption of 2,4-D than in 

 the light (see figure 64). Why absorption is greater in the dark is not 

 dear, but it seems possible that under the conditions of his experi- 

 ment, Rice may have been getting a longer period of wetting in the 

 dark. That is, the light may have caused more rapid evaporation of 



72 HRS. 



TIME 



Fig. 64. Absorption of 2,4-D by bean leaves in dark and in light, 900 f.c. (Rice, 1948). 



the water droplets from the leaf surface, resulting in a shorter period 

 of absorption. He observed no significant difference in absorption with 

 light intensities from 100 to 900 foot candles. It is also possible that 

 the physical characteristics of the leaf surface are altered by light and 

 dark (Rice, unpublished). Wettability of leaves is discussed in chapter 

 XVI. 



It is well-known that carriers such as Carbowax 1500 — a mixture 

 of polyalkylene glycols — or detergents or emulsifying agents can nota- 

 bly increase the effectiveness of an auxin spray applied to foliage. 

 From the data of Rice given in figure 63, it seems evident that one of 



