35 

 Ashton and Crafts (1973) summarized this issue by stating, "the mode 

 of action of the chlorophenoxy compounds must consist of a great number 

 of structural and biochemical reactions revolving around the central 

 theme of prolonged abnormal growth with failure of those changes charac- 

 teristic of maturity and senescence. In no other way may the great 

 number and diversity of structural and metabolic changes be reconciled." 

 (Ashton and Crafts, 1973, page 284). 



Translocation 



Polar salts of 2,4-D are absorbed more readily by the roots of 

 most species, whereas ester formulations are more readily absorbed 

 by leaves (Weaver, 1972; Weeds Science Society of America, 1979). 

 Foliar-absorbed 2,4-D is transported polarly within the phloem with 

 assimulated sugars, and root-absorbed 2,4-D moves upward in the xylem 

 during transpiration (Audus, 1972; Weaver, 1972). Movement is towards 

 rapidly growing tissues, such as developing flowers and meristematic 

 shoots and roots (Audus, 1972; Weaver, 1972; Ashton and Crafts, 1973). 

 Thus, thorough distribution of a herbicide, such as 2,4-D, is dependent 

 upon the active movement of foods in the plant. However, not all 

 species absorb and translocate 2,4-D at the same rate. Fang (1958) 

 demonstrated with radioactive labeled 2,4-D that the rate of transloca- 

 tion of 2,4-D was slower in peas and tomatoes than in bean plants. 

 Morphological characteristics that prevent 2,4-D absorption and translo- 

 cation and the plant's ability to conjugate or metabolize 2,4-D have 

 also been suggested as mechanisms of selectivity by tolerant species 

 (Fang, 1958; Mullison, 1982). 



