CHAPTER XVI 



Herbicides 



The first recognition of selective lierbicidal properties of auxins was 

 made in England by Slade et al (1945). These workers observed that 

 naphthaleneacetic acid killed Yellow Charlock in a stand of oats with- 

 out injuring the oats. The first experiment was carried out in 1940 and 

 was followed by a long series of researches under the auspices of the 

 Agricultural Research Council and under military secrecy. For secu- 

 rity reasons, their work was not published until 1945. Quite independ- 

 ently, Kraus in this country began investigating herbicidal qualities 

 of auxin in the fall of 1941. This work likewise was carried on under 

 military secrecy during the war, in the course of which it was found 

 again that auxins were not only herbicides, but they were also selec- 

 tive in their action (Kraus and Mitchell, 1947). Two other research 

 teams participated in the work in England, Nutman ct nl (1945), and 

 Blackman (1945). 



The first work was done with naphthaleneacetic acid, but while 

 the early experiments were being carried out Zimmerman and Hitch- 

 cock (1942) published the finding that some chlorinated phenoxyacetic 

 acids were much more powerful auxins and this led to the discovery 

 that such compounds were much more powerful selective herbicides 

 as well. 



The use of auxins as herbicides has four major advantages: 1) the 

 herbicidal action is highly selective and differences in molecular struc- 

 ture of the auxin, species or status of the plant lead to pronounced 

 differences in selectivity; 2) the herbicidal action is not permanent 

 in the soil, and after the passage of a few weeks no toxic residue 

 usually remains; 3) auxins exert their effects at extremely low concen- 

 trations making their use economical and 4) auxins are nontoxic to 

 animals and humans. 



PHYSIOLOGICAL BASIS OF HERBICIDAL ACTION 



For auxin materials to be effective in killing plants, the materials 

 must succeed in carrying out three steps. They must first enter the 

 plant, then be translocated through the plant, and finally carry out the 

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