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CHAPTER XI 



CHEMISTRY IN RELATION TO GROWTH 



Some Notions on Concentrations 



Inhibition and Stimulation 



The grovyth-substances can act in extremely high dilutions. 

 By the Avena test, auxin-a has been shown to be active at a 

 concentration of i part in 110,000,000 of water; though 

 ethylene in some circumstances is from 100 to 600 times more 

 potent for equivalent molecular weights (Crocker, Hitchcock 

 and Zimmerman (1935), as corrected by Hitchcock and 

 Zimmerman (1935, A) ). 



Acute disturbances not unlike the effects of poisoning can 

 be produced by the synthetic growth-substances in quite small 

 doses ; they are especially pronounced with young seedlings, 

 grown in solution-cultures, and the question has on that 

 account been asked whether it is not misuse of a term to call 

 the synthetics growth-promoting substances (Leonian and 

 Lilly, 1937). It is true that the substances do not always 

 promote growth, even in large plants (Hitchcock and Zim- 

 merman, 1935, A; Pearse, 1936-7A, ) though they may init- 

 iate or control it. The doubt evoked by the toxic phenomena 

 shown by seedlings is probably based on a misunderstanding 

 of results of an unrealized overdosage. 



This contention has received support from the work of 

 Macht and Grumbein (1937) who have considered the 

 question of toxicity of indole-acetic and indole-butyric 

 acids and a naphthalene-acetic acid, from the point of view 

 of the so-called diphasic biological effect familiar to toxi- 

 cologists and pharmacologists. Their work is discussed in 

 more detail below. In the present connexion it will suffice to 

 quote a few words from their paper: 



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