Extraction and Measurement Techniques 15 



ties obtained by diffusion, and extraction procedures must be used to 

 this end. Methods of detecting the presence of inhibitors are discussed 

 in the section dealing with the Avena test. 



Solvent Extraction 



A second method of obtaining the growth hormone from plant 

 tissues is by means of solvent extraction. Early studies using this tech- 

 nique employed chloroform as the solvent (Thimann, 1934). It was 

 found that if the plant material were acidified before extraction, larger 

 quantities of auxin were obtained. A serious drawback to the use of 

 chloroform, however, is the slow accumulation of a toxic substance, 

 perhaps an auxin inactivator, thought to be chlorine (Thimann and 

 Skoog, 1940). The most satisfactory solvent has been found to be 

 diethyl ether (Boysen-Jensen, 1936). The presence of spontaneously- 



8 7 6 5 4 3 2 



PH 



Fig. 3. The effect of pH on the partition of indoleacetic acid from water into 

 ether (Gordon and Nieva, 1949). 



formed peroxide in the ether will destroy easily oxidizable auxin, so 

 before use the ether should be redistilled over ferrous sulfate and 

 calcium oxide in a small amount of water. The redistilled peroxide- 

 free ether is made much more effective as a hormone solvent if a small 

 amount of water (5%) is added. 



Many other solvents have been used for auxin extraction. Ethyl 

 alcohol has been used, but yields smaller amounts than does ether. 

 Water has been used (Gorter, 1932), but inactivation of much of the 

 auxin may take place during the procedure. Terpstra (1953) has 

 nearly eliminated such inactivation during water extraction by adding 



