58 PHYTOHORMONES 



may be decreased by first placing the auxin-producing plant 

 parts on wet filter paper before transferring them to the agar. 



The second method is to extract the plant tissue directly 

 with a solvent (Thimann, 1934; Boysen Jensen, 1936a). For 

 this purpose, water, alcohol, ether, and chloroform have 

 been used. Extraction with water, however, usually leads 

 to inactivation, almost certainly by oxidizing enzymes 

 (Thimann, 1934). Chloroform is the most suitable solvent, 

 but it must be carefully freed from traces of peroxide or 

 chlorine; this is done by freshly redistilling chloroform of 

 anesthetic quality, adding about 0.5 per cent alcohol and 

 preserving in the dark. Ether, if used, should also be made 

 peroxide-free by distilling from a mixture of FeS04 and 

 CaO (method of Garbarini, 1909). 



The first evidence for actual production of auxin in the 

 plant was given by Paal's experiment (1919) with coleoptile 

 tips of Coix. The same procedure, namely, the causing of 

 curvature by placing the tip one-sidedly upon the stump, 

 was carried out on Avena by Nielsen (1924), and on hypo- 

 cotyls of the dicotyledon Helianthus — using the plumula as 

 tip — by Beyer (1925). Proof that auxin is produced in 

 coleoptile tips of Zea Mays was also given by Cholodny 

 (1926). The clearest evidence was, however, given by the 

 straight growth measurements of Soding. His work on 

 Avena (1923, 1925) has been previously discussed (III B). 

 In 1926 he extended these experiments to flower stalks of 

 Cardamine, Cephalaria, and some composites, showing that 

 the auxin-producing organ was in these cases the flower or 

 inflorescence. This was confirmed for Bellis by Uyldert 

 (1927, 1931) who also proved that the effect of applying the 

 inflorescence could be duplicated by agar on which Avena 

 coleoptile tips had stood. Gradmann (1928) showed that 

 auxin can be obtained from the tip of Convolvulus stems, but 

 only to a lesser extent from zones immediately below the 

 tip. Koning (1933) could only obtain small amounts of 

 auxin from buds of Ipomoea, but sections of the stem yielded 

 somewhat more. 



