64 



PHYTOHORMONES 



and of Helianthus decreases steadily during germination 

 from a relatively high value in the husked seeds almost to 

 zero. These observations disagree so completely with all 

 those above that they must be regarded as doubtful. 



B. The Auxin Precursor 

 The role of the seed in the formation of auxin in the other 

 parts of the young A vena seedling has been partly elucidated 



bySkoog(1937). If 

 the seed is removed, 

 auxin production in 

 the coleoptile tip 

 decreases steadily, 

 and if the plants are 

 decapitated about 

 12 hours later, re- 

 generation of auxin 

 production in the 

 stump does not oc- 

 cur. The plants, 

 however, as shown 

 by their sensitivity 

 to applied auxin, re- 

 main normal. The 

 auxin secreted by 



10 hrs. 



Fig. 29. Change of curvature of Avena coleop- 

 tiles with time. Curve A, indole-3-acetic acid, 0.1 

 mg. per hter, in agar; curve B, indole-ethylamine, 

 1 mg. per Hter. After a prehminary positive cur- 

 vature (cf. Figure 20), curve B shows a strong 

 negative curvature due to transformation of 

 indole-ethylamine into indole-acetic or indole- 

 pyruvic acid. 



the coleoptile tip does not come directly from the seed, as has 

 been claimed by Pohl (1935, 1936), because if so, it should be 

 possible to intercept it by removing the coleoptile and placing 

 an agar block upon the stump. Skoog could obtain no auxin 

 in this way, even with very young plants. This was shown by 

 the failure of such blocks to produce any curvature on Avena 

 within two hours. If, however, the blocks are left on the test 

 coleoptiles for 2 to 6 hours after application, they give rise 

 to distinct auxin curvature; this indicates that some sub- 

 stance (precursor) is being converted into auxin within the 

 tissues of the test plant (see Figure 29) . ^ For such tests, de- 



1 This may explain the observations of Beyer (1928a) who found that cyhnders 

 from 3 mm. below the tip of the coleoptile, if placed one-sidedly upon the stump, 



