68 



PHYTOHORMONES 



Nielsen, 1924; Went, 1928; Soding, 1929) is at least in part 

 due to the fact that the chloroform method determines the 

 total auxin present at the moment of extraction, while the 

 diffusion method is successful only if there is continued pro- 

 duction. 



It should be noted that Thimann's results bring for the 

 first time e\ddence that the auxin in the plant is present 

 in at least two different forms : the free mov- 

 ing auxin, which can be collected by the 

 diffusion method, and the bound auxin, 

 which is obtained, together with the free 

 auxin, by extraction of the tissues (see also 

 VIII F and Boysen Jensen, 1936a). 



Recently, however, the presence of auxin 

 in the lower parts of the coleoptile has been 

 confirmed by the diffusion method, this 

 being made possible by the development of 

 more sensitive tests. Thus, Soding (1935a) 

 using his Cephalaria test, detected auxin 

 diffusing from lower parts of green coleop- 

 tiles in amounts approximating 25 per cent 

 of that obtainable from the tip. However, 

 the fact that he also obtained, by diffusion 

 from the lower zones, auxin in amounts 

 sufficient to be detectable on Avena, which 

 no previous workers have been able to do, 

 suggests that his green coleoptiles are richer 

 in auxin than etiolated ones. By using the 

 de-seeded Avena test, which gives increased 

 sensitivity (c/. Ill C 4) Skoog has obtained 

 auxin from the lower zones of truly etiolated 

 coleoptiles by diffusion. The yield was less 

 than 5 per cent of that from the tips. Small amounts of auxin 

 could also be obtained by diffusion from the primary leaf. 

 Dollfuss (1936) has obtained small amounts of auxin by 

 diffusion from the zone 2-4 mm. below the tip of Sorghum 

 coleoptiles. 



Fig. 30. Auxin 

 distribution in Ave- 

 na seedling. Right- 

 hand figures, auxin 

 in plant units per 

 mm. (or AE per 

 2.5 mm.) of coleop- 

 tile or root; 

 left-hand figures, 

 lengths of the ex- 

 tracted sections. 

 (From Thimann, 

 /. gen. Physiol. 18: 

 23-34, 1934.) 



