74 



PHYTOHORMONES 



producing region. This is, however, extremely difficuh for 

 reasons which will become apparent later. Nevertheless, 

 Dolk (1930, mentioned by Went, 1928) practically suc- 

 ceeded in doing this by decapitating Avena coleoptiles, 

 waiting two hours and then decapitating again, when 

 growth came practically to a standstill (see Figure 31). 

 If at this point auxin in agar was applied, growth was at 



first decap. 



second decap. 



100 200 



first decap. 



300 



400 mm. 



second decap. 

 plus auxin 



200 



300 



400 



500 mm. 



Fig. 31. Gro^\^h rates of Avena coleoptiles, in mm. per hour. Upper figure, 

 two decapitations; lower figure, auxin applied after second decapitation. 

 (From Dolk, Diss., Utrecht, 1930.) 



once resumed. The conclusion is that "Ohne Wuchsstoff 

 kein Wachstum," without auxin no growth. It is perhaps 



necessary to mention that the reverse of this statement, 



namely "where there is no growth, there is no auxin," or 



even ''where auxin, there growth" (Fitting, 1936), is by 



no means necessarily true. 



A variant of this experiment was carried out by Went 



(1928). Continuous growth measurements were made on 



basal zones of Avena coleoptiles, and when they were found 



to have just stopped elongating, they were treated with 



auxin; as a result growth was restored, although slowly 



(c/. Figure 32). 



