52 



PHYTOHORMONES 



1932; Laibach, 1932; Laibach and Kornmann, 1933) and the auxanom- 

 eter of Koningsberger (1922) are also capable of application to auxin 

 problems. 



Quantitative measurements of the effect of auxin on 

 straight growth of coleoptiles have been made by Nielsen 

 (1930) and more extensively by Thimann and Bonner 

 (1933). They decapitated the plants, pulled out the leaf 

 completely, and placed blocks of agar containing auxin 

 symmetrically on the cut surface. Correction had to be 

 made for the growth of control plants treated with plain 



agar. Thimann and Bonner 

 found that the extra growth 

 caused by the auxin was 

 proportional to the concen- 

 tration in the block up to 

 a limit, above which it in- 

 creased less and less rapidly 

 with increasing concentra- 

 tion (see Figure 24). Thus 

 29 plant units produce 2.66 



20 



40 60 80 

 Plant Units 



100 120 



Fig. 24. Relation between auxin ap- 

 plied and straight growth produced, in 

 decapitated Avena coleoptiles. The ordi- 

 nate is the increase in growth over that j^^^ of coleoptile growth at 

 of untreated controls. (After Thimann ^ iroi i 



and Bonner, Proc. Roy. Soc. B. 113: 2o . At 15 , however, the 



126-149, 1933.) proportionality factor is 



different. This places the Avena test, based on curvatures, 

 on a firm foundation of actual growth increase. It also 

 provides independent confirmation of the view that a 

 second factor limits growth when auxin is in excess (but cf. 

 VIII E). 



If quite short cylindrical sections of coleoptiles are placed 

 in auxin solutions they also respond by an increase in growth, 

 which is approximately proportional, within limits, to the 

 logarithm of the concentration of the auxin (Bonner, 1933). 

 This makes possible a simple auxin determination by 

 straight growth measurements under the microscope, elim- 

 inating the necessity for humidity control. The sections, 

 usually 3-5 mm. long, are cut off from plants of about 

 30 mm. length. Only sections cut at the same distance from 



