DETECTION AND QUANTITATIVE DETERMINATION 35 



bend inward after about 1 hour of immersion at 25°C. The 

 higher the concentration the greater the bending. The final 

 state of curvature is reached in about 6 hours. In making this 

 test quantitative, the curvatures were evaluated in the following 

 way (Fig. 17) : 



= no auxin reaction, 2 halves concave at the outside over tlieir whole 



length (see figure). 



1 = slight auxin reaction, trace of convexity in limited region. 



2 = definite auxin reaction, ends of both halves approximately parallel. 



3 = fair auxin reaction, ends bent inward (see figure). 



4 = strong auxin reaction, ends just touching. 



5 = very strong auxin reaction, both halves crossing at end. 



6 = exceptionally strong auxin reaction, halves crossing midway or at base 



(see figure). 



Callus-forming Effect of Growth-substance Pastes. — Laibach and 

 Fischnich (1935a) used Vicia Faba as test plants. The seeds 



A B C 



Fig. 17. — Quantitative method (approximate) for measuring the concentration 

 of growth hormone present in solutions. Split sections of pea stems when 

 immersed in growth-hormone solutions exhibit a certain amount of curvature 

 depending upon the concentration of the active substance present. A represents 

 their appearance when immersed in water; jB in a solution containing 30 units per 

 CO.; and C in a solution containing 150 units per cc. (After Went, 19346.) 



are soaked overnight and then planted in 7 cm. pots in coarse 

 sand; they are kept in a greenhouse at a temperature of 25°C. 

 After 10 days the epicotyl is 20 to 25 cm. long, and the plants are 

 ready to use. They are decapitated just below the second node 

 on the tenth day and moved into the darkroom until the end of 

 the experiment. Here the temperature is maintained at 23 °C., 

 and the humidity at 70 per cent. 



To determine the concentration of a growth-substance solution, 

 it is mixed with lanolin paste and applied to the cut end of a 

 decapitated epicotyl. Increase in cross-sectional thickness of the 



