DETECTION AND QUANTITATIVE DETERMINATION 39 



under controlled conditions, e.g., at a temperature of 25°C. and 

 90 per cent relative humidity. The procedure ordinarily is car- 

 ried out in phototropically inactive light. 



When the coleoptiles have attained a length of 25 to 40 mm., 

 the tip is removed, and the object to be tested is placed on one side 

 of the cut surface of the coleoptile stump. Small portions of 

 plant organs may be tested for the presence of growth substance 

 by applying them directly to the decapitated coleoptiles, or they 

 may be placed on moist agar into which the hormone will diffuse 

 from the plant part. The agar is then cut into standard-sized 

 blocks and appHed unilaterally to the decapitated coleoptiles. 



If a minute amount of a growth hormone is present in the 

 object to be tested, it brings about increased growth on the side 

 of the coleoptile to which it is applied, thus producing a curva- 

 ture; this curvature may be taken as an indication of the presence 

 of a growth hormone. Curvatures, over a certain range, are 

 proportional to the concentration of the hormone present in the 

 object being tested. It is upon this fact that quantitative 

 determinations depend. 



For purposes of demonstration, the coleoptile tips removed by 

 decapitation may be replaced on one side of the cut surface with a 

 little gelatin. Curvature will follow. Numerous other plants 

 have been used in diverse ways for demonstrating the presence 

 of growth substances. 



