30 



GROWTH HORMONES IN PLANTS 



1 /50,000 WAE per hour. However, in practice if the d value is much greater 

 than 0.5 mm., the direct relationship between d and growth-substance con- 

 centration (characterized by d values of 0.1 mm. or less up to 0.5 mm.) no 

 longer holds. As a unit of the amount of growth substance given off into an 

 agar block, the " tip hour " has also been proposed. The amount that comes 

 from an Avena tip in one hour produces a curvature of about 15 deg. and 



A B 



Fig. 13. — Unilateral application of agar blocks containing growth hormone 

 to the cut surface of Avena coleoptiles, either in contact with a vascular bundle, 

 A, or parenchyma, B. (After Laibach and Kornmann, 19336.) 



corresponds therefore to about 1.5 AE (see duBuy and Nuernbergk, 1932). 

 (For a definition of AE see p. 33.) 



Wenfs Quantitative Method. — This method involves a deter- 

 mination of the angle of curvature of the decapitated coleoptile 

 on which has been placed the object to be tested. The measure- 

 ment is made by means of a protractor, equipped as shown in 

 Fig. 14. The concentration of growth substance necessary to 

 bring about a curvature of one or more degrees is designated in 

 various ways, as discussed on page 32. The method has been 

 modified slightly by several workers since 1928, and the current 

 procedure, assuming that the test plants have reached the proper 

 size, is as follows: 



