36 



GROWTH HORMONES IN PLANTS 



stump is measured after 4 days and is expressed in percentage of 

 the original thickness. 



A Vicia unit of effect is defined as that degree of callus forma- 

 tion after 4 days which corresponds to a 10 per cent increase in 

 thickness of a decapitated epicotyl of Vicia. The Vicia unit 

 corresponds to the effect of TSy 3-indole acetic acid in 1 g. of 

 paste. 1 The concentration of the unknown paste can be deter- 

 mined by comparing its callus-forming effect with the standard 



8 16 32 



Relative concentration 

 Fig. 18^1. — Relationship between concentration of 3-indole acetic acid applied 

 (in lanolin) to epicotyls of Vicia Faba, and per cent increase in their diameter; 

 a of the inset shows the appearance of a swelling caused by a dilute application 

 of growth hormone to the cut apical surface of a seedling stem; h, the effect of a 

 strong application. {After Laibach and Fischnich, 1935a, and Laibach, 1935.) 



curve (Fig. ISA). About 50 plants should be used for a single test. 



The accompanying standard curve was obtained from observa- 

 tions on three series of plants, 280 plants in each series. Accord- 

 ing to the graph published by Laibach and Fischnich, the direct 

 relationship between growth-substance concentration of the paste 

 and increase in thickness holds up to about 3 Vicia units, or 30 per 

 cent increase in thickness of the stump due to callus formation. 



Comparison of Avena with other Test Objects. — The Avena 

 coleoptile has proved to be a successful test object in most 



^ One gamma (7) = H 000 milligram. 



