DETECTION AND QUANTITATIVE DETERMINATION 23 



the end of which time the degree of curvature may be measured, 

 as described later. The temperature of the laboratory at 

 Utrecht is maintained at 22°C. and at a relative humidity of 

 90 to 95 per cent. If it is desirable to work with greater numbers 

 of test plants, the coleoptiles may be decapitated a second time 

 60 or 90 minutes after the first decapitation (van der Weij, 1931). 

 The time schedule mentioned is important only for the quantita- 

 tive work discussed later. 



Growth Curvature of the Coleoptile. — The appearance of a curva- 

 ture after the application of the unknown material is evidence for 

 the presence of growth substance. A negative curvature (bend- 

 ing away from the side with the applied object) indicates a 

 growth-promoting substance, whereas a positive curvature shows 

 that growth-retarding substances are present (Fig. 1) (Stark). 

 Quantitative methods for determining the amount of growth- 

 promoting substances present are described in the last part of this 

 chapter. If no growth curvature occurs, it means either that the 

 object being tested contains no growth substance or that there is 

 some factor which disturbs either the transfer of the growth 

 substance or its effectiveness after entrance into the Avena coleop- 

 tile. Mention of cases in which the latter is true is made at the 

 end of the chapter. 



Preparation of the Material to Be Tested. Direct Application 

 of Plant Parts to the Decapitated Avena Coleoptile. — Many plant 

 organs or parts of organs such as coleoptile tips, coleoptile 

 cylinder segments, root tips, etc., may be tested for the presence 

 of a growth substance by placing them (Figs. 6 and SB) unilat- 

 erally upon decapitated Avena coleoptiles (see Paal, 1918; 

 Stark, 19216; Nielsen, 1924). This is the simplest method and 

 the first one employed in attempting to detect growth substances 

 in some new object. 



Application of Material to Be Tested in Agar and in Lanolin. — 

 Frequently it is not feasible or desirable to apply the plant parts 

 to be tested directly to the Avena coleoptile. Other methods 

 depend upon the fact that growth substances are soluble in water, 

 alcohol, and ether, and that they are stable in agar and lanolin 

 (wool-fat) paste. With some of these methods it is possible to 

 concentrate the extract obtained from a quantity of plant material 

 before making the biological test and thus demonstrate that 

 growth substance is present even though only in small amounts. 



