Plant Qrowth'Suhstances 



blade over a length of i to 3 cm. If the cut is very far from 

 median the curvatures are smaller. Upon subsequent 

 immersion in the solution the two halves will bend out 

 because of the tissue tension. If no auxin is present in the 

 solution these original outward curvatures do not change 

 any more and are fixed, since they do not go back upon 

 plasmolysis. In the presence of auxin, however, about an 

 hour after immersion (at 25°) the free ends of the two halves 

 will start to bend inwards; the higher the concentration 

 the further they bend. After about 6 hours the final stage is 

 reached, and the originally most rapidly growing zone will 

 be convex at the outside. If the median slit is long enough, 

 the halves will each show an S-shape. Also, these curvatures 

 are not changed by plasmolysis. For the appearance 

 of the curvatures it is only necessary to have the apical cut 

 surface in contact with the auxin-containing solutions. 

 So either the shoots are reversed and placed upside down 

 in a small amount of solution (5 plants together in o-2-i 

 cc.) or they are left floating on the surface of the solution in 

 a shallow dish. If the auxin can enter only through the 

 surface of the radial cut no curvatures appear. 



"The simplest quantitative auxin determination with the 

 pea test is to place a number of pea shoots, apically slit 

 lengthwise over a distance of + 10 mm. in a whole range 

 of dilutions of the solution to be tested ; 5 plants per dilution 

 will be enough. After 12 hours it is determined in which 

 concentration a well-recognizable reaction occurs. In this 

 way either directly or by intrapolation the critical concentra- 

 tion may be calculated with an accuracy of 50 per cent." 



This critical concentration, or difference of concentrations, 

 corresponds to about 480 AE (Avena units), or i'4Xio~5 

 mg. of auxin-a, per ml. For purposes of comparison, it may be 

 mentioned that ordinary human urine contains, per litre, 

 about 2 mg. of auxins and indolyl-acetic acid together, each 

 having an activity of about 50,000,000 AE per mg. Hence, 

 Went's test is sensitive to the amount of growth substance 

 present in about one-hundredth of a millilitre of urine. 



130 



