40 PHYTOHORMONES 



The age and length of the plant at the time of its use 

 are of great importance in determining the resulting curva- 

 ture. As a matter of convenience plants shorter than 15 mm. 

 cannot easily be used, although their sensitivity is high. 

 Between 15 and 30 mm. above the guides of the holders, 

 there is little variation in sensitivity. In older plants, 

 however, the sensitivity falls off rapidly, due to the so- 

 called ''aging" studied by du Buy (1933; see V^). 



In testing impure preparations, it is an advantage of 

 the Avena test that it is very little affected by the 

 presence of salts, sugars, amino-acids, etc., all of which 

 are substances ordinarily influencing growth. This is be- 

 cause, through the rapid transport of auxin from the cut 

 surface to the reacting zones, a rigid selection occurs. Thus 

 M/1000 CUSO4 in the agar, a concentration which is ordinarily 

 highly toxic to plants, has no effect on the curvature. Ni 

 and Mn salts in the same concentration are equally without 

 effect, although all these metals reduce or inhibit the auxin 

 curvatures of immersed plant parts such as pea stems {u; 

 cf. also IX B). 



The effect of pH depends upon whether the auxin solu- 

 tion is buffered or not. Nielsen (1930) found that between 

 2,4 and 9.6 the pH of the agar mixture (unbuffered) does 

 not affect the curvature. Dolk and Thimann (1932), how- 

 ever, showed that auxin solutions buffered at pH 7 give 

 much smaller curvatures than at pH 5 (cf. VIII F). Kogl 

 and Haagen Smit (1931) also advise the acidification of 

 solutions before testing, but Jost and Reiss (1936) found no 

 change in activity on adding varying amounts of acetic acid 

 to pure auxin solutions, doubtless because these are not 

 buffered. 



If the tests are properly carried out, the curvatures 

 obtained will be linearly proportional to the concentration 

 of the active substance, within well-defined limits (see 



