130 PHYTOHORMONES 



appear. In presence of HCN it also disappears, so that the 

 inactivation must precede the HCN-inhibited process. 

 Thus the inactivation of auxin — when it is present in low 

 concentration and conditions are favorable — results in 

 growth, but when excess auxin is present, or when condi- 

 tions are unfavorable, most of the auxin disappears without 

 resulting in growth. Thus one stage of the growth process 

 is an inactivation of the auxin. 



The various steps in growth so far known can be sum- 

 marized as shown in Table X. 



F. The Effect of Acid on Growth 



Strugger (1932, 1933, 1934) has suggested a mechanism 

 for growth of an entirely different type. He considers the 

 physical and colloidal properties of the protoplasm, espe- 

 cially viscosity and hydi^ation, as the controlling factors. 

 These will be dependent upon the pH, and correspondingly 

 he finds that if hypocotyls be wounded on one side and 

 immersed in buffer solutions they curve, the amount of 

 curvature varying with the pH. Parallel with a two-peaked 

 curve for the variation of protoplasm \dscosity with pH, 

 he finds a similar two-peaked curve for the variation of 

 curvature (growth) with pH, the minimum for both growth 

 and \dscosity occurring at pH 5.1, which is considered to be 

 ''the isoelectric point of protoplasm." Bonner (1934) has 

 shown, however, that for growth there is no such two- 

 peaked curve: the growth caused by acid (see Figure 43) 

 increases steadily toward higher acidities. If we allow for 

 the fact that the pH within the tissues is not the same as 

 the pH of the buffer, then the curve of acid growth against 

 pH follows closely the pH dissociation curve of auxin 

 (pK = 4.8). This suggests that only the free acid form of 

 auxin has growth-promoting acti\dty, and that the applied 

 acid sets free the auxin acid from the salt form in which it 

 is present (pH of sap = 6.1). That the applied acid is not 

 in itself effective in producing growth is supported by the 

 fact that 2 hours after decapitation, when the auxin content 



