THE MECHANISM OF THE ACTION 



131 



has fallen to about one half (see p. 128), the curvatures 

 produced by acid are also reduced to about one half. It is 

 still more clearly borne out by the experiments of Thimann 

 (19356). He immersed coleoptile sections in acid buffer, 

 which caused temporary growth acceleration: when this 

 had ceased, immersion in fresh acid had no effect because 



100 



Fig. 43. Activity and dis.sociation of the weak acid, auxin. A, theoretical 

 dissociation curve of auxin, pK = 4.75; B, acid curvatures (in degrees) ob- 

 tained by peehng epidermis of Avena coleoptiles on one side, and placing in 

 acid buffers; C, same data plotted as per cent so as to meet curve A at pH 6.1. 

 White circles are the data of C corrected to internal pH in the cells, showing 

 close agreement with the dissociation curve A. (From Bonner, Protoplasma 21: 

 406-423, 1934.) 



all the auxin in the sections had been used up, but immersion 

 in auxin gave good growth. 



Thus it is clear that the action of acid is through its effect 

 on the auxin in the plant, and there is no evidence that the 

 colloidal properties of protoplasm control growth in the 

 way assumed by Strugger. 



It has been shown in IV A that there is good evidence for 

 the presence of at least 2 forms of auxin in the plant — one 

 form free-moving, and obtainable in agar by the diffusion 

 method, the other bound in some way in the cell, and 

 obtainable only by extraction with solvents like chloroform. 

 Boy sen Jensen (1936a) has suggested that the freely moving 



