THE MECHANISM OF THE ACTION 127 



in the growth process is probably sUght, and would take 

 much of the force out of the above arguments. It thus 

 seems impossible at present to arrive at a definite conclusion 



t t 



a b c 



Fig. 42. Scheme of the micellar structure of the growing cell wall, a, orig- 

 inal structure; b, the same after 50% stretching by artificial tension; c, the 

 same after 50% elongation bj- growth — there is no change in orientation of 

 micelles, as there is in b, but there is sliding of the points of attachment. New 

 micelles will become inserted in the spaces. (From Frey-Wyssling, Protoplasma 

 25: 261-300, 1936.) 



as to the constituent of the cell wall on which auxin exerts 

 its ultimate effect. 



E. The Intermediate Processes 



While the ultimate effect of auxin is thus upon the cell 

 w^all, we have little information as to the way in which this 

 effect is brought about. There is good e\ddence that there 

 must be a number of intermediate stages. This is clearly 

 indicated by comparison between the amount of auxin 

 applied to the Avena coleoptile and the total amount of 

 straight growth which it brings about (Thimann and Bon- 

 ner, 1933). From determinations of the growth, the amount 

 of auxin entering, and the analysis of the cell wall material, 

 it was calculated that at 25° one molecule of auxin brings 

 about the laying down of 3 X 10^ hexose residues in the 

 form of cellulose. Similar large ratios hold for pectin, hemi- 

 cellulose, and protein. At 15° more auxin is required for 



