THE MECHANISM OF THE ACTION 121 



laying in of new particles between the old ones of the wall, 

 or thirdly by the deposition of new material while the old 

 wall is held in an elastically stretched state. Combinations 

 of these processes may also be en\dsaged. 



C. Effect of Auxin on the Cell Wall 



Direct measurements on the effect of auxin upon the 

 stretching properties of the cell walls have been made by 

 Heyn (1930, 1931, 1931a, 1932, 1932a, 1933, 1934, 19346) 

 and by Soding (1931, 1932a, 1934), with similar techniques 

 and essentially the same results. Such experiments can 

 only usefully be made on plasmolyzed material, because 

 turgid cells show very little stretching, as was shown by 

 Soding (1931). The essential technique is to make two 

 marks on the plasmolyzed coleoptile or other object, suspend 

 it in front of a horizontal microscope, and determine the 

 distance between the marks before and after various loads 

 have been attached to its lower end. Part of the elongation 

 produced by such loads will be reversible; the reversible 

 stretching is a measure of the elasticity and the irreversible 

 stretching gives the plasticity. However, according to Buck 

 (1935), there is no difference in principle between these; 

 any sufficiently great and prolonged elastic stretching goes 

 finally over into irreversible, plastic extension. 



The first finding was that after decapitation both the 

 elastic (see also Horreus de Haas, 1929) and the plastic 

 extensibility of the coleoptile decrease. That this decrease 

 is connected with the decrease in auxin content of the plant 

 is indicated by the fact that after regeneration both elas- 

 ticity and, more especially, plasticity increase again. The 

 changes in plasticity follow very closely the changes in 

 growth rate, while the elasticity continues to decrease for 

 a number of hours after regeneration has occurred, and only 

 rises much later. Figure 40, plotted from Heyn's data 

 (1932), shows a typical group of experiments. Similarly, if 

 auxin is applied, the plasticity is greatly and the elasticity 

 slightly increased. Heyn concludes that the changes in 



