112 



GROWTH HORMONES IN PLANTS 



and stimulates the formation of the structure necessary for growth as 

 well as the whole process of intussusception. Since a greater plasticity 

 of the cell wall (dependent upon the intermicellar substance) occurs in 

 actively growing oat coleoptiles, it may be concluded that the increased 

 plasticity is conditioned physically by this "growth mechanism," i.e., 

 intussusception. It could be considered then that growth is prepared 

 for by the hormone and that increased plasticity of the walls follows. 

 This may be seen from numerous experiments by Heyn (1931a, b; 

 1932a, b). According to this hypothesis, the essential function of the 

 hormone is not to make the wall plastic (Heyn) — if this were the case, 

 only a subordinate process in growth would be influenced (at least in 

 flower stalks) — but its role lies in the regulation of intussusception 

 growth. 



Soding presented his views on the mechanism of stretching 

 growth in the following manner: 



stimulative substance: 



Stage of preparation: 



Stage of elongation: 



Main process 

 Hormone 



Preparation for growth 



Elongation by intussus- 

 ception 



Secondary processes 



Increase in wall extensibility 



Plasticity 



Plastic 

 turgor extension 



Elastic 

 extensibility 



Elastic 

 turgor extension 



Although a clear picture of the effect of growth substance must 

 await further evidence (see Bonner and Thimann, 1935), several 

 other points may be mentioned in this connection. It is unlikely 

 that growth substance spreads itself out in a monomolecular 

 layer over the growing cell wall and in this way influences growth ; 

 the computations of Thimann and Bonner (1933; see also Kogl, 

 1933, Mitt. Ill) show that insufficient growth hormone is present 

 to form even a monomolecular film on the growing cell walls; 

 hence, any kind of hormone action through increase of permeabil- 

 ity seems improbable. Further work on this problem by Bonner 

 (19346) has shown that cell elongation may not be attended 

 necessarily by a corresponding amount of wall formation. A 

 given amount of elongation may be accompanied by more than 

 the usual amount of wall deposition, as when the tissue is grown 

 in fructose solution; or practically no wall may be laid down, as 

 occurs at 2°C. He concluded that the increase of wall area 



