114 



GROWTH HORMONES IN PLANTS 



obtained also in neutral tap water covered with paraffin oil. 

 According to Strugger, the renewal of growth is brought about by 

 acidification ; in the case of tap water and paraffin, by acidification 

 resulting from the lack of oxygen. He summarized this view of 

 growth processes in the following scheme: 



The primary impetus in growth by elongation is: 



An acidity-gradient present within the tissue 



A plasmatic gradient as a result of the above 

 (change in ionization of the protoplasmic colloids) 



Change of swelling ' 



Nonosmotic increase of inter- 

 nal pressure by the pressure 

 of swelling 



1. Elastic extension 



Becomes fixed for the greater 

 part, by active growth of the 

 membrane. The possibility 

 of overextension exists to 

 some degree 



Change in characteristics of 



the membrane by the 



protoplasm 



Change in volume, the turgor 

 pressure remaining the same? 



2. Plastic extension 



Intervention of suction-force 

 regulation and of osmotic 

 regulation 



Growth of the cell wall? 



Strugger (1933) interprets the significance of growth substances 

 for growth by elongation as follows: "It is clear, therefore, that 

 growth substance obviously does not influence the protoplasm 

 and membrane directly but rather that it regulates the acidity 

 gradients in the course of metabolism and therefore the course and 

 intensity of stretching growth." 



Following up the work of Strugger, Bonner (1934a) showed that 

 in the Avena coleoptile the effect of acid on growth is not a direct 

 effect upon the physical properties of protoplasm. His study 

 indicates that the "acid-growth reaction" consists primarily in 

 setting free a certain amount of active auxin acid from the inac- 

 tive salt form of the growth hormone already present in the 

 coleoptile. The growth stimulation by acidification is propor- 

 tional to the concentration of the free auxin acid. It had been 



