GROWTH SUBSTANCES FOR NORMAL GROWTH 111 



during growth may be brought about by a softening of the 

 plastic intermicellar substance. 



3. GROWTH SUBSTANCES AND INTUSSUSCEPTION. If plastic 



extensibiUty is not the primary cause of growth, as Soding con- 

 tends, the only possibility remaining is that of intussusception. 

 Pfeffer tried to show that this is of decided importance in the 

 growth of the cell wall. He demonstrated that growth occurs in 

 the root only when the turgor pressure is compensated by an 

 opposing pressure; the growth force must be supplied, therefore, 

 by intussusception. 



Soding's hypothesis regarding the growth of the cell wall is 

 this: A viscous intermicellar substance embedded between the 

 micellae can be plastically extended by turgor. This extension, 

 although a part of the growth process, is of only secondary 

 significance. The essential step consists of the addition of new 

 glucose particles from the intermicellar substance to the micellae. 

 At the same time, new intermicellar substance is being formed 

 from the protoplasm, and this increase of matter in the cell wall 

 necessarily is associated with cell elongation. The forces effective 

 in this process are so great that turgor pressure is of secondary 

 significance. 



It is clear that we have not yet been successful in determining 

 the primary cause of growth, and it seems best to leave this 

 question open for the present. With regard to the effect of 

 growth substances upon wall growth, there are apparently two 

 possibilities : They influence either plastic extensibility or growth 

 by intussusception. 



The first possibility is upheld by Heyn. His view is sum- 

 marized in the following statement : 



In view of the evidence, it might be expected that the protoplast acts 

 as a mediator in the process of growth-substance activity on the wall. 

 Therefore, the protoplast may be important for the supply of growth 

 substance to a particular portion of the wall, or growth substance may 

 have an effect not directly upon the cell wall but upon the protoplasts 

 which subsequently produce changes in the condition of the wall. 



The other possibility — that the growth substance is concerned 

 in growth by intussusception — is upheld by Soding. He assumed 



. . . that the hormone has an effect upon the intermicellar substance 

 directly or indirectly (perhaps through the mediation of the protoplasm) 



