108 GROWTH HORMONES IN PLANTS 



substance which can take place later either by intussusception or 

 by apposition. According to this hypothesis, then, a change in 

 the rate of growth can result from changes in either turgor pres- 

 sure or plastic extensibility. 



3. ACTIVE GROWTH. — In the third place, the active growth of 

 the cell wall has been considered as the primary step, a theory 

 propounded by Pfeffer (1904). According to this hypothesis, 

 new particles are laid down between those already present in the 

 wall. Although the turgor pressure is significant in so far as it is 

 necessary to keep the protoplasm in connection with the cell wall, 

 it is not regarded as the source of the energy; rather, this is the 

 result of forces that are active in the secretion of new ingredients 

 for the cell wall. The first step in growth is concerned, there- 

 fore, with an increase in the substance of the cell wall; it is not a 

 reversible process and is not influenced by changes in turgor 

 pressure. 



Discussio7i of Hypotheses. — A consideration of the influence of 

 growth substances upon these processes may make it possible 

 to determine the primary step in growth. 



1. GROWTH SUBSTANCES AND ELASTIC EXTENSIBILITY. HcyU 



(19316) and Soding (1931), at practically the same time, found 

 that a far-reaching parallel exists between growth and elastic 

 extensibility in the oat coleoptile, and Soding made the same 

 observation on flow^er stalks. Elastic extensibility remains 

 practically constant during the normal growth of the coleoptile, 

 but it decreases when growth is retarded either by decapitation or 

 by complete removal of the coleoptile from the seedling. In 

 decapitated seedlings, the decrease in extensibility may be 

 partly annulled by the addition of growth substance. From 

 this it might be concluded that elastic extension is the primary 

 step in growth and that growth substances make growth possible 

 by increasing the elastic extensibility. Heyn, however, did not 

 interpret his results in this way. The elastic extensibility of 

 excised coleoptiles (removed from the seed and not growing), 

 with growth substance applied, in some of the experiments was 

 greater than the extensibility of excised coleoptiles without 

 applied growth substance. The difference was not great, how- 

 ever, and in both cases the elastic extensibility was far less than in 

 growing coleoptiles. The fact that extensibility in nongrowing 

 coleoptiles cannot be increased to any great extent by the addi- 



