GROWTH SUBSTANCES FOR NORMAL GROWTH 109 



tion of growth substance led Heyn to conclude that changes in 

 elastic extensibility are not the primary cause of growth. Soding 

 (1931, 19326, 1934) came to the same conclusion. He investi- 

 gated the extent to which curvature, produced by the unilateral 

 application of a tip to a decapitated Avena coleoptile, can be 

 removed by plasmolysis. It was found that while the early part 

 of the growth curvature persists almost entirely, the later part 

 disappears to some extent. Soding concluded from this that the 

 first step in growth is not reversible; hence it cannot be brought 

 about by a difference in elastic extensibility of the cell walls or by 

 changes of turgor pressure or of osmotic concentration. This 

 conclusion is sound as far as can be judged at present. 



2. GROW'TH SUBSTANCES AND PLASTIC EXTENSIBILITY. Accord- 

 ing to Pfeffer's original idea, the limit of elasticity is not reached 

 in turgor extension, thus excluding plastic growth. More recent 

 investigations by Overbeck (1926), Pringsheim (1931), and Heyn 

 (19316) have shown that saturation with water produces an 

 overextension of the cell wall. The question arises whether this 

 plays a part in normal growth. Went (1928a) suspected that 

 the effect of growth substances involves an increase in the plastic 

 extensibihty of the cell walls; Heyn (1930, 19316, 1934c) came to 

 a similar conclusion. The latter determined plastic extensibility 

 according to the method outlined above. It was found that the 

 amount of bending was far greater when the coleoptiles were 

 treated with growth substance before the experiment (see Heyn 

 and van Overbeek, 1931). In coleoptiles which were not growing, 

 plasticity was increased following the addition of growth sub- 

 stance. Similar results were obtained from experiments with 

 hypocotyls of Lupinus (Heyn, 19316, 1934a) (Fig. 36). He 

 found, furthermore, that the turgor pressure itself is sufficient 

 to produce an irreversible increase in the surface of the cell wall 

 when the growth substance has increased the plasticity suffi- 

 ciently. Heyn concluded, therefore, that the primary cause of 

 growth is the plastic expansion of the cell wall. This brings 

 about a decrease in the thickness of the cell wall, which is com- 

 pensated for by an increase in substance. Gessner (1934) has 

 shown, also, that a close relationship exists between growth and 

 wall extensibility in Helianthus. He concluded that change in 

 wall extensibility is the cause for a change in the rate of growth, 

 not the result of it or an accompanying phenomenon of it. 



