124 PHYTOHORMONES 



the bud at first is largely due to differences in elastic stretch- 

 ing on the two sides, presumably caused by osmotic changes ; 

 the subsequent straightening is accompanied by increase 

 of plasticity on the more rapidly growing side. 



Friedrich (1936) has confirmed the findings of Warner 

 (1928) and Metzner (1934) that upon geotropic stimulation 

 the lower side of wilted Helianthus seedlings has a higher 

 content of reducing sugars than the upper. Not only is it 

 doubtful whether this difference is established as soon as 

 is the growth difference, but also no such difference could 

 be detected after application of indole-acetic acid. However, 

 Friedrich ascribes this to the different beha\dor of this 

 substance and the auxin a probably present in the plant 

 (see VII D), and concludes that the difference in sugars is 

 caused by the difference in auxin a content between the 

 upper and lower sides. The increased sugar concentration 

 is then considered to cause the increased rate of growth on 

 the lower side. Unfortunately the evidence is as yet too 

 weak to substantiate this suggestion. 



D. Structure of the Growing Cell Wall 



To explain the changes in plasticity and elasticity during 

 growth, attempts have been made to take the structure of 

 the cell wall into account. Heyn (1933), on the basis of 

 stretching measurements with the automatic load-extension 

 apparatus of Denham and Lonsdale (1928), has concluded 

 that the cell wall of Avena consists of at least 2 layers, the 

 outer of which has much greater extensibility than the inner; 

 on plasmolysis the inner layer becomes wrinkled. Thus, 

 according to conditions, the properties of the wall should 

 be determined mainly by one layer or the other; under 

 natural conditions this is the inner, difficultly extensible, 

 layer and hence the action of auxin would be mainly on 

 this layer. 



Analysis of the cell walls of growing Avena coleoptiles 

 gives the following figures in per cent of the dry weight: 

 cellulose 42 per cent, pectin 8 per cent, hemicellulose 38 per 



