THE MECHANISM OF THE ACTION 



123 



this account he has concluded that changes in neither plas- 

 ticity nor elasticity can be the primary cause of growth; 

 growth, he considers, is therefore caused by active intus- 

 susception. Thus, according to Soding, growth must be 

 accompanied by an increase in dry weight of the walls. 

 However, other experiments indicate that this is not so. 

 Heyn and van Overbeek (1931) found that the increase in 

 plasticity and the increase in length caused by auxin take 

 place also if the plants are kept at 4°. Bonner (1934a) 

 therefore measured the diy weights of the walls of coleoptile 

 sections in auxin and in water, and found that at 25° elonga- 

 tion was paralleled by an increase in weight, but at 2° there 

 was considerable elongation but no increase in weight (see 

 Table IX). 



TABLE IX 



(After Bonner, 193-4a) 



He further found that, if placed in 1 per cent fructose solu- 

 tion, the increase in weight, i.e. the cell wall formation, 

 exceeded the rate of growth. From these measurements 

 the processes of wall thickening and elongation would 

 appear to be independent, as Heyn concluded on other 

 grounds. Thus the evidence is consistent with the view 

 that auxin allows growth by increasing the plasticity of the 

 wall, rather than that it causes active intussusception of 

 wall material. The exceptions remain, but there are a num- 

 ber of possible explanations for them which will require 

 further investigation. The work of Amlong on roots appar- 

 ently provides definite contradiction (see IX D) . Zollikofer 

 (1935) has studied the curvatures of the flower-stalks of 

 Tussilago and Papaver during the floral movements, of 

 which unfortunately the auxin relations are not known. 

 Her measurements, however, suggest that the nodding of 



