122 



PHYTOHORMONES 



growth rate are due to changes in plasticity, while the 

 elasticity changes only as a result of growth. Increased 

 plasticity allows increased elongation, the actual force 

 causing elongation being the turgor. 



The elasticity can be independently measured by plas- 

 molyzing the plants after they have undergone auxin curva- 

 tures (Soding), the decrease in curvature giving that part 

 of it which was purely elastic. Correspondingly, normal 



30- 



-»- growth rate 

 — o— plasticity 

 -•- elasticity 



^ 5 hours 



after decapitation 



Fig. 40. Changes in growth rate (dots), plasticity (circles), and elasticity 

 (crosses) of an Avena coleoptile after decapitation. Ordinates, arbitrary units. 

 (Data of Heyn, 1932.) 



plants can be plasmolyzed, when the decrease in length 

 gives that part of the straight growth which was purely 

 elastic (Heyn, 193 lo). Further, though turgid coleoptiles 

 show httle stretching, they show appreciable bending if 

 they are supported horizontally at one end and riders are 

 placed on the other. Both of these techniques give results 

 similar to the above; the auxin curvatures are almost 

 entirely due to increase in plasticity. 



The flower-stalks of a number of plants behave in essen- 

 tially the same way (Soding, 19325; Heyn, 19346), but 

 Soding found that while decapitation of these caused a de- 

 crease in growth it caused little change in plasticity. On 



