Metabolism and mode of action 



external osmotic concentration in the hypotonic region. This fact suggests 

 that the cell elongation of such roots is not due to passive stretching of the cell 

 wall but favours some hypothesis relating active wall synthesis to cell 



OS ov- 



External concn. f1 



0-6 



Figure 3. Elongation of Avena 

 coleoptile sections as a function of 

 external osmotic concentration. All 

 media contain 0-09 M sucrose and 

 mannitol in varying concentration. 

 Incubation time 20 hours. After Ordin 

 et al. (1955). 



enlargement. The relations found by Burstrom do not obtain for Avena 

 coleoptile sections. The irreversible component of elongation like the total 

 elongation is an inverse function of external osmotic concentration. In the 



I 



/^^ 



/^v? 



-OS 



-OS 



(a) 



•\. 



\ 



\. 



^.. 



02 01 0-6 08 10 02 01 06 0-8 10 

 flolarltf of mannitol 



Figure 4. Length changes of Avena. coleoptile sections as a function of external mannitol concentration. 

 Incubation for 1-2 hrs under anaerobic conditions, (a) Freshly cut sections, (b) Sections previously 

 incubated for 20 hours in 0-5 M solution. After Ordin et al. (1955). 



case of the Avena coleoptile the greater the turgor pressure the greater the 

 elongation rate. This implies that mere contact between protoplasm and cell 

 wall is not of itself sufficient to support maximum elongation rate. 



In order to ascertain to what extent purely osmotic considerations govern 

 auxin-induced water uptake in the Avena coleoptile it is necessary to have 

 measurements of diffusion pressure deficit (d.p.d.) and of internal osmotic 

 concentration during the elongation process. The simplified method of 



262 



