Metabolism and mode of action 



mcreased and the solution is no longer hypertonic. At the same time wall 

 pressure must increase above zero since tissue d.p.d. remains constant. 

 Measurements of internal osmotic concentration were carried out in the 

 present work by the plasmolytic method since the cryoscopic method has 

 been shown by Le Gallais (1955) to be inaccurate and unsuitable. The 

 plasmolytic method consists in examination of coleoptile sections from the 

 d.p.d. determination solutions under the high power of the microscope. 

 Internal osmotic concentration is taken as equal to the external osmotic 

 concentration of that solution in which 50 per cent of the subepidermal cells 

 of the section are plasmolysed. The internal osmotic concentration of the 



Figure 5. Avena coleoptile sections do 

 not lose water immediately they are 

 transferred from aerobic to anaerobic con- 

 ditions, (a) Sections previously incubated 

 in 0-4 M solution in presence of I A A, 

 5 mg/l. (b) Sections previously incubated 

 in 0-5 M solution in presence of lAA 5 

 mgjl. (c) Sections previously incubated in 

 0-6 M solution in presence of lAA 5 

 mgjl. Sections transferred to distilled water 

 at arrow. Lack of elongation indicates 

 death of section. After Ordin eta\. (1955). 



cells of freshly excised coleoptile sections was found to be 0-42 M (corrected 

 to initial volume). The internal osmotic concentrations of sections incubated 

 for 20 hours under various conditions are summarized in Table 2. Sections 



Table 2 



Internal osmotic concentration of Avena coleoptile sections after 20 hrs. Incubation in varied media. 



Initial osmotic concentration 0-42 M 



incubated in lAA but in the absence of other external solute decrease in 

 internal osmotic concentration. This effect is apparently due to dilution of 

 the cell content by the water taken up under the influence of auxin. Sections 

 incubated in mannitol alone show small increases in internal osmotic con- 

 centration over the 20-hour period. The addition of sucrose to the medium, 



264 



