414 GROWTH OF PLANTS 



of sampling, and no important change was observed in total carbohydrate 

 (sum of sugar and polysaccharide) . Previous reports of extensive losses of 

 substances from leaves during the interval preceding frost (autumnal 

 migration) were confirmed only for the nitrogenous substances in these 

 species, and, even in this case, only the Viburnimi results should be empha- 

 sized, as the nitrogen losses from lilac were small and of doubtful signifi- 

 cance." 



Concerning the Determination of the Isoelectric Point 



OF Protoplasm 



Attempts had been made to determine the isoelectric point of living 

 plant tissue by placing pieces of the tissue in buffer solutions of low con- 

 centration but of various H-ion concentrations. The buffer solution in which 

 the tissue did not change the pH was interpreted as the isoelectric point 

 of the tissue. It was assumed that in this pH the amphoteric substances 

 (especially proteins) of the protoplasm did not react differentially mth 

 either basic or acid ions of the buffer solution and that consequently the 

 pH of the buffer solution was not changed. It was assumed, on the other 

 hand, that if the buffer solution had either a higher or lower pH than the 

 living tissue the amphoteric substances reacted differentially with basic 

 ions in the first case and acid ions in the second, moving the pH of the 

 buffer toward that of the tissue. 



Youden and Denny ^^^ find that it is the substances that leach out of 

 plant tissues when soaked in the buffer solutions (phosphate, phthalate, 

 and borate) that mainly determine the change m the pH of the solutions, 

 and not the ion absorption from the solutions by the insoluble amphoteric 

 substances of the protoplasm. Tissue soaked in distilled water gave solu- 

 tions of the same pH as the buffer solution that was not changed in pH 

 when the tissue was soaked in it. The substances that leach from the tissue 

 and mainly determine the pH of water or buffer solutions are heat-stable, 

 diffuse through collodion membranes, and are soluble in acid alcohol. 

 This shows that proteins or other colloidal substances do not play an impor- 

 tant role in causing the change in pH. Fig. 166 shows how several plant 

 tissues soaked in buffer solutions of a considerable range of pH values 

 change the pH of the solutions in the direction of the pH of water m which 

 the tissue has been soaked for the same length of time. One exception is 

 mentioned in the description of the figure. 



The authors make the following statements: ^^^^ ^^^^^ °^ p-^^^'" ^■^- ^- ^ 

 "Most of the effect upon the buffer solution was not due to absorption of 

 ions from the buffer by the tissue, but was caused by substances leaching 

 out of the tissue into the buffer. On the acid side of the isoelectric point 

 only about 5 per cent of the change in reaction undergone by the buffer 

 was caused by the tissue itself; on the alkaline side the tissue was more 

 effective, causing about 25 per cent of the change." ^'''-'''^ - p.286-287 in 

 "It is not our purpose to claim that plant tissue does not contain 



C. B. T. I. 



