Hoagland — 52 — Plant Nutrition 



bolic activities of the cell by which cellular energy 

 was made available for ion transport. Effects of light 

 and temperature on the process of ion accumulation 

 were in accord with this conclusion. Another interest- 

 ing and significant fact was that the Nitella cells, as 

 long as they remained uninjured, maintained an ap- 

 proximately constant hydrogen ion concentration in 

 the vacuolar sap at the value of pK 5.2, even when 

 the outer medium varied in its reaction over so wide 

 a range as nearly 5 pB. units. At the same time this 

 sap, chemically considered, was not highly buffered. 



I shall not dwell on these early studies on Nitella, 

 useful as they were at the time they were made. Rather 

 I should like to interject here a brief discussion of 

 some recent experiments on Nitella cells. After many 

 years investigators are in a position to return to re- 

 search on large cells of this kind with the aid of radio- 

 active isotopes as tracers. I believe that my colleague, 

 Dr. S. C. Brooks, was the first to utilize isotopes in 

 experiments on Nitella cells. He has carried on studies 

 especially with radioactive rubidium (Brooks, 1940). 

 In these studies a rough separation was made of the 

 cell wall, the protoplasm, and the vacuolar sap so that 

 these several parts of the cell system could receive 

 separate examination. The rubidium was found to 

 enter the protoplasmic phase extremely rapidly and 

 reached there a high net concentration, much higher 

 than that of the external solution. Entry into the 

 vacuole was, however, very slow and during the ex- 

 perimental periods the concentration of rubidium in 

 the vacuole did not reach nearly so high a value as 

 that of the protoplasm. 



Mr. Broyer and I (1942) have also made experi- 

 ments on Nitella with radioactive isotopes, from the 

 point of view of our own problems, especially as they 

 are related to the investigation of salt absorption by 

 roots, which I shall discuss presently. We have found 

 that radioactive bromide can accumulate at first in the 

 protoplasmic phase of Nitella cells, but subsequently 



