892 ADVENTURES IN RADIOISOTOPE RESEARCH 



for example, that of potassium. Mullins and Brooks^^^ placed cells of 

 Nitella coronata first in a solution containing radioactive potassium and, 

 later, in solutions of different chlorides. The curves for the loss of radio- 

 active ions from Nitella were found to be exponential and following the 

 equation. 



Table 2. — Replacement of 

 PoTAssiXTM Ions Present in the 

 Protoplasm of Nitella by Other Ions 



where C ^ is the concentration of radioactive ions in the protoplasm, Cq 

 the initial concentration of ions in the protoplasm, k a constant giving 

 the slope of the logarithmic curve, and t the time of immersion of the 

 cell in the solution of inactive ions. The values oilc which were found are 

 given in Table 2; they represent the rate of loss of ions from the cell. 

 As seen in the table, the loss of labelled potassium is greatest in the 

 solution containing non-labelled potassium or rubidium and is minimal 

 in pure water. Sodium and lithium are much less effective in replacing 

 the labelled potassium than is potassium. 



A similar result was thus obtained as in the case of lead. It was further- 

 more shown by Jenny and Oveestreet^^^ and Broyer and Overstreet^^^ 

 that ionic exchange movements may take place during periods of and 

 under conditions favourable to active solute uptake. 



Jenny, Overstreet and Ayers,^*^ who studied the contact inter- 

 change of cations between plant roots and soil colloids, found that barley 

 plants containing radioactive potassium retain this isotope against 

 distilled water, but release it to salt solutions and clay suspensions. 



Hevesy, Linderstr0m-Lang and Olsen^^^ placed sunflower and 

 maize seedlings in culture solutions containing radio-phosphorus. After 



(1) L. J. Mullins and S. C. Brooks, Science 90, 256 (1939); comp. also S. C. 

 Brooks, Trans. Faraday Soc. 33, 1002 (1937); Proc. Soc. Exp. Med. Biol. 38, 

 856 (1938). 



(2)H. Jenny and E. Overstreet, /. Phtjs. Chem. 43, 1185 (1939). 



(^)T. C. Broyer and R. Overstreet, Amer. J. Bot. 27, 425 (1940). 



(*) H. Jenny, R. Overstreet and A. D. Ayers, Soil Sci. 48, 9 (1939). 



<^> G. Hevesy, K. Linderstr0m-Lang and C. Olsen, Nature 137, 66 (1936); 

 139, 149 (1936). 



