92 Mineral Nutrition of Plants 



various ions were characteristically different. This fact will be apparent 

 from Figure i, which gives the curves for Rb+, Sr++, FLP0 4 ~, and I - . 

 Also, it was found that in each case the isotopic exchange curve was 

 characteristic of the living root. With ether-killed tissue, all the ele- 

 ments were released very rapidly by isotopic exchange. This point is 

 illustrated in Figure 2 for the case of I~. 



50- 



I 

 I 



^ 



5 



10 



TIME IN MINUTES 



15 



20 



Figure i. Graphs showing the release at O C. of P 32 , I 131 , Sr s5 , and 

 Rb S6 by apical root segments in exchange for the inactive isotopes in the 

 culture solution. Apical segments of roots (2 cm.) containing the absorbed 

 radioelements were placed in 0.005 N solutions of H 2 P0 4 ~, I - , Sr++, 

 and Rb+, and counted at selected intervals to determine the loss of the 

 radioactive elements by exchange. The percentage of the initially absorbed 

 activity remaining in the root is plotted against time.— Overstreet and 

 Jacobson (38), and Jacobson and Overstreet (23). 



A third point of interest was the fact that, at least in the neighbor- 

 hood of o° C, the different elements showed rather widely different 

 longitudinal absorption patterns in the root tips. That is, for example, 

 regions in the tissue which absorb H 2 P0 4 - very rapidly do not neces- 

 sarily absorb I - rapidly. This finding is illustrated in Figure 3. 



In general, the isotopic exchange experiments seem to indicate that 



