APPLICATIONS OF ISOTOPIC INDICATORS 937 



EARLY BIOLOGICAL APPLICATION 



In contradistinction to the animal body, the uptake of mineral con- 

 stituents by the plant is not followed by a loss of such constituents, 

 and it was formerly considered that the ions taken up by the roots 

 of the plant did not migrate in the opposite direction at all. The appli- 

 cation of isotopic indicators, however, has shown that this is not the 

 ease. Ions taken up by the plant can be removed by an exchange process 

 under the action of other ions present in the soil or in the nutrient 

 solution. It was already found in 1923 that minute amounts of lead, 

 labelled by the admixture of the lead isotope thorium B, when taken 

 up by the roots of V icia J aba, could to a large extent be removed by an 

 excess of non-labelled lead added to the nutrient solution^"^\ Most 

 other ions were found to be much less effective in removing the labelled 

 lead ions from the plant. 



In recent years, the behaviour of essential constituents of plants 

 has been investigated, making use of artificial, radioactive ions as indi- 

 cators ; similar results were obtained. Mullins and Brooks^^^^ placed 

 cells of Nitella corovata first in a solution containing radioactive potas- 

 sium and later in solutions of different chlorides. Sodium and lithium 

 were found to be much less effective in removing labelled potassium 

 than potassium itself, whereas rubidium was more effective. Jenny and 

 OvERSTREET^-*^ and Broyer and Overstreet*-^^ found that ionic 

 exchange could take place during periods of, and under conditions 

 favourable for, active solute uptake. It was also observed^^^^ that for 

 each six phosphate ions taken up by the roots of growing wheat seed- 

 lings, one phosphate ion migrated from the roots into the nutritive 

 solution. 



Early in the twentieth century, the application of bismuth compounds 

 in syphilis therapy came to the fore. This induced Christiansen, 

 LoMHOLT and Hevesy^^^^ to investigate the absorption, circulation 

 and excretion of labelled bismuth preparations. Lomholt^^"^ succeeded 

 in showing that, of all the preparations investigated, bismuth hydroxyde 

 suspended in oil was most suitable for therapeutic application. 



Successes achieved by Blair-Bell in cancer therapy, using lead com- 

 pounds, induced the investigation of the partition of labelled lead com- 

 pounds between normal and tumourous tissue. ^^^) Though this work 

 gave a negative result, it nevertheless proved to be of great importance 

 in the future development of isotopic indicators. It was in the course 

 of these investigations that Schoenheevier became familiar with the 

 method of isotopic indicators, which he applied several years later with 

 such great success in the study of fat and protein metabolism and of 

 numerous related problems. Never were more beautiful investigations 

 carried out with isotopic indicators than those of the late Professor 



