y96 ADVENTURES IN RADIOISOTOPE RESEARCH 



While the above figures show that an easily measurable percentage of 

 the radioactive phosphate ions and, correspondingly, of all phosphate 

 ions added to the solution contained in the inner cylinder passed through 

 the plant into the outer solution, it is difficult to estimate the quanti- 

 tative significance of these figures. During their passage through the 

 plant the active phosphate ions get more and more "diluted" by none- 

 active phosphate ions present in the plant. By this dilution process 

 the sensitivity of the isotopic indicator much increases. Let us assume 

 that we add 10^ radioactive units to the inner solution wdiich contains 

 1 mgm phosphorus; the presence of 1 radioactive unit in the outer liquid 

 will then not indicate the exodus of 1 • 10~^ mgm from the "inactive" 

 roots into the outer solution, only, but a much larger amount of phos- 

 phate in view of the above described increase in sensitivity of the radio- 

 active indicator during the passage of the ^^P atoms through the plant. 



To determine the sensitivity of the radioactive indicator the following 

 procedure is applied. The labelled phosphate found in the outer solution 

 can be assumed to have migrated mainly as free phosphate from the 

 roots into the solution (cf. p. 903); therefore, we isolate the free phosphate 

 from the inactive roots at the end of the experiment and determine its 

 specific activity. Let us assume the specific activity to be 10,000 activity 

 units per mgm P. In this case, the presence of 1 activity unit in the "in- 

 active" outer solution indicates the exodus of lO'^ mgm from the "inac- 

 tive" roots into the inactive solution. 



Amount of phosphorus given off by the roots 



The results of experiments in which the procedure described above is 

 applied are to be seen in Tables 5, 6, and 7. 



Table 5. — Amount of Phosphokus Migrating from 



THE "Inactive" Roots into the Outer (Inactive) 



Nutrient Solittion in the Course of 1 Day. 



Average Temperature 20°C 



Free P content of 



active roots 



inactive roots 



Percentage of '^P added present in 

 the free P of 



active roots 



inactive roots 



67.5 Y 



150 -/ 



4.02 



0.84 



To 1 y of free P present in the inactive roots thus corresponds 0.0056 

 p. c. of the 32p added to the inner solution. After the lapse of one day, 

 0.037 p. c. of the ^sp added was found to be present in the "inactive" 

 solution. Consequently, in the course of one day 6.6 y of P originally 

 present in the roots migrated into the culture solution. 



