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ADVENTURES IN RADIOISOTOPE RESEARCH 



(b) the phosphorus atoms migrate. In case (a) labelled phosphorus atoms 

 should only be found in the upper leaves; in case (b) the labelled phos- 

 phorus atoms should be equally distributed between the upper and 

 lower leaves. 



As is seen from Table 1, the lower leaves contain an appreciable 

 amount of labelled phosphorus atoms, clearly showing that in the case 

 of the sunflower, just as in that of the maize plant, a very considerable 

 migration of the phosphorus atoms takes place, and the probability of a 

 labelled phosphorus atom being found in a preformed leaf is not very 



days 



different from the probability of its being found in a leaf grown in a 

 labelled solution. In comparing the figure obtained for the lower and 

 upper leaves, we have to take into account the fact that the lower leaves 

 increase in size to some extent while the plant is in the labelled solution. 

 However, this increase was found to be very slight, as to be seen from the 

 third vertical column of Table 1 and also from Fig. 1, which illustrates 

 the plant at the three different stages of its growth. 



From the above, it follows that the greater part of the phosphorus 

 atoms are not fixed in position in the leaves but can migrate. The bulk 

 of the phosphorus atoms found in plants are presumably present as 

 inorganic phosphate, and carried about with the circulation stream in 

 the plant. To what extent the phosphorus atoms present in organic 

 combination are permanently localized is under investigation. 



In connexion with the experiments discussed above, we tried to 

 ascertain if any exchange of phosphorus atoms takes place between 

 an isolated leaf and a nutritive solution. These experiments were carried 

 out with the leaves of a sunflower grown in labelled nutritive solutions. 

 Some of the leaves were ignited and analysed at once after separating 

 them from the stem, while other leaves were placed for 48 hours in a 

 nutritive solution containing unlabelled phosphorus to give a chance 



