906 ADVENTURES IX RADIOISOTOPE RESEARCH 



of the ^^N found in all plants, if the nitrogen atoms were released from 

 the molecules in which they were incorporated and replaced by other 

 nitrogen atoms carried to the place of synthesis by the circulation 

 stream. A comparison of the ^^N content of the nitrogen prepared from 

 the upper and the lower leaves, which was most kindly made with the 

 mass-spectrograph by Professor Schoenheimer and his collaborators 

 .(RiTTENBERG, Keston, Rosebury and Schoenheimer, 1939), lead 

 to the result that the ^^N content of the total nitrogen prepared from 

 the "old" leaves amounted to about nearly half of that present in the 

 "new" leaves. Thus, a large part of the nitrogen atoms present in the 

 "old" leaves was replaced by other nitrogen atoms during the second 

 stage of the experiments^). 



In our later experiments we have separated the protein nitrogen from 

 the non-protein nitrogen present in the leaves. Furthermore, we have 

 compared the nitrogen exchange in three different kinds of leaves: 

 those which have not grown further while in a culture solution containing 

 ^^N, those which have grown to some extent and, finally, those which 

 were formed during that period alone. 



EXPERIMENTAL PROCEDURE 



Sets of 4 sunflower plants were grown in jars containing 5 liters of a complete 

 culture solution with a content of 28 mgm normal nitrogen as ammonium sulphate 

 per liter. After the lapse of three weeks, the solution was replaced by another 

 solution containing, instead of normal nitrogen, nitrogen with 2.5 per cent i^N 

 ■content. After the lapse of 6 days, this solution was renewed. After the lapse 

 of a further 6 days, the experiment was broken off. The leaves were sorted into 

 three groups: (a) old leaves, which did not grow after the plants were transferred 

 into the culture solution containing heavy nitrogen; (b) young leaves developed 

 solely after the transfer; (c) intermediary leaves which could not with certainty 

 be classified in one of the former groups. That the old leaves did not grow further 

 in the solution containing heavy nitrogen was ascertained by measuring their 

 size before the transfer into the solution containing the heavy nitrogen and at the 

 end of the experiment. The new leaves were not yet visible at the time of transfer. 

 As to the intermediary leaves, the size of one of the youngest leaves was measured 

 before and after the experiment. The size of this leaf was found to have increased 

 by nearly one half of its original value. The increase in size in the case of most 

 of the other intermediary leaves was much less. 



The leaves were dried at room temperature in ^'acMo over sulphuric acid. They 

 were, subsequently, ground in a ball mill, and the powder obtained extracted 

 with ice-cooled 2 per cent sodium chloride solution containing phosphate buffer 

 (pH = 6.8). The operations were carried out in a room kept at 0°. To the extract 



"'These results were communicated at the meethig of the Semaine internationale centre le Cancer, 

 Paris, November 1938. (Comp. ACTA lUnio internationalis contra cancnun], Vol. IV, No. 1 — 2, p. 175, 

 1939 1. Since, contrary to our later experiments which are described in this paper, in our earlier plants 

 some growth of the "old" leaves took place during the experiment, the '"N content of the old leaves 

 was only partly due to replacament of old protein molecules by new ones. 



