The Metabolism of Senescent Leaves 



205 



dicarboxylic amino acids and in the amide N, again emphasiz- 

 ing the high activity of glutamic and aspartic acids and their 

 amides in the protein metabolism of the leaves. 



It may be concluded from the evidence outlined above that 

 the protein content of leaves is the result of continuous 



Table I 



Incorporation of 15 N into Proteins of Barley Leaves 



Protein separated from leaves 24 hours after supplying them with 

 0-04 M-NH 4 H 2 P0 4 containing 29-3 Atom % excess 15 N. 



synthetic and catabolic activities. In ageing tissues the loss 

 of protein is attributable to a decline in synthetic capacity, 

 leading to an adverse balance and the predominance of 

 catabolic changes as yellowing of the leaf proceeds. 



Respiratory Activities of Senescent Leaves 



The changes of respiratory activity in yellowing leaves 

 have been recently reviewed by James (1953). Much of the 

 available data with leaves of different species has been ob- 

 tained with detached leaves often kept for long periods in the 

 dark. As already noted, senescence occurs rapidly under these 

 experimental conditions, but in the present context it is of 

 interest to examine briefly the general relation between the 

 rate of respiration and yellowing of the tissues. The pioneer 

 research of F. F. Blackman (see James, 1953), carried out 

 with leaves of Cherry Laurel and Tropaeolum, showed that 

 during yellowing of mature leaves a marked increase in the 

 rate of respiration took place. In leaves of different species 



