74 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



From the experiments of Saposchnikowi it appears that excised 

 leaves rapidly take up asparagine and in the light show an increase 

 in protein-content. 



The increase of amino-acids in leaves kept in the dark offers the 

 key to the interpretation of some observations of long standing 

 which have never been adequatelj^ explained. In an excellent paper 

 on the chemistry and physiology of foliage leaves, published by 

 Brown and Morris^ in 1893, determinations are reported of the 

 increase of diastatic activity of leaves kept in the dark. Thus, for 

 example, in experiments with Hydrocharis morsusrance they found : 



Table 53. 



The explanation given for this phenomenon by Brown and Morris 

 is, in brief, that the protoplasm elaborates diastase according to 

 the requirements of the leaf. 



"As long as conditions are favorable for assimilation, the leaf-cells are supplied 

 with an abundance of newly assimilated materials and so plentifully that the supply 

 exceeds their powers of metabolism and translocation. The excess of nutritive 

 material is in part at least deposited as starch. At this period there is little or no 

 elaboration of diastase of the protoplasm, probably none at all in those cells in which 

 starch deposition is in active progress. When the light fails, and assimilation falls 

 off, the living cells speedily use up or translocate the excess of the soluble assimilative 

 products, e. g., cane sugar, and begin to draw their supplies from the reserve of starch. 

 To enable them to do this effectually, the somewhat starved protoplasm now com- 

 mences to elaborate the needed diastase more rapidly, and this secretion becomes 

 still more marked as the starvation point of the cell is neared." 



It seems to us that this argument ascribes to protoplasm final 

 causes beyond the justification of the experiments, in a manner 

 already briefly considered in the introductory discussion of this 

 paper. The fact that amino-acids increase in the leaves kept 

 in darkness has been established repeatedly. The accelerating 

 influence of amino-acids on the diastatic activity as determined by 

 Sherman and others offers a more direct explanation of the periodic 

 variations of diastase in leaves than the one originally advanced 

 by Brown and Morris. Thus an increase in diastatic activity of 

 leaves which had been kept in the dark would simply mean that the 

 amino-acids in the leaves had increased and therebj^ produced 

 conditions which are favorable to diastatic activity. 



' Saposchnikow, W. Bot. Zentrbl., 63, 246 (1895). 



' Bbown, H. T., and G. H. Morris. Jour. Chem. Soc. London, 63, 644 (1893). 



