THE ASSIMILATION OF ORGANIC ACIDS I27 



between the plant and the external air. The plant has 

 difficulty in getting what C0 2 and 2 it requires from the 

 air. 



In green plants during day-time, photosynthesis pro- 

 duces 2 in the tissue, and so the respiratory needs are more 

 or less provided for. But in darkness the supply of 2 is 

 insufficient to allow the formation of C0 2 in the oxidative 

 catabolism. Consequently less highly oxidised organic acids 

 are formed, which accumulate in the cells. 



The Assimilation of Organic Acids 



The succulent plant which has become charged with 

 organic acids in darkness becomes less acid on exposure to 

 light, and evolves 2 . The question is, How is this effected ? 

 Aubert (1912) and Gerber (1897) argue that under the in- 

 fluence of light the organic acid is split into sugar and C0 2 . 

 Mayer found that malic acid in watery solution gave off 

 C0 2 in light. It is contended that the C0 2 so produced 

 is the material for the photosynthesis of the plant when 

 exposed to light. According to this view, the organic 

 acids stored during the night provide indirectly the 

 material for photosynthesis during the day, in the form 

 of C0 2 . 



It seems unreasonable to suppose that when the plant 

 can assimilate or reduce so highly oxidised an acid asH 2 C0 3 , 

 necessitating a large expenditure of energy, it should not 

 reduce the less oxidised organic acids. The view that the 

 organic acids serve directly for photosynthesis has been held 

 by Treboux and others. The Hydrilla plant appeared to be 

 most suitable for further investigation on this subject, and 

 I carried out parallel experiments with two specimens which 

 had been rendered acid by the influence of seasonal varia- 

 tion and the prevailing high temperature. Having first 

 observed the effect of increasing strengths of C0 2 -solution 

 on photosynthesis (cf. Table XVI.), I tried the effect of 

 substituting malic acid for carbon dioxide. 



