136 PLANT RESPIRATION 



tion at the same time as respiratory products. In the oxidation 

 of these substances very unstable carboxyHc acids could be 

 formed, since it is most probable that CO2 is spht off from car- 

 boxyl groups. In this respect the following observation of 

 Euler and Bolin^ is worthy of note. It was found that laccase 

 preparations from Medicago sativa contain a considerable quan- 

 tity of glycolic acid, CH2OH— COOH, glyoxylic acid, CHO— 

 COOH, and particularly mesoxalic acid, COOH— CO— COOH. 

 It is not unlikely that glycoHc and glyoxylic acids are to be 

 regarded as products of the oxidation of acetaldehyde. Mesox- 

 alic acid may represent the oxidation product of the ''three- 

 carbon substance," the formation of which precedes that of 

 pyruvic acid, and which would be considered as the normal 

 intermediate of alcoholic fermentation as well as of oxygen 

 respiration. Mesoxalic acid can be easily oxidised to CO-, and 

 H2O and is therefore even more probably a normal intermedi- 

 ate of oxygen respiration. 



It is evident that such direct products of the oxidation of the 

 substances arising from the anaerobic cleavage of sugar appear 

 only in small quantities, since they are continually consumed 

 and could never accumulate in large amounts. Quite different 

 is the physiological role of those plant acids which are formed 

 in large quantities in fleshy fruits and succulent parts and which 

 are also found in other plants. Of these acids we shall consider 

 only the most common — oxaHc, malic, citric and tartaric acid. 

 The latter appears in plants of southern regions while the rest, 

 especially malic acid, are contained in most plants and some- 

 times in high concentration. 



These acids have also been regarded by various authors as 

 intermediates of oxygen respiration at a time when the nature 

 of oxidation processes in plant cells was still unintelligible and 

 the connection of alcohoHc fermentation with oxygen respira- 

 tion was not yet a subject of experimental investigation. From 

 the present viewpoints, such theories are neither theoretically 

 nor experimentally justified. Years ago they found much favor 

 and are still admitted to modern texts. For example, there is 

 a consideration of the physiological transformations of plant 



' Euler, H. und J. Bolin. Z. f. physiol. Chem. 6i : i. 1909. 



