THE METABOLISM OF PLANTS. 22/ 



The mode of formation of acids in the plant is a subject 

 of considerable difficulty. The first attempt to explain it was 

 made by Liebig. He considered that the highly oxidised 

 acids were formed as the first products of constructive 

 metabolism from carbon dioxide and water in the cells con- 

 taining chlorophyll: that, for example, oxalic and formic 

 acids are produced according to the equations, 



CO 2 + H 2 O = CH 2 O 2 +O. 



It has been already pointed out (p. 144) that a synthetic 

 formation of formic acid from carbonic dioxide and water 

 may, according to Erlenmayer's view, take place as the first 

 stage in the formation of organic substance, but the formic 

 acid so produced is at once converted into the corresponding 

 aldehyd, and this in turn into a polymer. It is doubtful, 

 therefore, if Liebig's theory will satisfactorily account for the 

 presence of free formic acid, even in cells which contain chlo- 

 rophyll, and it certainly fails to account for the presence of 

 acids in Fungi and in etiolated plants. We are led to con- 

 clude, therefore, that the acids present in plants are products 

 not of constructive but of destructive metabolism. 



The next point to be considered is as to the substances 

 from which they are derived. It is commonly held that the 

 acids are produced by the oxidation of carbohydrates, espe- 

 cially sugar, but there is no direct evidence to show that 

 this is actually the case, though, from the facility with which 

 acids can be formed artificially from carbohydrates, it may 

 be assumed to be quite possible. This oxidation is usually 

 supposed to be more or less direct ; for instance it is fre- 

 quently expressed by the equation 



sugar oxalic acid 



C 6 H 12 6 + 9 = 3 (C 2 H 2 4 ) + 3H 2 0. 



But, even admitting that carbohydrate is thus directly oxi- 

 dised, it is hardly probable that the process is so simple as 

 this. It was pointed out in the last lecture (p. 202) that 

 the effect of oxidation is to diminish the stability of complex 

 organic substances and thus to facilitate their decomposition, 



152 



