230 LECTURE XII. 



formed from protoplasm (p. 218), we necessarily assume that 

 fatty acids are among the products of its decomposition. 



It must be borne in mind that acids may be readily derived from each 

 other in various ways. To the examples already given (see p. 228) 

 the following may be added. Under the action of reducing agents, 

 succinic acid is produced from malic or tartaric acid : and conversely, 

 since malic (monohydroxysuccinic) and tartaric (dihydroxysuccinic) acids 

 are oxidised derivatives of succinic acid, it is quite possible that they 

 may be formed from succinic acid in the plant. Again, on heating malic 

 acid fumaric acid is formed, and fumaric acid is converted into succinic 

 acid under the influence of nascent hydrogen. Further, as has already 

 been pointed out, acids undergo decomposition by fermentation : thus 

 malic acid yields on fermentation either (a) succinic acid, acetic acid, 

 carbon dioxide and water, or (b) propionic acid, acetic acid, and succinic 

 acid, or (c) butyric acid, carbon dioxide, and water : the gradual con- 

 version observed by Schindler, of citric into tartaric acid in lemon-juice 

 which had been allowed to stand for a long time, is probably another 

 illustration of the transformation of one acid into others by fermen- 

 tation. 



Now as to the significance of the acids in the metabolism 

 of the plant. It has been already mentioned that Liebig 

 regarded the highly oxidised acids, especially the oxalic, as 

 being the first products of the constructive metabolism of 

 the plant, and he was further of opinion that by gradual 

 reduction of them carbohydrates and even fats were formed. 

 The most important piece of evidence which he offered in 

 support of his views was the fact that, as fruits ripen, they 

 become less sour, a fact which he interprets to mean 

 that acid is converted into sugar. This may, however, be 

 explained in other ways : the sugar may be produced from 

 starch, and the diminution of the acidity may be attributed 

 to the neutralisation of the acid by bases. Still, it appears 

 from Beyer's analyses, that the proportion of mineral matters 

 diminishes in fruits as they ripen, and hence it may be 

 inferred that the acids do not become neutralised ; Beyer 

 also finds that the acids diminish and that the sugar increases 

 in plucked fruits. Again, it seems probable, as stated above 

 (p. 206) that the starch which makes its appearance in the 

 embryo of a germinating oily seed, is formed indirectly, 

 through protoplasm, from the fatty acids set free in the seed by 



