ORGANIC ACIDS 487 



store up malic acid as reserve food -material 1 . Organic acids function as 

 reserve material only in a few special cases, although many of the higher 

 plants seem to have the power of reassimilating malic, citric, and even 

 oxalic acid, while calcium oxalate, which usually remains intact, may in 

 certain cases be dissolved and assimilated. According to Kraus 2 calcium 

 oxalate disappears from rhizomes, bark, &c. only when there is a lack of 

 calcium, so that apparently this salt is utilized mainly for the sake of the 

 calcium it contains. But little energy can be obtained by the consumption 

 of oxalic acid, and experiments with fungi show that the amount of growth 

 in a fluid culture remains almost the same, whether oxalic acid has been 

 produced in abundance or only in minimal amount as an end-product of 

 metabolism in the place of carbon dioxide 3 . Salts of organic acids are 

 commonly employed in the maintenance and regulation of turgor. It 

 must, however, always be remembered in this connexion that the same 

 substance may frequently serve a variety of purposes. 



SECTION 86. Organic Acids (continued}. 



Organic acids are usually products of katabolism, but may probably 

 also be produced synthetically, while they may arise either during aerobic 

 or during anaerobic existence, as when fermentation or intramolecular re- 

 spiration takes place. In the former case they are apparently formed directly 

 in the process of respiration, but nevertheless in a regulatory manner and 

 not because only imperfect combustion is possible, for even when oxygen 

 is present in great excess, respiration and also the production of acid proceed 

 in the same manner both in the cases of fungi 4 and of the higher plants 

 (Sect. 56). Organic acids are not produced under all circumstances, but 

 only when the conditions which regulate their production are satisfied, and 

 it is simply a special instance of regulatory protection when Citromyces 

 consumes the citric acid produced by its own metabolism as the supply of 

 more suitable nutriment decreases. Aspergillus, Penicillium, and Citromyces 

 generate a specific degree of acidity in a culture-fluid, which however in 

 each case is less than suffices to affect injuriously the plant's own vital 

 activity. The same is the case with regard to the cell-sap of the higher 

 plants, for the sap only attains a certain degree of acidity when plants of 

 the Crassulaceae are kept in darkness. Similarly the production of salts 

 of organic acids takes place in a self-regulatory manner, for wherever, 

 as is often the case, the same constant turgidity is maintained both 



1 G. Kraus, I.e., 1886, p. 15. 



2 G. Kraus, Flora, 1896, p. 54. Cf. also Kohl, I.e., p. 48. On the solubility of calcium 

 oxalate, see Wehmer, Versuchsst., 1892, Bd. XL, p. 456. 



3 Wehmer, Bot. Zeitung, 1891, p. 553 ; Pfeffer, Studien z. Energetik, 1892, p. 197. 

 * Wehmer, 1. c., 1891, p. 537 ; 1893, p. 50. 



