486 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



accti 14 per cent, of acetic acid under similar conditions (Sect. 103). Certain 

 organisms, especially various mould-fungi, can withstand large quantities of 

 free acid, whereas others, including most bacteria, have not this power, 

 and hence the fermentative activity of most lactic and butyric bacteria is 

 inhibited when the amount of acid present rises to 0-5 per cent. (Sect. 103), 

 so that to permit of continued fermentation the acid must be neutralized 

 as fast as it is formed. An over-accumulation of organic salts also usually 

 inhibits vital activity, but in many Crassulaceae as much as half of the 

 dry weight may consist of soluble salts of malic acid *, whereas the total 

 amount of organic acid in other plants never exceeds one per cent. 



With the exception of the oxalic acid present in the form of calcium 

 oxalate, the organic acids and their salts are for the most part held in 

 solution 2 . Crystals of calcium oxalate are of very common occurrence, 

 and in a few plants this substance may form 50 per cent., in some Cactaceae 

 even 80 per cent., of the dry weight 3 . Similarly during certain fermenta- 

 tions as much as one-half of the fermentable material may be converted 

 into organic acid, the latter finally surpassing the total weight of the 

 fermentative organisms 4 . 



Organic acids are produced in many different ways and subserve 

 a variety of purposes. Thus during certain fermentations lactic or acetic 

 acids form end-products of metabolism which must be excreted and 

 removed to permit of the continuance of vital activity (Sect. 77). This is, 

 however, unnecessary in other plants such as Aspergilhis and Citromyces, 

 in which further production ceases as soon as a certain amount has ac- 

 cumulated. In such cases the acids may serve as plastic material, as 

 osmotic substances, as solvent or neutralizing agents, or as aids to the 

 action of enzymes. Moreover certain plants may be able to suppress or 

 kill others by means of the acids which they excrete (Sect. 92), and an 

 acid sap or the presence of acicular crystals of calcium oxalate may protect 

 certain plants against the depredations of animals 5 . Fungi are not only 

 able to use organic acids as food-material but may also consume in part the 

 organic acids which they themselves produce, and similarly the Crassulaceae 



1 G. Kraus, Stoffwechsel bei d. Crassulaceen, 1886, p. 6 (Abh. d. Naturf.-Ges. zu Halle, 

 Bd. xvi). 



2 Crystals of magnesium oxalate, calcium tartrate, and calcium citrate are occasionally, but 

 rarely, found. Cf. Zimmermann, Mikrotechnik, 1892, pp. 61, 65; Wehmer, Ber. d. Bot. Ges., 1893, 



P- 338. 



3 Literature : Kohl, Kalksalze u. Kieselsaure i. d. Pflanze, 1889, p. 35 ; Zopf, Pilze, 1890, p. 193 ; 

 Schimper, Bot. Zeitung, 1888, p. 80, and Flora, 1890, p. 237 ; Wehmer, Bot. Zeitung, 1891, p. 149, 

 and Versuchsst., 1892, Bd. XL, p. 113; Errera, Bull. d. FAcad. roy. d. Belgique, 1893, iii. sen, 

 T. xxvi, Nr. 7 ; G. Kraus, Flora, 1897, P- 54- 



4 Regarding mould-fungi, see Wehmer, Bot. Zeitung, 1891, pp. 342, 534, on oxalic acid, and 

 1893, 1. c., p. 41, for citric acid. For fermentations, see Sect. 103. 



* Stahl, Pflanzen u. Schnecken, 1888, pp. 40, 84. 



