198 METABOLISM 



In a corresponding experiment with ammonium tartrate only about half the 

 amount of oxalic acid possible was formed, whilst free tartaric acid was com- 

 pletely oxidized into carbon-dioxide and water. Since, however, it is obvious 

 that the respiratory material is used up to a considerably less extent in an 

 incomplete oxidation such as that which takes place when oxalic acid is formed 

 than when carbon-dioxide is produced, it becomes a question whether this in- 

 complete utilization of respiratory material does not make itself felt in the growth 

 of the fungus. The increase in the dry weight of the fungus is, however, the 

 same whether oxalic acid be formed or not ; the oxalic acid lost has no great 

 nutritive or respiratory value (compare Lecture XVII). Further, under certain 

 conditions, the oxalic acid formed may be still further used for respiratory 

 purposes by the fungus itself. [WEHMER (1891, a) shows that at high tempera- 

 tures (above 30 C.) the oxalic acid is always oxidized by the fungus.] 



Just as Aspergillus (and Penicillium) form oxalic acid, so Citromyces 

 (WEHMER, 1894) manufactures citric acid and can itself make use of it again. 

 Citromyces glaber can acidulate its substratum up to 4 per cent, of citric acid ; 

 it can withstand as much as 20 per cent, of citric acid, although it is very 

 sensitive to the presence of inorganic acids. 



In addition to proving that the formation of the acid is useful to the fungus 

 in acidifying its substratum, WEHMER' s research is also specially important 

 as showing that the formation of the acid is not due to a deficiency of oxygen, 

 as was formerly thought to be the case in this and other instances. 



Indeed a formation of acid takes place in almost all plants, and although 

 this occasionally perhaps occurs during the process of synthesis it is associated, 

 for the most part, with the process of respiration. BENECKE (1903, Bot. Ztg. 

 61, 79) has shown clearly that in the higher plants also, where it is very widely 

 distributed, oxalic acid is produced under the same conditions as in Fungi. Since 

 it is possible to grow certain plants, such as maize, both with and without oxalates 

 it is manifest that here also oxalic acid is not an essential product of meta- 

 bolism. Generally speaking, however, the formation of organic acids during re- 

 spirationis quantitatively less than that of carbon-dioxide, and it is only amongst 

 succulents that one finds organic acids produced in such large quantities that 

 the formation of carbon-dioxide is, at least at first, completely inhibited. It has 

 long been known that the amount of acid in the leaves of these plants increased 

 greatly at night, but it is to the comprehensive researches of AD. MAYER (1875- 

 87), G. KRAUS (1886), WARBURG (1886) and AUBERT (1892), that we owe 

 a knowledge of the details of the case. In darkness these plants absorb oxygen 

 without giving off equal quantities of carbon-dioxide ; the atmosphere sur- 

 rounding them decreases in volume. Malic acid occurs in the Cactaceae, 

 isomalic acid in the Crassulaceae, oxalic acid in the Mesembryanthemaceae, 

 and the formation of the acid takes place so freely that one may detect its 



CO 

 presence by tasting the leaves. The extreme case is when -^ = o, that 



^2 



is to say, when no carbon-dioxide is formed at all. In continued darkness and 

 at higher temperatures the coefficient increases in value but never reaches unity. 

 Continued formation of these acids would lead to serious injury, which the 

 plant avoids by gradually having recourse to normal respiration and the forma- 

 tion of carbon-dioxide when a certain limit has been reached. This fact shows 

 that we have here to deal with a special peculiarity of succulents, and that the 

 formation of acids cannot depend upon an insufficiency of oxygen. Possibly, 

 as in the case of Fungi, succulents may obtain certain advantages from the forma- 

 tion of acid : that is indeed true, but the purpose of the formation of the acids is 

 naturally very different in the two cases. The acids break down in sunlight, not 

 only when they are exposed in pure solutions, but also more especially in the 

 presence of certain accelerating bodies which act catalytically ; then carbon- 



