488 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



during and after growth, every increase in the volume of a cell must be 

 accompanied by a corresponding production of organic salts or other 

 osmotic substances 1 . 



If the attainment of the required maximum is prevented by the 

 continual removal of the acid as fast as it is formed, an almost unlimited 

 amount may be produced. Wehmer has shown that Aspcrgillus niger and 

 Penicillium glaucum produce in a sugary nutrient solution an amount of 

 oxalic acid sufficient to render the culture medium feebly acid, whereas if 

 .the acid is continually neutralized by the addition of chalk, the fungus 

 ultimately produces a quantity of oxalic acid several times exceeding 

 its own body-weight. 



The same is the case with Citromyces, although here the further pro- 

 duction of citric acid ceases only when the medium becomes strongly 

 acid, and a similar result would probably be obtained with the higher 

 plants were it possible to continually remove the free acids or their salts 

 from the living cells The condition of equilibrium at which further pro- 

 duction ceases may be modified by changes in the external conditions : 

 thus a rise of temperature to 34* C. lowers the limit at which Aspergillus 

 niger ceases to produce free oxalic acid 2 , and not only a rise of tempera- 

 ture, but also illumination, causes the percentage of free acid to diminish 

 in plants of the Crassulaceae (Sect. 56). Similarly the accommodation to 

 concentrated solutions, and frequently also the performance of work against 

 resistance 3 , may involve an increased turgidity, and hence also an increased 

 production of organic acids or their salts as far as these are responsible 

 for the regulation of turgor. 



It seems that the turgor-producing salts of organic acids are aplastic 

 products in the higher plants, so that the renewed production induced by 

 a deficiency must cease as soon as a condition of equilibrium is again 

 reached. The same may also hold good for the free organic acids, though 

 here the condition of equilibrium may simply represent the balance between 

 production and decomposition 4 , for both fungi and plants of the Crassulaceae 

 are able to reassimilate the organic acids they themselves have produced. 

 Carnivorous plants (Sect. 65) afford examples of the way in which the 

 secretion of acid may be primarily induced by chemical stimulation, or 

 the rate of production increased. 



By means of this self- regulatory power of increasing or decreasing the 

 production of acid, an excessive formation of basic or alkaline substances 



1 Pfeffer, Druck u. Arbeitsleistung, 1893, p. 428. Cf. Sect. 24. 

 J Wehmer, Ber. d. Bot. Ges., 1891, p. 165. 

 3 Pfeffer, 1. c., pp. 296, 428. 



* Cf. Pfeffer, Sitzungsb. d. Sachs. Ges. d. Wiss., 1891, p. 36. On the decrease of acids during 

 the ripening of fruits, cf. Sect. 109. 



