ORGANIC ACIDS 485 



It is not surprising that the protoplast is able to modify or to dissolve 

 its cellulose investment. The latter process is usually induced by the action 

 of enzymes, the former by the infiltration of substances produced by the 

 protoplast. The modifications which a cell-wall may undergo are dependent 

 upon the external and internal conditions and upon the reactive powers of 

 the protoplast. It is for this reason that only particular tissue-elements 

 lignify; that cork is produced normally in certain regions only, and is 

 formed after an injury over all the exposed tissues; that the amount of 

 cuticularization varies according to the external conditions ; and that under 

 certain circumstances a pathological formation of gum may occur. 



None of these alterations, nor even the fact that the cell-wall is capable 

 of growth, indicate that it is to be regarded as a living structure, nor need 

 we regard as well-founded the supposition of Wiesner, according to which 

 the cell-wall is permeated throughout by the protoplasm \ for neither the 

 presence of combined nitrogen in certain cell-membranes, nor the existence 

 of localized protoplasmic communications, suffice to prove that his con- 

 clusions have any general application. 



SECTION 85. Organic Acids. 



Even excluding the fatty acids, every plant produces some organic 

 acid or other, either free or in the form of a salt. Oxalic, malic, citric, and 

 tartaric acids occur most frequently. Formic and acetic acids are also 

 often present, and like butyric and lactic acids may form part of the main 

 products of particular fermentative processes (Sect. 105). Certain acids, 

 such as fumaric acid, aconitic acid, &c., occur only in a few plants 2 , and 

 it seems that none even of the commonly occurring organic acids is of 

 universal occurrence. The different acids belong to quite distinct series, but 

 for the most part are derivatives of methane; the metabolic products 

 never seem to include acids of more than tribasic value. 



Organic acids may be present as neutral or acid salts, or in uncombined 

 form : thus 6 to 7 per cent, by weight of a fresh lemon consists of citric acid, 

 while in the Crassulaceae malic acid constitutes as much as a per cent, 

 of the weight when fresh 3 . Similarly Citromyces* may cause as much as 

 8 per cent, of citric acid to accumulate in a culture-fluid, and Bacterium 



1 Wiesner, Elementarstructur, 1896; Pfeffer, Energetik, 1892, p. 252; Correns, Jahrb. f. wiss. 

 Bot., 1894, Bd. xxvi, p. 587.- On the so-called Dermatosomes, cf. Sect. 13. 



2 Cf. Husemann, Pflanzenstoffe, 1882, and Ebermayer, Physiol. Chemie, 1882. On the distri- 

 bution of formic and acetic acids, see Bergmann, Bot. Zeitung, 1882, p. 784. 



3 Ebermayer, I.e., p. 273; Wehmer, Bot. Zeitung, 1891, p. 373. 



' * Wehmer, Beitr. z. Kenntniss einheim. Pilze, 1893, i, p. 77. On fungi which grow in very acid 

 solutions, see Wehmer, I.e., 1891, p. 296, and 1895, II, p. 143. 



