324 PHYSIOLOGY OF NUTRITION. 



IV. THE BYE-PRODUCTS OF VEGETABLE 

 METABOLISM. 



130. The Organic Acids in Plants. 



The organic acids in plants are not to be regarded as products 

 of assimilation. They originate for the most part, as will be 

 more specially shown in 131, from carbohydrates by processes of 

 oxidation. The organic acids of plants (Oxalic, Citric, Malic, etc.) 

 are present in the cell-sap either in the free state or, as perhaps 

 more generally, combined with bases to form acid or neutral salts, 

 which may be readily soluble or soluble only with difficulty. The 

 cell-sap in parenchyma very generally contains more or less con- 

 siderable quantities of free organic acids or of acid salts of organic 

 acids, and we can readily satisfy ourselves of this by applying blue 

 litmus paper to the fresh-cut surface of any plant structure. 

 The reddening of the paper indicates the presence of the acid. In 

 many cases the acid character of the cell-sap may be detected 

 even by the sense of taste. 



The free acids and their acid salts have numerous functions in 

 the cells, which we have already in part considered. The acids 

 very considerably intensify the turgescence of the cell contents, 

 they accelerate the transformation of starch by diastase, they 

 serve in many cases to protect plants against the attack of 

 injurious animals, they decompose the nitrates absorbed from the 

 soil by the roots, a process of great importance in the formation 

 of proteids, they combine with the excess of lime absorbed by the 

 plants, and they decompose the chlorides present in the plant with 

 liberation of Hydrochloric acid. 



Oxalic acid is very widely distributed in the vegetable kingdom. 

 It is found free, and in acid salts which are soluble in the cell- 

 sap, but also very frequently in combination with lime. Crystals 

 of Calcium oxalate are found in special cells, and we have referred 

 to them (see 24). Further examples may now be mentioned. 



We prepare a longitudinal section, perpendicular to the surface, 

 from the leaf of Aloe arborescens. The epidermis, the green 

 parenchyma, and the aqueous tissue free from chlorophyll are 

 easily made out under the microscope. In the green tissue we 

 notice further tubular cells, running parallel to the long axis of 

 the leaf, which are crowded with large quantities of needle-shaped 

 crystals of Calcium oxalate. These bundles of raphides lie in a 

 mucilaginous matrix, and in the preparation of sections it fre- 



