CHEMICAL COMPOSITION OF PLANTS 49 



widespread in plants. Their chief significance is to increase the 

 impermeabihty of the cell wall to water. For this reason, they 

 are mostly found on the surface of stems, leaves, and fruits of 

 plants, where they impregnate and cover the cuticle and cork 

 tissue and increase their protective function against water losses. 

 Dibasic organic acids are very widespread in the cell sap of 

 plants, both in a free state and in the form of salts. Those most 

 frequently met with are oxalic acid, COOH-COOH; malic acid, 

 COOHCH2CHOHCOOH; and tartaric acid, COOHCHOH, 

 CHOH-COOH. Of the tribasic acids, the most widespread is 

 citric acid. 



CH2COOH 



C(OH)COOH 



CH2COOH. 



Especially large amounts of acids are found in fleshy leaves 

 of succulents, as well as in such plants as Rumex and Oxalis. 

 Malic, citric, and tartaric acids are present in fruits, usually 

 when not quite ripe. The presence of organic acids produces 

 an acid reaction, which can be demonstrated in the expressed 

 juice of most plants. In some plants, the actual acidity due to 

 the presence of hydrogen ions may attain as much as pH 1.6 

 to 1.3. The exudate from phloem cells is frequently alkaline. 

 Oxalic acid in the presence of calcium ions very often forms 

 various-shaped crystals of calcium oxalate, such as druses, 

 raphides, etc., described in detail in courses of plant histo- 

 chemistry. Calcium oxalate crystals usually appear in cells 

 that are approaching senescence; they are especially abundant 

 in dying tissues. The oxalates seem to have a toxic action that 

 may kill the cells in which they are depositing. 



The formation of organic acids in plant tissue is closely asso- 

 ciated with the process of respiration, and they may be regarded 

 as by-products of oxidation processes. They are formed from 

 pyruvic acid and acetaldehyde, which are intermediate products 

 of the oxidation of sugars, w^hich, with incomplete oxidation, 

 form dibasic acids (Butkevich, Bernhauer). Such partial oxida- 

 tion may be due to an insufficient access of oxygen. Organic 

 acids accumulate chiefly in plants with thick leaves, succulents, 

 as well as in the flesh of ripening fruit. In later stages of matur- 



