126 CHEMISTRY OF PLANT LIFE 



both acid and neutral esters of all such acids are possible. Exam- 

 ples of all of these different types of derivatives of organic acids 

 are frequently found in plant tissues. 



The occurrence, properties, and functions of a particular type 

 of glycerol, and other esters of organic acids, which are known as 

 fats and waxes, are not taken into consideration in the following 

 discussions, but reserved for a subsequent chapter dealing specially 

 with them. 



> 



SOME COMMON ORGANIC ACIDS 



Free organic acids, or their mineral salts or volatile esters, 

 sometimes occur as separate and characteristic individual com- 

 pounds in particular species of plants, or fruits; but much more 

 commonly, two, three, or even more acids or their derivatives, are 

 associated together. 



Formic acid, H-COOH (H^CCfe), occurs in free form and in 

 considerable proportions in the leaves of several species of nettle, 

 where' it is responsible for the unpleasant effects of the " sting." 

 It may be detected in small amounts in the vegetative parts of 

 many, if not all, plants, especially during periods of rapid growth, 

 and is probably one of the intermediate products in the photo- 

 synthesis of carbohydrates (see Chapter III). 



Higher members of the formic acid series (as acetic, CHa COOH ; 

 propionic, C2Hs-COOH; butyric, CsHs-COOH; etc.) are often 

 found in small quantities in the leaves of many plants and seem to 

 be characteristically present in certain species. They are easily 

 produced from carbohydrates by bacterial action and, hence, are 

 always present in fermenting tissues, such as silage, sauerkraut, 

 etc. Furthermore, the glycerol esters of higher members of this 

 and other monobasic acid series are constituents of all natural fats 

 and oils (see Chapter X). 



Oxalic acid, HOOC-COOH (H 2 C 2 04), is found in small 

 amounts in nearly all plants and in relatively large proportions 

 in those of Oxalis, rhubarb, etc. It occurs both as the free acid 

 and as neutral, or acid, oxalates of calcium, potassium, and, per- 

 haps, of magnesium and sodium. Solid crystals of insoluble 

 calcium oxalate are often found in plant cells, and it has been 

 shown that when so deposited the calcium cannot become again 

 available for metabolic uses. It is stated, further, that such 



