THE PLANT 91 



tbcin are hydrocarbons — i.e., compounds of hydrogen and 

 carbon; others contain, in addition, oxygen or sulphur. Tlie 

 hydrocarbon essential oils or terpenes have the general 

 formula (C^Hg)". Many vegetable perfumes consist mainly of 

 these terpenes — e.g., oil of turpentine, of lemon,of orange, or of 

 eucalyptus. Of oxygenated essential oils, many different vari- 

 eties are known, e.g., oil of bitter almonds contains benzoic alde- 

 hyde, CgH^CHO ; camphor has the empirical formula Cjj,HjgO ; 

 oil of lavender contains linalyl acetate, Cj^Hj^CgHjOj. Of essen- 

 tial oils containing sulphur, allyl isothiocyanate, C3H5NCS, 

 found in oil of mustard, and allyl sulphide, (OjHj)^^, present 

 in oil of garlic, may be taken as typical. 



The Renins may be regarded as oxidation products of ter- 

 penes. Their constitution is complex and not well under- 

 stood. In plants they oft«n occur associated with terpenes. 



IV. Organic Acids and their Salts, — Many organic acids 

 have been detected in various vegetable products. They 

 generally occur as potassium, sodium, or calcium salts, though 

 sometimes as free acids. All organic acids contain the group 

 COOH, ** carboxyl." 



Among those commonly occurring in plants the following 

 may be mentioned : 



Oxalic acid, COOH.COOIT. 



Tartaric acid, COOH.CHOH.CHOH.COOH. 



Malic acid, COOH.CHOH.CH^.COOH. 



Citric acid, CH,(COOH).C(OH)(COOH).CH,(COOH). 



Gallo-tannic acid, C,n^(0}i),.C0.0.C,ll,(0B.),.C001I. 



Gallic acid, C,H2(OH)3.COOH. 

 The acidity of fruits is often due to the presence of free 

 acids — e.g., malic acid, which occurs in apples, gooseberries, red 

 currants, blackberries, and sour cherries — but sometimes to 

 the occurrence of acid potassium or calcium salts ; thus grapes 

 contain acid potassium tartrate, sweet cherries acid potassium 

 malate. In many fruits two or more acids may occur to- 

 gether ; thus gooseberries contain both malic and citric acids. 



