208 AGRICULTURAL CHEMISTRY 



The organic acids of plants are valuable mainly because 

 they impart palatability to foods and exert a favorable 

 influence upon digestion by stimulating the secretion and 

 flow of the digestive fluids. Many of the organic acids 

 have medicinal properties, and some, as oxalic acid, are 

 poisonous. The organic acids cannot be considered as 

 heat- or flesh -producing nutrients, but simply as food 

 adjuncts. In plants, they take an important part in the 

 assimilation of the mineral elements of plant food, and 

 the production of new tissue. The acid sap comes in 

 contact with the soil particles, dissolving the plant food 

 which is then absorbed by osmosis. 



Essential Oils 



281. General Properties. The essential or volatile 

 oils are the compounds which impart characteristic taste 

 and odor to plants. They differ from the fixed oils or 

 fats by completely volatilizing when heated, and leaving 

 no permanent residue on cloth or paper. They also have 

 an entirely different chemical composition from the fats. 



282. Occurrence. Volatile or essential soils are found, 

 in some form, in nearly all plants, particularly during 

 growth. In some fruits, and seeds, they impart the 

 characteristic flavor and give individuality to the mate- 

 rial. Oil of lemon, oil of cedar, and oil of nutmeg are ex- 

 amples of essential oils. In nearly every plant, one or 

 more of the essential oils is present at some period of 

 growth. 



283. Chemical Composition and Properties. The 



essential oils are mixed bodies, many of them belonging 



