108 CHEMICAL COMPOSITION OF THE PLANT 



are used for food, medicine, soap making and many other pur- 

 poses and are among the most important articles of commerce. 



Proteins. Plants also contain a third group of foods, known 

 as proteins or proteid foods which are composed of carbon, 

 hydrogen, oxygen and nitrogen, and are among our most im- 

 portant food substances. They are especially abundant in seeds 

 and the food value of many plants over others depends almost 

 entirely on their percentage of protein. They may be associated 

 with the carbohydrates, but in some cases are borne in separate 

 cells. The great value of clover, cowpeas, soybeans and related 

 plants for stock feed, and of beans and peas as food for man 

 depends primarily on their protein content. 



Plants contain many other compounds, some of which are 

 referred to as waste products. Among the most important of 

 these compounds are the following: 



(a) The essential (or volatile) oils, which are entirely 

 different from the true oils previously referred to. They may 

 occur in any part of the plant, but are especially abundant in 

 the foliage and flowers. Most of the odors of plants are due to 

 these essential oils, many of which are extracted and used in 

 the manufacture of medicines, perfumes, soap and other articles 

 of commerce. 



(b) Gums and resins of various kinds are familiar articles 

 of commerce. Among the most important are turpentine, resin, 

 balsams, gum-camphor, gum-arabic and gum-tragacanth. 



(c) Organic acids are very numerous and are especially 

 abundant in certain fruits. Among the most common organic 

 acids of commerce are oxalic, malic, tartaric and citric acids. 



(d) Tannins are abundant and are found in the great ma- 

 jority of the higher plants. They are used for the making of 

 hides into leather, in medicines and for other purposes. 



(e) Alkaloids are abundant plant products from which we 

 obtain many valuable medicines and some of our most danger- 



