PLANT CONSTITUENTS — FATS, PROTEINS 287 



skatol, indigo, and certain essential oils like vanillin, are without 

 doubt formed in this way. 



Amino-acids. — In regard to the occurrence of amino-acids 

 themselves in plants it may be stated that about eight have 

 been isolated. A recent view in regard to the occurrence of these 

 acids and other probable products of protein metabolism in 

 plants and soil is, that these excretion products, when present 

 in the soil, act as toxins to the plant itself. As previously 

 stated also (p. 250) amino-acids in the soil may, in some cases, 

 be used by plants as a direct source of nitrogen for the synthesis 

 of plant protein. The amino-acids present in the plant itself 

 also function in the resynthesis of plant protein. 



Alkaloids, Essential Oils, Terpenes, Tannins, etc. — Of these 

 different groups of compounds some are of importance medici- 

 nally, some have a very great value as perfumes, etc., and others 

 in various ways. They include substances known as alkaloids, 

 e.g. quinine, morphine (from opium), nicotine, strychnine, 

 brucine, etc. These are all valuable medicines. Also sub- 

 stances known as essential oils, e.g. oil of peppermint, oil of 

 cloves, oil of rose, heliotrope, vanillin (vanilla), etc. These are 

 used as constituents of perfumes or flavors and some of them 

 also medicinally. Then we have the group known as terpenes, 

 including le7non oil, turpentine and camphor, and finally the 

 tannins, used in leather manufacture and obtained from the 

 bark of hemlock, oak and other trees. 



These various compounds belong mostly to the more complex 

 groups of organic compounds and in most cases are physiolog- 

 ically related to the plant as end products of metabolism, as 

 just discussed. They are of great economic value, and the plants 

 producing them are grown as agricultural crops in certain more 

 or less restricted localities. In general, however, they are not 

 important agriculturally. 



