CHAPTER I 



INTRODUCTION 



This chapter should be re-read after the remaining chapters have been studied. 



ALL plants are made up of a complex organized mixture of chemical 

 substances, both organic and inorganic. As a preliminary to the study 

 of plant chemistry, the student should realize that the chemical 

 compounds which make up the living plant may be approximately 

 grouped into the four following classes. Thus, in later chapters, when 

 reference is made to any plant product, it will be understood, broadly 

 , speaking, to which class it belongs, and what relationship it bears to 

 other chemical compounds. 



The main classes may be enumerated as follows : 



(1) Carbohydrates. These contain only carbon, hydrogen and oxygen. 

 The simplest members are the sugars, which are aldehydes and 

 ketones of polyhydric alcohols of the methane series of hydrocarbons. 

 The more complex carbohydrates, such as starch, cellulose, dextrins, 

 gums and mucilages, are condensation products of the simpler sugars. 

 The sugars are found in solution in the cell-sap of living cells throughout 

 the plant. Cellulose, in the form of cell-walls, constitutes an important 

 part of the structure of the plant, and starch is one of the most widely 

 distributed solid "reserve materials." 



(2) Fats. These also contain only carbon, hydrogen and oxygen. 

 Chemically they are glycerides, that is glycerol esters of fatty acids. 

 These acids are derived from two series of hydrocarbons, i.e. the 

 methane series and the olefine series, and they usually contain a large 

 number of carbon atoms. The fats occur as globules deposited in the 

 cells, especially in the tissues of seeds where they form reserve materials, 

 though they also occur in other parts of plants. The acid components of 

 the fats are also found in the free state. 



The carbohydrates and the fats both belong to the aliphatic series of 

 organic compounds, that is to the series in which the carbon atoms are 

 united in chains. 



(3) Aromatic compounds. These are characterized by having the 

 carbon atoms united in a ring, as in benzene. They may contain more 

 than one carbon ring, and, moreover, aliphatic groupings may be attached 



o. 1 



