CHAPTEE 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 com- 

 pounds which make up the living plant may be approximately grouped 

 into the six 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. The simplest members of this class 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) Vegetable acids. This term is usually applied to acids and hydroxy- 

 acids derived from the lower members of the methane, olefine and 

 acetylene series of hydrocarbons. Such acids as formic, acetic, valeric 

 and caproic are not readily detected in the plant. Nevertheless, it is 

 more than likely that they play an important part in metabolism, for 

 their amino derivatives, glycine, valine, etc. (see section 5) form con- 

 stituents of practically all proteins. The dibasic and hydroxy-acids, e.g. 

 oxalic, succinic, glutaric, malic, etc., are probably products of oxidation 

 of the sugars in respiration. Aspartic (amino-succinic) and glutaminic 

 (amino-glutaric) acids are also constituents of proteins. 



(3) Fats. Chemically these are glycerides, that is glycerol esters, of 

 acids derived from the higher members of the methane and olefine series 

 of hydrocarbons, and they usually contain a large number of carbon atoms. 

 The fats occur as very fine 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. 



