CHAPTEE X. 

 THE CHEMICAL CONSTITUENTS OP PLANTS. 



IN the introductory chapter an account of the elements which enter 

 into the composition of plants has been given, and in the succeeding 

 chapters on the atmosphere, soils and manures, the sources from 

 which plants obtain the necessary supplies of their food have been 

 discussed at some length. The ultimate constituents of plants have 

 thus already been considered. Their proximate constituents, i.e., the 

 actual chemical compounds existent in the various parts of a plant, 

 remain to be described. In this chapter, a short account of the 

 chemistry of the chief compounds which are found in most plants will 

 be given. To some of these substances brief allusion has already been 

 made under " Carbon " in Chap. I., p. 8. 



The following is a list of the various classes into which these com- 

 pounds may be conveniently divided : 



I. Carbohydrates. 

 II. Fats and waxes. 



III. Organic acids and their salts. 



IV. Essential oils and resins. 

 V. Inorganic salts. 



VI. Nitrogenous substances, 

 (i) Albuminoids. 



(ii) Amino-acids and amino-cdmpounds. 

 (in) Alkaloids. 



(iv) Cyanogenetic glucosides. 

 VII. Chlorophyll and other colouring matters. 



I. THE CAEBOHYDEATES. 



An important group of compounds, the members of which consti- 

 tute the larger portion of the dry matter of most plants. They are 

 neutral bodies and contain only the elements carbon, oxygen and 

 hydrogen, the two latter being generally present in the proportion of 

 16 to 2, i.e., the same as in water. As a rule they contain 5 or 6, or 

 some multiple of 5 or 6, carbon atoms, and many of them exhibit 

 optical activity, i.e., they rotate the plane of polarised light to the right 

 ( + ) or to the left (-). 



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