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CHAPTEE IX. 

 THE CONSTITUENTS OF 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 allu- 

 sion has already been made under " Carbon" in Chap. I., p. 8. 



The following is a list of the various classes into which 

 these compounds 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.) Amides, 

 (iii.) Alkaloids. 

 VII. Chlorophyll and other colouring matters. 



I.- THE CAEBOHYDEATES. . 



An important group of compounds the members of which 

 constitute 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 



