THE VOLATILE PAET OF PLANTS. 53 



peculiar to itself, and usually the same plant in its different 

 parts reve.als to tlie senses of taste and smell the presence 

 of several individual substances. In tea and coffee occurs 

 an intensely bitter " active principle," thein. From tobacco 

 an oily liquid of eminently narcotic and poisonous proper- 

 ties, nicotin, can be extracted. In the orange are found 

 no less than three oils • one in the leaves, one in the flow* 

 ers, and a third in the rind of the fruit. 



Notwithstanding the great number of bodies thus occur- 

 ing in the vegetable kingdom, it is a few which form the 

 bulk of all plants, and especially of those which have an agri- 

 cultural importance as sources of food to man and animals. 

 These substances, into which any plant may be resolved by 

 simple, mostly mechanical means, are conveniently termed 

 proximate elements, and we shall notice them in some de- 

 tail under six principal groups, viz : 



1. Watee. 



3. The Cellulose Gkoup oe Amyloids — Cellulose, 

 (Wood,) Starch, the Sugars and Gums. 



3. The Pectose Group — the Pulp and Jellies of Fruits 

 and certain Roots. 



4. The Vegetable Acids. 



5. The Fats and Oils. 



6. The Albuminoid or Peotein Bodies. 



1. VFater, H^ O, as already stated, is the most abundant 

 ingredient of plants. It is itself a compound of oxygen and 

 hydrogen, liaving the following centesimal composition : 



Oxygen, 88.88 



Hydrogen, 11.11 



100.00 

 It exists in all parts of the plant, is the immediate cause 

 of the succulence of the tender parts, and is essential to 

 the life of the vegetable organs. 



In the following table are given the percentages of water in some of 

 the more common agricultural products in Vsxa fresh state, hut the pro- 



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