51 



CHAPTER IV. 



SUBSTANCES INTO WHICH PLANTS CONVERT THE SIMPLE 

 ELEMENTS UPON WHICH THEY LIVE. 



58. We have for so far strictly confined our attention to 

 the consideration of tlie store of materials which a bountiful 

 Providence has placed in the air, the water, and the earth, 

 for the nourishment of the vegetable tribes. We have seen 

 that four elementary bodies, — Oxygen, Nitrogen, Hydrogen, 

 and Carbon, — constitute the great bulk of every plant, and 

 that the remaining portion is composed of a few mineral com- 

 pounds with most of which you are familiar. The materials 

 employed by nature are few in number, yet how varied are 

 the fonns they are made to assume in the plants and flowers 

 that cover the earth! 



59. You will now inquire, what is the nature of those sub- 

 stances into which plants convert the raw materials of their 

 food, and which are discovered in the structure of the vege- 

 table and in the various forms of nutritive matter stored m 

 their seeds and roots? The question is natural, and leads to 

 one of the most interesting parts of our subject. 



The compounds which plants contain, produced by the 

 union of the simple elements that I have described, are almost 

 innumerable. The greater number of them, however, exist 

 in exceedingly minute quantities: thus, the bitter substance 

 Quinine, which is found in Peruvian Bark, and which at 

 present is so extensively employed in medicine ; the bitter prin- 

 ciple which chemists extract from the bark of the root of the 

 apple-tree ; the curious element Iodine, which is obtained from 

 the ash of sea- weeds, and the various colouring matters which 

 almost all plants contain, form so small a proportion of the 

 entire bulk of vegetables, that for the practical farmer in this 

 country these considerations would be without any real ad- 

 vantage. But in every plant which it is the object of the 

 farmer's care to bring to perfection, we find about half-a-dozen 

 of compound bodies, distinguished by a remarkable similarity 

 of composition, and upon the presence of which their value as 

 food depends. To the consideration of these forms of matter 



