THE TL.V^'T. 39 



CIIAPTEE YI. 



B T A E C IT , V7 D Y - F I B K E , G I- U T E N , ETC. 



We have hitherto examined the raw niatcrial of 

 plants. That is, wc have looked at eacli one of the 

 elements separately, and considered its use in vege- 

 table growth. 



We will now consider another division of plants. 

 We know that they consist of varions substances, such 

 as wood, gum, starch, oil, etc., and on examination 

 we shall discover that these substances are composed 

 of the various atmospheric and earthy ingredients do- 

 scribed in the preceding chapters. They are made 

 up almost entirely of atmospherio matter, but their 

 ashy parts, though very small, are (as we shall pres- 

 ently see) of great importance. 



These compounds may be divided into two classes. 



The first class are composed of cai'hon, hydrogen^ 

 and oxygen. 



The second class contain the same substances and 

 nitrogen. 



The first class (those compounds not containing ni- 

 trogen) comprise the wood, stare) i, gum, sugar, and 

 fatty matter, which constitute the greater part of all 

 plants, also the acids which are found in sou;- fruits, 

 etc. Yarious as are all of these things in their char- 

 acters, they are entirely composed of the same ingre- 

 dients (carbon, hydrogen, and oxygen), and usually 

 combined in about the same proportion. There may 



