CHAPTER IX 



CHEMICAL COMPOSITION OF THE PLANT 



WE HAVE examined both young and mature plants and have 

 studied the various organs of which they are composed. We 

 have also learned something of the structure of these plant 

 organs. Let us learn something of their chemical composition ; 

 something about the qualities that make them useful or unfit 

 for food and other purposes. Of course, we have reason to believe 

 that the different parts of the plant are unlike in chemical com- 

 position, for we know that they are different in structure and 

 texture, and that we use them for radically different purposes. 



Water, which is so essential for plant growth (Chapter I) 

 is the most abundant and one of the most important compounds 

 in the plant. We know that fruits and vegetables are juicy and 

 we have seen the bleeding of trees when pruned too late in the 

 season, and, therefore, it is not necessary to demonstrate that 

 plants contain water. The quantity of water in the different 

 parts of the plant varies from the very large amounts in juicy 

 fruits to the very small amounts in dry grains. The following 

 table shows some of the extremes : 



Percentage of water content 

 Plant by weight 



Cucumber (fruit) 96 



Cabbage (leaves) 90 



Beets, red (roots) 88.5 



Apples ( fruit) 83.2 



Potato ( Irish ) . 78.9 



Potato (sweet) 71.1 



Corn (green fodder) . . . '. 79.8 



Corn ( dry grain ) 10.9 



We also know that plants contain carbon, for we have seen 

 the charcoal which is formed as a result of burning a piece of 



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