CHAPTER IV 

 THE FOOD REQUIREMENTS OF BACTERIA 



THE tissues of higher plants contain a large propor- 

 tion of carbon, as may be readily ascertained by charring 

 the substances. Rye straw contains more than half of its 

 weight of fiber, of which 40 per cent or more is carbon. 

 It has been estimated that an acre of beech forest assi- 

 milates nearly a ton of carbon annually. All of this 

 carbon is obtained from the invisible gas, carbon dioxid, 

 which constitutes from .03 to .04 per cent of the total 

 volume of dry air. 



By means of the green coloring matter, chlorophyl, 

 contained in their leaves, the higher plants decompose 

 the carbon dioxid, and cause its carbon to enter into 

 various combinations, such as starches, sugars, fats, 

 gums and proteins. Lower plants, among them the 

 microscopic algse, are enabled by virtue of their chloro- 

 phyl, or of other coloring matters allied to chlorophyl, 

 to decompose the carbon dioxid of the air, and to build 

 out of the carbon various and numerous complicated 

 organic substances. 



The decomposition of the carbon dioxid, by means 

 of chlorophyl, can be effected only in the presence of 

 sunlight, for this furnishes the energy for the breaking 

 up of the carbon dioxid molecules. In this way, simple 



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