CHAPTER XIV VriV ^ 



THE CHEMICAL CYCLE OF LIFE ^s^^t^ 



87. The carbon cycle. An understanding of the behavior 

 of green plants in relation to food-making shows us how closely 

 the living things in the world depend upon each other. Let 

 us take the case of carbon, which is an essential constituent 

 of all living matter. The carbon in our bodies came from the 

 proteins, fats, and carbohydrates which we ate. We obtained 

 these either from the bodies of plants or from the bodies of 

 animals. In the latter case they were still derived from plants, 

 for the cows or pigs or chickens that we used as food got 

 the carbon in their bodies from the plant food which they in 

 turn ate. 



Now the plant gets its carbon from the carbon dioxid in the 

 air. (Water plants can get carbon from the carbon dioxid 

 dissolved in the water.) But what is the source of the carbon 

 dioxid } We saw (p. 1 2) that the proportion in the air is very 

 small. A few warm, sunny days in August would enable the 

 plants of this country to use it all up, and that would be the 

 end of everything. But the winds are all the time stirring up 

 the atmosphere, so that new supplies of this important material 

 are brought to the plants ; and there are certain rocks — lime- 

 stone and marble especially- — that are capable of yielding a 

 small quantity of this gas when they decompose. But this 

 amount is very small indeed when we consider what is being 

 used up by the plants from hour to hour. There is, however, 

 still another source. 



We have seen (p. 29) that all living things, while using up 

 oxygen from the air, are at the same time throwing off carbon 

 dioxid. Moreover, every fire throws off quantities of carbon 



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