114 



PLANTS AND MAN 



the most abundant of plant tissues; in it are locked up vast 

 amounts of carbon. When the wood decays (which indicates that 

 the colorless plants are at work), the cellulose is changed to 

 glucose and organic acids, and eventually into carbon dioxide 

 and water. The carbon molecules are returned to circulation, to 

 be used over again by green plants in another carbon cycle. 



CO, 



GREEN PLANTS 



BACTERIA 



ANIMALS 



Fig. 76. — Bacteria are important in the cycle of carbon which occurs in nature. 



Thus if there were no decay, life would come to a standstill 

 through deficiency of carbon. In the carbon cycle, the colorless 

 plants are the depositors in the universal account, while the green 

 plants are the spenders; animals look on interestedly since their 

 own existence depends upon the continual amounts that the 

 green plant spenders can withdraw. 



Just as there is a limited amount of carbon in nature, so is 

 there a definite quantity of nitrogen available for protein syn- 

 thesis. Four fifths of the atmosphere is nitrogen gas, but few plants 



