756 



ANIMALS AND THEIR ENVIRONMENT 



roles of animals, green plants and bacteria in this cyclic use of the ele- 

 ments can be gained from a consideration of the details of the more 

 important cycles. 



329. The Carbon Cycle 



There are about six tons of carbon (in the form of carbon dioxide) 

 in the atmosphere over each acre of the earth's surface. Yet each year 

 an acre of luxurious plant growth, such as sugar cane, will extract as 

 much as twenty tons of carbon from the atmosphere and incorporate 

 it into plant protoplasm. According to one estimate, the green plants 

 would use up the entire supply of atmospheric carbon dioxide in about 

 35 years. Carbon dioxide fixation by bacteria and animals is another, 

 but quantitatively minor, drain on the supply of carbon dioxide. Carbon 

 dioxide is returned to the atmosphere by respiration. Plants carry on 

 respiration continuously and green plant tissues are eaten by animals 

 who, by respiration, return more carbon dioxide to the air. But respira- 

 tion alone would be unable to return enough carbon dioxide to the air 

 to balance that withdrawn by photosynthesis; vast amounts of carbon 

 would accumulate in the dead bodies of plants and animals. The carbon 

 cycle is balanced by the decay bacteria and fungi which break down 

 the carbon compounds of dead plants and animals and convert the car- 

 bon to carbon dioxide (Fig. 37.2). 



When the bodies of plants are compressed under water they are not 

 destroyed by bacteria, but undergo a series of chemical changes to form 

 peat, then brown coal or lignite, and finally coal. The bodies of certain 

 marine plants and animals may undergo somewhat similar changes to 

 form petroleum. These processes remove some carbon from the cycle 



in Air or 

 Ived in Water 



imals 



Compounds 



of 



ol Bodies 



Decay Bacteria 

 and Fungi 



Dead Organisms 



Corbon Compounds of 

 Dead Plant* and Animals 



^^/>/, 



^O/s 



Proteins, Fats and 

 Other Carbon Compoundt 



/ 



Plant Porosites 



Carbon Compounds 

 of tlie Bodies 

 of Parasites 



o » jifuiL^ yt 



Figure 37.2. The carbon cycle in nature. See text for discussion. 



