THE ROLE OF CARBON DIOXIDE 34i 



The Role of Carbon Dioxide. — All of the carbon dioxide used by green 

 cells in the process of photosynthesis reaches the chloroplasts in solution in 

 water or as carbonic acid which is dissolved in the water. In land plants the 

 atmosphere is the only important source of carbon dioxide. Carbon dioxide 

 released in the process of respiration may be utilized in photosynthesis with- 

 out leaving the plant, but this seldom constitutes an important part of the 

 total amount consumed. The carbon dioxide utilized in photosynthesis by 

 submerged water plants diffuses into them from the surrounding water. 



The atmosphere is composed chiefly of two gases, nitrogen (about 78 per 

 cent) and oxygen (about 21 per cent), but also contains, in addition to a 

 variable but never large amount of water-vapor, small quantities of other 

 gases. One of its minor constituents, carbon dioxide, which constitutes on 

 the average only about 0.03 per cent by volume of the atmosphere, plays a 

 role of the greatest significance in the biological world. As a result of the 

 photosynthetic activity of green plants, the carbon dioxide from the air becomes 

 chemically bound for periods of indefinite length in the organic molecules of 

 living organisms. In view of its important biological role the proportion of 

 carbon dioxide in the atmosphere seems precariously small. The actual amount 

 present, however, is enormous. The best estimates, necessarily very approxi- 

 mate, place the total quantity of carbon dioxide in the atmosphere at about 

 2 X ioi5 kilograms. According to the estimates of Schroeder (1919) the 

 quantity of carbon dioxide used annually in photosynthesis by all of the plants 

 on the earth's surface is about 60 million million kilograms. 



I. Sources of the Atmospheric Carbon Dioxide. — While green plants are 

 continually removing carbon dioxide from the atmosphere, other processes 

 are continually replenishing the atmospheric reservoir with this gas. Carbon 

 dioxide is continually being returned to the atmosphere as a product of the 

 respiration of plants and animals. Contrary to popular opinion plants are 

 undoubtedly more important producers of carbon dioxide than animals. Car- 

 bon dioxide is released into the atmosphere as a result of the respiration of 

 both green and non-green plants. The relatively great importance of the 

 latter group of organisms as generators of this gas is not always appreciated. 

 The organic residues of plants and animals are decomposed as a result of the 

 activities of bacteria and fungi. During such decay processes the carbon of 

 these residues is mostly released in the form of carbon dioxide as a result 

 of the metabolic activities of these organisms, and escapes into the air. The 

 evolution of carbon dioxide gas by soils is often very considerable and is fre- 

 quently referred to as "soil respiration." Most of the carbon dioxide lost from 

 a soil into the atmosphere results from the metabolic activities of soil micro- 

 organisms although smaller quantities are released in the respiration of roots 



