76 THE STUDY OF PLANT COMMUNITIES ■ Chapter V 



measurable quantities of several rare gases, which have no part in 

 our discussion. Within the limits of the atmosphere that can affect 

 plants directly, there is but slight variation in the proportions of 

 these gases whether over the ocean or land, at sea level or on high 

 mountains. Minor but rather consistent variations have been found 

 over large industrial cities where quantities of carbon dioxide are 

 constantly being produced. 



Whenever an organism respires or a fire burns, oxygen is re- 

 moved from the atmosphere and carbon dioxide is added to the 

 air. Decomposition of organic matter also liberates carbon dioxide, 

 and photosynthetic activity of plants removes carbon dioxide 

 and liberates oxygen. When these processes are not in balance, 

 there may be local variations in the composition of the air, but so 

 long as the air is not strictly quiet, the least motion, combined with 

 diffusion, is sufficient to eliminate gaseous differences almost at 

 once. 



Thus, regardless of its terrestrial environment, the organism is 

 almost certain to be plentifully supplied with these gases that form 

 a relatively constant part of the atmosphere; therefore, these need 

 not be considered as variable factors in the environment. 



GASES OF THE SOIL ATMOSPHERE 



Although normally there is never a shortage of oxygen in the 

 air above ground, such a shortage sometimes occurs in the soil. 

 Air space in the soil is limited and is partially, or sometimes wholly, 

 occupied by water. Any change in the composition of the soil at- 

 mosphere is only slowly readjusted from the atmosphere above, 

 for here air movement and diffusion are relatively slow. 



Since all living structures in the soil respire, and this includes 

 small animals and other microorganisms as well as roots of large 

 plants, the supply of oxygen is constantly reduced and carbon 

 dioxide is released. As a result, the soil atmosphere always contains 

 less oxygen and more carbon dioxide than the air above. Oxygen 

 decreases with depth, and carbon dioxide increases. In the soil un- 

 der closed stands of vegetation, carbon dioxide often equals 5 

 percent and has been found in much higher concentrations. The 

 constant use of oxygen and its extremely slow rate of diffusion 

 when soils are saturated soon result in oxygen deficiency. Tern- 



