190 



ANALYSIS OF THE ENVIRONMENT 



atmosphere near its region of origin (p. 

 131), though it is a relatively heavy gas 

 Uke carbon dioxide. 



In addition to the gases listed in Table 

 14, various amounts of different trace con- 

 centrations appear in the air. These include 

 heavy water, ammonia, nitrous and nitric 

 acid and their compounds, sulfurous and 

 sulfuric acids and their compounds, and 

 oxides of nitrogen. Droplets and frozen par- 

 ticles of water condense about dust particles 

 or minute bits of sea salt evaporated from 

 spray. These tiny particles, together with 

 pollen grains and spores of many kinds, are 

 carried aloft from the earth, and, in the re- 

 versed direction, outer space contributes an 

 invisible shower of cosmic dust. Radioactive 

 products of radium and other elements con- 

 tinually enter the air and make up a part of 

 its electrified particles (Humphreys, 1931). 

 Certain of the environmental roles of these 

 atmospheric gases and impurities have al- 

 ready been suggested (see Index). We need 

 to discuss primarily the ecological relations 

 of the nitrogen complex, oxygen, and car- 

 bon dioxide, both in the atmosphere and 

 when dissolved in water. 



NITROGEN 



Nitrogen is chemically inert. It is difficult 

 to get atmospheric nitrogen into stable 

 chemical combination, and it often escapes 

 from artificial compounds with explosive 

 violence. The great reservoir of nitrogen in 

 the air acts as a diluent of the chemically 

 active oxygen and of carbon dioxide. Atmos- 

 pheric nitrogen may be "fixed" as nitrites 

 or nitrates by electric discharges, and the 

 products are washed to earth by rain or 

 snow in small but measurable amounts. 



Nitrogen-fixing bacteria are important 

 agents in the nitrogen cycle (p. 497), espe- 

 cially those that five symbiotically with 

 legumes. Some evidence suggests the pos- 

 sibiUty of the fixation of free nitrogen by 

 green plants, though this is still highly de- 

 batable. There is also evidence that the 

 aerial parts of plants absorb significant 

 amounts of nitrogenous compounds from 

 rain and dew. E. C. Miller (1938) reviews 

 both these points critically. 



OXYGEN 



Oxygen is present in the atmosphere in 

 sufficient amounts so that it does not be- 

 come a hmiting factor for animal or plant 

 life except in the carbon dioxide-rich death 



valleys and at the low partial pressures pre- 

 vailing in higher altitudes. For some proc- 

 esses, the normal partial pressure of oxygen 

 in the atmosphere is not optimal. The early 

 growth of the chick embryo is accelerated 

 by exposure for five days to concentrations 

 of oxygen above normal; fastest growth oc- 

 curs at an initial concentration of about 30 

 per cent, though continuous exposure dur- 

 ing the whole period of incubation gives 

 the highest percentage of hatching at 21 

 per cent (Cruz and RomanofiF, 1944; Barott, 

 1937). Animals can use oxygen taken di- 

 rectly from the atmosphere or that obtained 

 as a by-product of photosynthesis canied 

 on by indwelling symbionts. Animals hav- 

 ing anaerobic respiration either obtain their 

 oxygen or otherwise carry on oxidative 

 processes without the use of free oxygen 

 (von Brand, 1946). Normal oxygen pres- 

 sure is a limiting factor for many anaerobic 

 organisms. 



CARBON DIOXIDE 



The general contribution of carbon diox- 

 ide to environmental control has been dis- 

 cussed in earlier pages (76 and 173). Now 

 we are interested in its role as the main 

 source of carbon in the tissue of plants and 

 animals, obtained through photosynthesis, 

 in the function of this gas in regulation of 

 the respiratory activity of vertebrates and 

 insects, and in its relation to other bio- 

 logical processes. The percentage of car- 

 bon dioxide in the atmosphere, 0.03 per 

 cent, is remarkably constant over land and 

 sea. It is increased near certain escape vents 

 from the Hthosphere, near industrial plants, 

 and in cities where it is released in large 

 amounts. The partial pressure of carbon 

 dioxide may also be increased somewhat 

 near decaying matter or just above well- 

 fertihzed soil, especially if the soil surface 

 is loosely pulverized, as in land under good 

 cultivation. A gradient from 0.053 to 0.28 

 volume per cent at the surface, to 0.04 to 

 0.067 just above the leaves, has been found 

 in a well-cultivated beet field. The partial 

 pressure of the carbon dioxide in soil atmos- 

 phere is always relati\'ely greater than that 

 above ground. 



Within the eflFective range of other fac- 

 tors, such as light, temperature, and mois- 

 ture, green plants in greenhouses and even 

 in fields can increase their rate of photosyn- 

 thesis if they are supphed with an atmos- 

 phere enriched by additional carbon diox- 



