CLIMATIC FACTORS : THE AIR 77 



porary saturation may not be serious, but, when prolonged, it re- 

 sults in death of the vegetation through inhibition of root growth 

 and absorption. Under these conditions, several soil organisms may 

 carry on anaerobic respiration for a time, but such activity results 

 in chemical changes of several kinds, which may affect fertility of 

 the soil or actually inhibit plant growth. 



Available oxygen in an aquatic habitat probably is somewhat 

 higher than in a saturated soil because of the movement of the 

 water and because the oxygen is more readily replaced by solution 

 from the atmosphere. If, however, the water is solidly frozen over, 

 it is not uncommon for the oxygen supply to fall so low that many 

 of the fish die. When such conditions develop in well-stocked 

 fishing lakes, it is now common practice to cut several holes 

 through the ice and to pump air through the water until the de- 

 pleted oxygen supply has been replaced. The mud at the bottom 

 of a shallow pond is probably the least favorable habitat for plant 

 roots. Most plants growing well in such places are of the emergent 

 type, having at least part of their structure in the air and charac- 

 terized by lacunar tissue, which permits gases to accumulate in, 

 and move freely through, the plant. 



WATER CONTENT OF THE ATMOSPHERE 



In addition to the gases constituting the atmosphere, water is 

 always present as vapor but in widely varying amounts. Since at- 

 mospheric moisture represents the indirect source of the plant's 

 water and likewise controls the amount and rate at which water 

 is lost by the plant, it is an environmental factor deserving some 

 attention. 



The capacity of air to hold water vapor increases as tempera- 

 tures rise or pressure is reduced. The air is said to be saturated 

 when it contains as much moisture as it can hold at a given tem- 

 perature and pressure. If for any reason the temperature is raised 

 or the pressure is decreased, the amount of water remaining con- 

 stant, the air is no longer saturated. On the other hand, if the tem- 

 perature of saturated air decreases, the capacity is reduced, and 

 some of the vapor precipitates as a liquid. Thus air that is not sat- 

 urated will become so without change of vapor content if its tem- 

 perature is lowered, and, when saturation is reached, the air is said 



