CHAPTER XII 



AIR 



All plants need air for respiration and green plants need it also 

 for photosynthesis. Air in many of its relations to plants is nearly 

 as universal as gravity. We shall not deal with it, therefore, in many 

 of its physiological relations but only in those relations that are 

 variable enough to be of ecological significance. 



86. Atmospheric Composition and Pressure.— The composition 

 of the atmosphere does not vary materially from place to place 

 except in the amount of dust and smoke that it contains. In large 

 cities there is often enough dust or smoke, or both, in the air to 

 exclude certain kinds of plants entirely. Most evergreen trees, for 

 example, are unable to endure urban conditions and remain healthy 

 for any great length of time. This is perhaps even more true of the 

 majority of lichens and for that reason lichens ordinarily are not 

 found in or near cities of even moderate size. 



In the vicinity of volcanoes and hot sulphur springs, and also in 

 the vicinity of smelters and other places where large quantities of 

 coal are burned, there are often poisonous gases, mostly sulphurous 

 acid, in the atmosphere which are very injurious to vegetation. The 

 areas thus affected, however, are relatively limited. 



Experiments have shown that ordinary air pressures are not at the 

 optimum for vegetative growth of common green plants used in the 

 experiments. It was found in all cases that a decrease in atmospheric 

 pressure, other conditions remaining the same, was accompanied by 

 increased growth. We have no experimental data, however, to 

 indicate whether the normal atmospheric pressure is at the ecological 

 optimum for the sum total of the plant's activities. Probably it is. 

 A decrease in atmospheric pressure increases the relative humidity 

 of the air and by so doing has a tendency to check transpiration. 

 The decreased pressure at higher altitudes, however, is ordinarily 

 more than counterbalanced by a decrease in temperature, and this 

 has the opposite effect on humidity. Altogether, therefore, atmos- 

 pheric pressure is not of very great importance from an ecological 

 view-point. 



87. Air in Water.— Air dissolves in water very slowly when the 

 latter remains still. Therefore, running water is usually well 

 aerated, while standing water often is not. Practically all seed 



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