INTRODUCTION 27 



(c) Air. 



The atmosphere or air contains four gases — oxygen, 

 nitrogen, water vapour, and carbonic acid gas. The 

 amount of the last is •03-'04 per cent., by volume, 

 but sufficient to enable plants to obtain their carbon 

 from it, and when this is realised one is inclined to 

 marvel at the extraordinary activity of the plant. 

 The effect of solar energy upon all life, however, ex- 

 plains this. In air also there is a greater or lesser 

 amount of water, in the form of water vapour. The 

 carbonic acid gas in the atmosphere is obtained by the 

 plant to carry out the process of assimilation. The 

 carbon is turned into carbohydrates and the oxygen is 

 given off. The processes of respiration and assimila- 

 tion are compensatory in animals and plants. Plants 

 respire, as do animals, taking in oxygen and giving off 

 carbon dioxide, noticeable chiefly at night, whilst 

 assimilation is only possible during the day. In 

 respiration, energy is liberated as the starch, pro- 

 teids, etc., are broken down into carbonic acid gas 

 again. 



Nitrogen in the air is not directly available to 

 plants. They obtain it, however, from the soil, which 

 derives it from the air in the first place. 



This is an outline of some of the more striking 

 chemical characteristics of the atmosphere or air. 



Physically the air acts upon the plant through its 

 force or motion in the form of wind. This, as has 

 been seen, promotes evaporation. As a result the 



