190 



THE VITAL PROCESSES 



and by chemical means, the latter being the more impor 

 tant of the two methods. Plants supply the means for 

 storing it chemically (Fig. 83). Attention has already been 

 called to the fact (page 112) that growing plants are con- 

 tinually taking carbon dioxide 

 into their leaves from the air. 

 This they decompose, adding 

 the carbon to compounds in 

 their tissues and returning 

 the oxygen to the air. It is 

 found, however, that this pro- 

 cess does not occur unless the 

 plants are exposed to sunlight. 

 The sunlight supplies the 

 energy for overcoming the at- 

 traction between the atoms of 

 oxygen and the atoms of car- 

 bon, while the plant itself 

 serves as the instrument 

 through which the sunlight 

 acts. The energy for decomposing the carbon dioxide 

 then comes from the sun, and through the decomposition 

 of the carbon dioxide the sun's energy is stored becomes 

 potential. It remains stored until the carbon of the plant 

 again unites with the oxygen of the air, as in combustion. 



The Sun's Energy in Food and Oxygen. Food is derived 

 directly or indirectly from plants and sustains the same 

 relation -to the oxygen of the air as do the plants them- 

 selves. (The elements in the food have an attraction for 



surface, but on condensing into a liquid, it falls as rain. It then finds its way through 

 streams back to the ocean. All water above the sea level is in such a position that 

 gravity can act on it to cause motion, and it possesses, on this account, potential or 

 siored-up energy. It is because of this energy that rapids and waterfalls are such 

 important sources of power. 



FIG. 83. Nature's device for 

 storing energy from the sun. See 

 text. 



