312 The Realm of Nature CHAP. 



of the species is secured in spite of the death of the 

 individual. 



399. Constructive Plant Life. Plants alone are able 

 to raise inorganic substances, such as water, oxygen, carbonic 

 acid, into the sphere of life-wrought or organic material. 

 They cause the elements to combine into proteids^ the raw 

 material of protoplasm. This power in its entirety is con- 

 fined to those plants which possess green leaves, and is 

 exercised by them only when the energy of sunlight falls 

 on the green colouring matter known as chlorophyll. Then 

 the leaf is able to break up carbonic acid derived from 

 the atmosphere, to restore the oxygen to the air, and 

 cause the carbon to combine with the elements of water, 

 forming starch which is at first stored up amongst the 

 cells of the leaf. Subsequently the starch is transformed 

 into sugar, which dissolves in the sap and is carried through 

 the whole plant. On meeting the nitrates, sulphates, phos- 

 phates, and other salts of lime or potash, absorbed from the 

 soil by the roots, the sugar combines with them, producing 

 proteids and various waste products in a manner not yet 

 discovered. The influence of green leaves on the air in 

 sunlight is to unburn or decompose ( 44) the carbonic acid. 

 The solar energy used up in this work is converted into 

 potential energy of chemical separation, which is restored 

 to the kinetic form when wood or coal ( 347) unites with 

 oxygen. The oxygen given out by the action of chlorophyll 

 in the leaf laboratory is more than enough to supply the 

 ceaseless respiration of the plant in daylight and darkness 

 so that, on the whole, green plants diminish the proportion of 

 carbonic acid and increase that of oxygen in the air. 



400. Destructive Animal Life. Contrasted with the 

 constructive processes of plants, changing lifeless into living 

 matter and kinetic into potential energy, animals are wholly 

 destructive. They cannot utilise solar energy, but derive 

 all their power of doing work from oxidation of their own 

 substance. They cannot manufacture proteids, so that all 

 their food has to be prepared for them by plants. Animal 

 life would indeed be impossible if plant life did not precede 

 it. In their respiration animals are always removing oxygen 



