THE PRINCIPLES OF BIOLOGY 3 



into organic substances. The oxygen of the carbon dioxide 

 is set free and returns to the air (Fig. 2). Starch, sugar, 

 oil, and woody fibre are examples of substances thus 

 formed. Can you think of any fuel not due to plants ? 

 How Animals obtain Energy. — You have noticed that 

 starch, oil, etc., will burn, or oxidize, that is, unite with the 

 oxygen of the air ; thus the sun's energy, stored in these 

 substances, is changed back to heat and motion. The 

 oxidation of oil or sugar may occur in a furnace; it may 

 also occur in the living substance of the active animal. 



Fir.. 3.— Colourless plants, as Mush- A green LEAF, even after it is cut, gives 

 rooms, give off no oxygen. off oxygen (O) if kept in the sun. 



Fortunately for the animals, the plants oxidize very little 

 of the substances built up by them, since they do not move 

 about nor need to keep themselves warm. We notice that 

 animals are constantly using plant substances for food, and 

 constantly drawing the air into their bodies. If the sun- 

 light had not enabled the green plant to store up these 

 substances and to set free the oxygen (Fig. 3), animals 

 would have no food to eat nor air to breathe; hence we 

 may say that the sunlight is indirectly the source of the 

 life and energy of animals. Mushrooms and other plants 

 without green matter cannot set oxygen free (Fig. 3). 



