THE PRINCIPLES OF BIOLOGY 3 



into organic substances. The oxygen of the carbon dioxid 

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

 oil, and woody fiber are examples of substances thus 

 formed. Can you think of any fuel not clue 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. 



FlG. 3. — Colorless 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. 



ery little 



not move 



notice that 



r food, and 



If the sun- 



Fortunately for the animals the plartts 

 of the substances built up by them, sine 

 about nor need to keep themselves warm 

 animals are constantly using plant substancj 

 constantly drawing the air into thj^ftbodie 

 light had not enabled the green ^Hnt to store up these 

 substances and 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). 



