318 THE GROWTH OF PLANTS AND ANIMALS 



phorus, together with iron, sulphur, chlorine and sodium. 

 They are found, however, only in comparatively small 

 amounts, about 2 to 4 per cent of the plant weight, and 

 constitute the ash or unburned portion of the plant. 



Plant growth. The most fundamental form of 

 growth, upon which depends the food supply of all 

 animals, is that which takes place in green plants. 

 These require for their growth certain substances as 

 carbon, hydrogen, nitrogen and oxygen. By means of 

 the energy of the sun's light and heat, they are able to 

 separate carbon from its union with oxygen in the carbon 

 dioxide of the air. In the same manner, they are able 

 to separate the hydrogen in water from its union with 

 oxygen. The carbon, hydrogen and oxygen they then 

 transform into such entirely new and complex substances 

 as starch, 1 woody fibre or cellulose, and sugar. 



Non-destructibility of matter. When plants decay 

 or are burned up, the starch, cellulose and sugar are 

 again converted into the original simple substances from 

 which the plants formed them. For example, the carbon 

 in the starch again unites with the oxygen of the air to 

 form carbon dioxide; the hydrogen and oxygen in the 

 starch form water. 2 We thus see that the substances used 

 by the plants for their growth and development are not 

 destroyed even by burning or decay, but are used over 

 and over again for thousands of years and will continue 

 to be so used as long as plants grow. 



Conservation of energy. In the growth of plants, 

 energy is required to separate out the materials which 

 they use and to build them into their proper places in 



1 Growth : Carbon dioxide + water = starch + free oxygen 



6 CO 3 +5 H a O=C 6 H,oO 6 -h 6 O 2 



2 Decay : Starch + oxygen = carbon dioxide + water 



C 6 H 10 O5+60 2 = 6CO a +5H 3 O 



