LIGHT AND CHLOROPHYLL 441 



the green meadow grasses, the cultivated fruits and vegetables, 

 we realize that the amount of carbon dioxide used by them is 

 enormous. This supply comes partly from the breath of 

 people, and partly from creatures, which, like birds, fly 

 through the air, or, like cattle, roam over the surface, or, 

 like worms, burrow in the soil. Everything that breathes, 

 whether it be plant or animal, large or small, sends out car- 

 bon dioxide into the air, and is a source of food to plants. 

 Decaying matter gives off carbon dioxide, and the decaying 

 grasses of meadows, the rotting fruits of orchards, the decay- 

 ing leaves and tree stumps of forests are sources of carbon 

 dioxide. Carbon dioxide also comes from smoldering logs and 

 burning fuel. The carbon dioxide given out by breathing 

 organisms, by burning substances and decaying matter is 

 caught up by the wind, and is scattered far and wide over the 

 land. So much of this carbon dioxide is removed from the 

 air by the plants, that ordinary fresh air contains only about 

 .04 per cent of it. The air of a city or town is never quite as 

 free from carbon dioxide as is the air of the open village and 

 country. 



A working plant needs light and chlorophyll. Plants 

 grown out of doors or in bright rooms are healthier than those 

 grown in dark and ill-lighted places. In our homes we place 

 the winter plants near the windows, knowing that they grow 

 faster and stronger and keep fresher and greener there than in a 

 more shaded position. Unless a plant has light, it starves to 

 death, because without light a plant cannot combine soil water 

 and carbon dioxide into food substances, such as starch and 

 sugar. Unless a plant receives light, its leaves turn yellow 

 (Fig. 308) and fall off, and it dies. In order to be strong and 

 independent, a plant must have in its leaves a substance called 

 leaf green, or chlorophyll, and leaf green cannot form unless 

 light shines upon the leaf. But a plant does not necessarily 



