FOOD THE PLANT TAKES FKOM THE AIR 01 



which means that the amount of carbon dioxide in the 

 air remains always about the same. 



The leaves of plants absorb carbon dioxide only in 

 direct sunlight; on cloudy days the amount of gas ab- 

 sorbed is small, and at night the plant not only does 

 not absorb any gas, but actually gives off carbon 

 dioxide just as animals do in breathing. The amount of 

 carbon dioxide thus given off is, however, very small 

 compared with what is taken up under the influence of 

 light. 



60. Oxygen. — While the leaves of plants under the 

 influence of sunlight take up carbon dioxide and give 

 off oxygen, the roots of growing plants require a con- 

 stant supply of free oxygen in order that they may 



Seeds, you remember, require a supply of air 

 before they will sprout, and it is the oxygen of the air 

 that enables them to begin to grow. A seed supplied 

 with only nitrogen gas will not sprout, no matter how 

 perfect the other conditions. When plants are in bloom, 

 and later on when they are forming seed, a constant 

 supply of oxygen is necess;i is then neces- 



sary for all growing plants from the beginning of their 

 life to the end. The air, of course, contains an abundant 

 supply of oxygen for all the needs of the plant above 

 ground — that is, for the buds, flowers, and leaves. The 

 spaces in soils, as you will presently learn, contain air, 

 which supplies oxygen to the roots of plants. If for any 

 reason the supply of air in the soil becomes exha 

 the plant growth is cl 



61. Ammonia Gas. — Air contains minute quantities 

 of this gas, which supplies to plants a small part of 



