22 AGRICULTURE. 



so it is. But in the air there is a gas called carbonic acid gas. 

 It is formed wherever carbon is burned. The carbon unites 

 with the oxygen gas of the air and forms a compound, a gas, 

 that is called carbonic acid gas. This is the source from which 

 the plant gets its carbon. 



There is only a very small quantity of this carbonic acid gas 

 in the air, but the plants have a large number of leaves and 

 they are broad and thin, and the air is moving more or less all 

 the time, so that the plant has no difficulty in getting all the 

 carbon that it requires. The carbonic acid gas of the air goes 

 in through the leaves ; the plant takes up the carbon for its 

 own use and sets free the oxygen gas with which the carbon 

 was united. Just here we might mention that all animals are 

 constantly breathing out carbonic acid gas from their lungs, 

 and that when too much of it is present the animals will be 

 smothered. We feel the effect of it when shut up in a close 

 room. Plants take up this carbonic acid gas, keep the carbon 

 and set free the oxygen, so that plants are constantly purifying 

 the air for animals, and animals are constantly producing car- 

 bonic acid gas to feed the plants. Nature has in this way 

 made plants and animals dependent upon each other. 



The starch of potatoes, the sugar of beets, the jelly of cur- 

 rants and apples, the oil of flaxseed and the fibre of flax and 

 of all parts of plants are made up entirely of the three elements 

 carbon, hydrogen and oxygen (C H and O). 

 . The plants get all the carbon from the air, and the hydrogen 

 and oxygen can all be got from water, which, as we have said, 

 is a compound of hydrogen and oxygen, so that starch, sugar, 

 jelly, oil and fibre are made up by the plant from what comes 

 from water and the air. When a farmer sells from his farm 

 sugar or butter (oil) or fibre he is selling what in the first place 

 came from the rain and the air, and thereby he does not rob 

 the soil so much as when he sells grain or hay, since these 

 contain mineral or soil material. 



