CHAPTER XXII 



AIR 



Nine-tenths of all the dry matter of plants comes 

 from the air. This is a very important reason why 

 we should be careful that the plants have all the air 

 surrounding them that they need. If their air supply 

 is cut off, their food supply is lessened, which, of 

 course, will affect their growth. If plants are sepa- 

 rated sufficiently for them to get enough sunlight, there 

 usually will be enough air. Some experiments can be 

 made in the garden to show the effect on plants of 

 shutting off their air supply in different amounts. 



Instead of dark covers, such as were used in the 

 sunlight experiments, use covers of glass which will 

 let the sunlight through, but which will stop the air 

 supply. One cover can be a glass lantern chimney, 

 which will have a large opening at the top, which will 

 admit a good deal of air, but will stop the air from 

 blowing around the plant. Another cover can be a 

 fish globe or a milk bottle, and this will shut off 

 practically all of the air without shutting off the sun- 

 light. The children can then watch these plants for 

 a week or more, and see how they act when they have 

 plenty of sunlight, but not enough air. 



The air is made up of oxygen, nitrogen, water, 

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