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leaf mouths it breathes in oxygen and breathes out 
carbonic-acid gas. 
There is only one key to unlock the matter, and that 
is this,—to find out whether the plant does most 
towards poisoning or towards purifying the air. 
And that has been found out already. 
Wise men say that Leaf Green and Sunbeam do 
much more good to the air than the little breathing 
mouths do harm. The two good fairies take away a 
great deal of poison, and send back a great deal of the 
helpful oxygen; while the tiny mouths neither rob the 
air of much oxygen nor give it much poison. In- 
deed, the harm they do is so small compared with the 
great good accomplished by Leaf Green and Sunbeam, 
that even at night you need not worry at the thought 
that you have plants in your room. 
Perhaps you wonder that a plant does these two 
things that are so exactly opposite to each other. 
But a plant must breathe as well as eat; for when it 
breathes, it takes in the precious oxygen which is just 
as necessary to its life as to ours. 
In summer, by the dusty roadside, you see plants 
almost white with dust, looking quite ill and lifeless. 
And they are both ill and lifeless; for their little leat 
throats are so choked that they cannot breathe in the 
oxygen they need, and in consequence they are being 
slowly suffocated. 
