They take in or
absorb out of the air carbonic acid gas which we have given out
of our mouths and then by the help of the sun-waves they tear
the carbon and oxygen apart. Most of the oxygen they throw back
into the air for us to use, but the carbon they keep.
If you will take some fresh laurel-leaves and put them into a
tumbler of water turned upside-down in a saucer of water, and
set the tumbler in the sunshine, you will soon see little bright
bubbles rising up and clinging to the glass. These are bubbles
of oxygen gas, and they tell you that they have been set free by
the green cells which have torn from them the carbon of the
carbonic acid in the water.
But what becomes of the carbon? And what use is made of the water
which we have kept waiting all this time in the leaves? Water,
you already know, is made of hydrogen and oxygen, but perhaps
you will be surprised when I tell you that starch, sugar, and
oil, which we get from plants, are nothing more than hydrogen
and oxygen in different quantities joined to carbon.
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