158 THE FAIRY-LAND OF SCIENCE. 
phyll," will have nothing to do with the green waves 
and throws them back, so that every little grain of this 
protoplasm looks green and gives the leaf its green 
colour. 
It is these little green cells that by the help of the 
sun-waves digest the food of the plant and turn the 
water and gases into useful sap and juices. We saw 
in Lecture III. that when we breathe-in air, we use 
up the oxygen in it and send back out of our mouths 
carbonic acid, which is a gas made of oxygen and 
carbon. 
Now, every living thing wants carbon to feed upon, 
but plants cannot take it in by itself, because carbon 
is solid (the blacklead in your pencils is pure carbon), 
and a plant cannot eat, it can only drink-in fluids and 
gases. Here the little green cells help it out of its 
difficulty. They take in or absorb out of the air the 
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 
Fig> 4a 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 
Oxygen-bubbles rising from laurel- water, and Set the 
leaves in water. tumbler in the sunshine, 
you will soon see little bright bubbles rising up 
and clinging to the glass. These are bubbles of 
