One of these remains
white, but the other kind, near the surface, is altered by the
sunlight and by the help of the iron brought in by the water.
This particular kind of protoplasm, which is called "chlorophyll,"
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 things 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.
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