"Let me tell you about this. Ethel has in her hands two little plants.
The one in her right hand has been growing in the light; the other, in
her left hand, has been put away in the dark to grow. The absence of
green colour is very marked in this latter plant. So you see it takes
light to form this green, or chlorophyll as it is called. The
chlorophyll-saturated cells, absorbing carbonic acid and the
water-diluted food from the soil, literally break them up. And when
broken, food is found suitable for plants to absorb. Wonderful, is it
not?

"I spoke of carbonic acid; well, this is a gas, as some of you have
found out before, made up of carbon and oxygen. It is a gas which we of
the animal kingdom breathe out as waste from our bodies. The plant takes
it in through the leaf--and, by the way, I ought to explain that. It is
this way: if we had a magnifying glass we should find over the inner
surface of leaves, pores, or stomata as they are called. They open in
the presence of light; and from these openings what the plant has no use
for passes out, and gases from the air may pass in. Some call these
openings breathing pores.

"Quantities of water pass out through these pores. When this process
goes on too rapidly a plant will wilt.

"So, to go back, we will suppose that carbonic acid gas has passed into
the leaves. Straightway the chlorophyll bodies get to work. The gas is
broken up, and oxygen and carbon are left. The carbon is wood the plant
builds. Some of the oxygen passes out into the air and some is kept for
plant food use.