In Fig. 5 you see a piece of a liverwort leaf cut down through the mouth, and in Fig. 6 another kind, a blue-flag (l l, lips; h h, hollow of the mouth).
Fig. 6.
Air is a curious mixture. It is a gas made of several gases stirred together as you stir tea and milk and sugar. One of these gases is called oxygen (don't be afraid of the hard names); that is what keeps us alive. I won't give you the name of the next, because it is only used, like the milk, to weaken the tea. The third is a very disagreeable and dangerous gas, called carbonic acid gas. It is this last that makes your head ache in a crowded room or car. This is what you hear of every now and then as choke-damp, which suffocates people down in mines and deep wells. It is this which comes from burning charcoal, and makes it sure death to burn it in a closed room. There is very little of this dangerous stuff even in close air. Carbonic acid gas, though so poisonous, is made up of two things, which are very good and perfectly harmless when they are separated—carbon and the life-giving oxygen. Carbon is coal, or something like coal. United together, these two harmless things make a dreadfully dangerous one, just as innocent saltpetre, sulphur, and charcoal unite to form the deadly gunpowder.
Now notice how beautifully plants and animals are made to live together and help each other. Animals breathe in the air, and help themselves to the oxygen which keeps them alive, but breathe out the deadly carbonic acid gas. Plants breathing the air separate by some wonderful power of their own the carbonic acid gas into carbon and oxygen, help themselves to the carbon, and breathe out the oxygen. What plants consume we throw away as useless, and what plants breathe out sustains our life. That is the reason why the country is apt to be so much more healthy than the city. The air that is poisoned by people and fires becomes purified by plants.
Unlike the fairies of the story-book, who do all their good deeds by night, these little plant fairies work only by the light. The sun is their master, and his first ray is their call from sleep. They set to work in a minute, separating the dangerous carbonic acid gas into carbon and oxygen; and while they use the carbon and grow by it as you do by your food, they are giving back the sweet pure oxygen to the air. All day long they are at their good work; but when the sun sinks behind the hills, they do not need any sunset gun to tell them their time of rest has come. They drop work at once, and drop their fairy ways; they begin right away to behave as the animals do—to breathe in oxygen and breathe out the hateful carbonic acid. That is the reason it is not healthy to sleep in a room with flowers at night, though they are so good to have in the daytime.
We owe our lives to the plants—the food we eat, the pure air we breathe, as well as much of the rain that falls from heaven. They are ministering angels, and the loving, tender heavenly Father has appointed them their work to do—to beautify the earth and purify the air under the guidance of the glorious sun, which He has created, and which He keeps in its appointed path.