You cannot make things out of thin air, people say. There is no such thing as thin air, if by that is meant something empty. It is really very thick and powerful, and from it all things are made that are made, or without it cannot be made, whether tree, plant, person, or army tank.

What is the atmosphere? It is an air ocean. We walk at the bottom of an air ocean at a depth of from two hundred to five hundred miles. We cannot see it, we cannot touch it, and yet it presses down upon us with a pressure of a ton to every square foot. Each acre of land sustains forty-six thousand tons of air. It is possible to carry this surprising weight on our heads owing to the way it equalizes the pressure all around us.

The atmosphere is composed, as everything is composed, of small items called molecules. They are not all of the same kind. Some contain oxygen, others carbon dioxide, some nitrogen, others argon; some ozone, others nitric acid; some water vapour, others ammonia. In quantity nitrogen heads the list and oxygen seconds it, while in importance the carbon dioxide is second to none. When we grasp that water, carbon, nitrogen, nitric acid, and ammonia contribute ninety per cent of all the materials that are built into the tissues of plants, it is easy to see how necessary it is that they should have roots in the air as well as roots in the soil.

The leaves are these upper roots or mouths — plants are pretty well all mouth.

— John Stewart Collis in The Worm Forgives the Plough