They breathe when the leaf mouths take from the air the oxygen, and give back to it carbonic-acid gas.
The veins and veinlets, of which you see so many running through a leaf, act in something the same way as the water pipes of a city; for through these veins the watery food, the earth broth, is carried to the different cells.
When one knows all that we know even now about a plant, one looks at a tree covered with leaves with a good deal of admiration.
Just think of what is being done inside that quiet-looking tree! Think of the millions of cells that go to make it up, each cell having its own work to do! Think of the immense amount of business being carried on within the trunk, inside the branches, and especially in each green leaf! And when you have the chance, notice how hard each leaf tries to get just as much sun and air as it possibly can.
In the first place, the thin, flat leaf blades are so spread out that every part is exposed to the light and air.
Then notice how the leaves are placed in reference to one another.
Almost every single one is fastened to the tree so as to get its fair share of sunshine.
When you think of the many thousands of leaves borne by one tree, it astonishes you to see how seldom one leaf gets in another’s light.
And the shapes of leaves are always suited to their arrangement on the tree.
If you should take the leaves of a chestnut tree and replace them by the leaves of a maple, you would find the maple leaves all getting in each other’s way, or else you would see that they were taking up a great deal more room than necessary.