The roots and stems and leaves are all full of little passageways running upward and branching and dividing until they reach the leaves. Fig. 2 shows a corn stalk cut across. You see some roundish holes, marked a; these are the ends of tubes that run through the stalk. When the corn begins to grow, take a stalk about two feet high, and cut it across; you will see little white spots all over the cut place. This figure is one of those white dots magnified.
Fig. 3.—Plant Mouths.
A, Corn Leaf: B, Bean Leaf, with Mouth: C, Mouth seen Sidewise.
When these tubes come into the leaves, they open into little spaces just under the outside skin of the leaf. These spaces are like the hollow of a mouth, and each one has generally two lips, that are sometimes open and sometimes shut. Through these tiny mouths (Fig. 3) the plant breathes. It draws in air, and it sends out, as you do, a mixture of air and water. If you want to know how much water there is in your own breath, try holding a piece of cold glass before your mouth.
Plants are not wasteful of the water so necessary to their lives. What they do not use they give back to the air from which it was received, as we make our thank-offerings to God of what He has given us. The roots suck up the water, and each little cell takes a drink as the water passes it, and hands on the rest to the cell just above it. And so the water takes its course, supplying each thirsty cell with drink as it passes, spreading through every part of the plant until it reaches the little mouths. And there all that is left is breathed out in a fine steam which you can not see until it touches some cold substances, and is turned into water again.
Some one who wanted to know exactly how much water was given back to the air by growing plants carefully examined a number of them, and found that a single sunflower gave off in twelve hours a pound and a quarter—enough to fill nearly to the brim three common table goblets. Another plant, the wild cornel, was found to breathe but more than twice its own weight of water in a day and a night.
Fig. 4.—Water-carrying Plants.
In order to find out what parts of the flowers were the principal water-carriers, a deutzia, one of our most delicate and beautiful spring flowers, which you probably know by sight if not by name, was put into some very blue water, colored with a mixture of what is called aniline, and in a little while every vein of the flower was a beautiful dark blue. The poor little blossom was, however, poisoned with its dose, and wilted away in a few minutes (Fig. 4).
The quantity of water that plants breathe off is so great that it makes an entire change in the climate when forests are cut down. Plants, like grasses and small weeds that grow on the surface, of course do not make the same difference, for their roots only go down a little way. But trees are very important: unless the air is kept damp by the sea or some large body of water, it depends very much upon trees for its moisture. Where there are no trees, the rain that does fall sinks into the earth, and runs away in little under-ground currents, and is lost. There are no deep roots to stop this waste, to suck up the water, and restore a large part of it to the air.