Yet the earth broth seems to have no difficulty in making this long, steep climb.
Now, even wise men have to do some guessing about this matter, and I fear you will find it a little hard to understand.
But it is believed that the roots drink in the earth broth so eagerly and so quickly, that before they know it they are full to overflowing. It is easier, however, to enter a root than it is to leave it by the same door; and the result is, that the broth is forced upward into the stem by the pressure of more water or broth behind.
Of course, if the stem and branches and leaves above are already full of liquid, unless they have some way of disposing of the supply on hand, they cannot take in any more; and the roots below would then be forced to stop drinking, for when a thing is already quite full to overflowing, it cannot be made to hold more.
But leaves have a habit of getting rid of what they do not need. When the watery broth is cooked in the sun, the heat of the sun’s rays causes the water to pass off through the little leaf mouths. Thus the broth is made fit for plant food, and at the same time room is provided for fresh supplies from the root.
Fig. 137
If you should examine the lower side of a leaf through a microscope, you would find hundreds and thousands of tiny mouths, looking like the little mouths in this picture (Fig. [137]).
Some of the water from the earth broth is constantly passing through these mouths out of the plant, into the air.