CHAPTER XIX.
THE SAP IN LEAVES.
I have told you about the ribs of leaves. Let us see what makes them so firm and strong. Look at a large grape-leaf on the vine. It spreads out very firmly. If the wind blows it very hard it bends, but it stands out again as firmly as ever. But break the leaf off, and see what happens. In a little time it wilts. If you hold it up by the stem its edges droop down all around. The leaf does not stand out as it did when it was on the vine. The ribs are all there, but they have lost their strength. How do you think they lost it? I will tell you.
Wilting of leaves explained.
When you broke off the stem, the sap could no longer get to the leaf. It is just as no water can get into a house when the water-pipe is cut off outside. The sap goes to all parts of the leaf from the stem through the ribs. The ribs, like the stem, have little fine pipes in them for the sap to run in. Now, if the ribs are not full of the sap they are not firm, and they bend easily. When these ribs and the net-work between them are not full of sap the leaf is wilted, as we say.
But when the leaf is picked it is full of sap. How does any of the sap then get out of it so as to make it wilt? It does not leak out of the stem. If it did, you could see it drop as you hold the leaf up. Where, then, does it get out? This I will explain to you. There are little holes, or pores, as they are called, all over the leaf. They are so small that you can not see them without a strong microscope. The watery part of the sap escapes into the air through these pores.
The quantity of moisture that comes from leaves.
There is a great deal of moisture that comes from leaves. You can see that this is so if you put a cluster of leaves under a glass vessel. A large tumbler will answer. You will, after a little time, see the moisture in drops on the inside of the glass. This moisture is the water that comes from the pores of the leaves.
You remember what I told you in the last chapter about the leaf of the pitcher-plant. The water in that leaf comes from its pores on the inside. If, instead of its having a pitcher-shape, the leaf was laid open and spread out like common leaves, the moisture would all go off in the air. But as it is a pitcher with a lid, the moisture that comes from all the pores is shut in. It can not fly off in the air. And after a while enough moisture collects to fill the pitcher. This shows how much water commonly goes from leaves into the air. If any leaf that you see spread out could be changed into a pitcher or cup shape with a lid, it would in a little time be full of the water that comes from its pores.
Now you can understand why a leaf wilts after it is picked. It does not wilt as soon as you pick it, for the sap is all in it then. But let it be a little while. The watery part of the sap is going out of the pores of the leaf all the time, and there is no sap coming to it through the stem. So the leaf wilts.
Keeping flowers from wilting.
You can keep a leaf from wilting for a long time by placing the stem in water. When you do this the water goes up through the little pipes in the stem. This takes the place of the water that goes out of the pores of the leaf.
When you put flowers in water, you know that the water is less the next day. This is because so much of the water goes up in the stems to the leaves and blossoms.
You know that if you have a plant in a flower-pot, the earth gets dry in a day or two. This is chiefly because the water in the earth is sucked up by the roots, and runs up all through the plant, and goes out of the pores of the leaves and blossoms. Some of the water goes up directly from the earth into the air, but most of it goes through the plant.
Much water in the air, but not seen.
You can not see the water that comes out of the leaves and blossoms into the air. There is a great deal of water in the air that you can not see. You have often seen in a hot day the water stand in drops on the outside of your tumbler. Just think how these drops come there. People sometimes say that the tumbler sweats, just as if the water came through the glass. But this, you know, can not be. Water can not get through glass. The drops come there in this way. The cold water in the tumbler makes the glass very cold. And the water in the warm air around the tumbler, therefore, gathers upon it. Sometimes there is much more water in the air than there is at other times. Then the tumbler is very wet. Now a great deal of the water in the air comes from the leaves of the trees and the plants all about us. The leaves may be said to be breathing moisture into the air all the time. I shall tell you more about the water that is in the air in Part Third.
This moisture that is breathed out from the leaves makes the air soft, while the fragrance of the flowers makes it balmy. Each leaf yields but a little water, and so does but little good in this way. But there are so many leaves that a great deal of water comes from all of them. It puts me in mind of the Scotch proverb, “Many a little makes a mickle.”
Lesson that can be learned from the leaves.
Those who want to do good in the world may learn a lesson from the leaves. A large amount of good may be done when a great many do each a little. Let those who can do but little think of this. Let them do every day what they can, just as each leaf does. Great men, that excite the wonder of the world, can do a great deal of good; but they can not do any thing like as much as is done by a great many people together that do each a little in a noiseless way. Every child, in doing little kind things, may, like the small leaf, do his part of the good that is to be done in the world. And if much of the good that he does is not noticed by others, God sees it all, just as he sees all the moisture that is breathed out by each little leaf.
Questions.—What makes the ribs of leaves firm? What happens to these ribs when a leaf wilts? How does the watery part of the sap get out of a picked leaf? What is said of the quantity of water that comes from leaves? Tell about the water in the leaf of the pitcher-plant. How does a picked leaf wilt? How does putting a leaf in water keep it from wilting? What makes the earth in a flower-pot become dry? Can you see the water that goes into the air from the leaves and other things? Tell about water settling on tumblers in hot weather. What lesson can we learn from the leaves?
CHAPTER XX.
THE USES OF LEAVES.
Refreshing moisture from leaves.
One use of leaves, as I told you in the last chapter, is to supply the air with water. In the hot weather the air would be very dry and uncomfortable to us if the leaves did not breathe out moisture from their pores. You can see how this is if in a hot day you walk across a sandy plain where there are no leaves except those of the scanty grass and weeds. Here no moisture is breathed out upon you, to lessen the heat that you suffer from the burning sun.
Another use of the leaves is this. They are pleasant and beautiful to the sight. I have told you about this use of them in the beginning of the seventeenth chapter.
Their shade.
Another use of leaves is to give shade. We know how refreshing this is to us in a hot day. When in a city we walk through streets where there are no trees, how delightful it is to come out of the blazing sun into a square that is full of trees! How comfortable are the cows in the pasture lying under the trees at mid-day, chewing the cud!
But the shade given by leaves does good not merely to man and animals. It does good to fruits, if there is not too much of it. The sun would very often be too hot for the fruits, if it shone full on them all the time. So the leaves partly shade them.
The grape-vine stripped of its leaves.
The chief use of leaves is to keep plants and trees alive and make them grow. If you should strip off the leaves