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.