The Child's Book of Nature. Worthington Hooker
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I have told you that the sap goes up in a plant or a tree in certain pipes. Now when it gets to the leaves it turns about and goes back again down toward the ground by some other pipes.
The difference between the sap that goes up and that which comes down.
So there is a set of pipes for the sap to go up, and a set of pipes for it to go down. In a tree, the pipes for it to go up are in the wood. Now where do you think the pipes are for it to go down? They are in the live part of the bark. The sap is all the time going up to the leaves in the one set of pipes, and coming down in the other set. And this is what we call the circulation of the sap.
The sap that goes up has a great deal of water in it. Much of this water is got rid of when the sap comes to the leaves. You remember that I told you, in the chapter on leaves, that water is let off into the air from their pores. For this reason the sap that comes down from the leaves has much less water in it than the sap that goes up.
The sap that goes up is not perfect sap. It has to make a visit to the leaves and get an airing there before it can be of much use. After it is aired it goes to all parts of the plant, down to the very roots.
It is this aired sap from which generally every part of the plant grows, or is made. You remember that I told you in the last chapter that in trees the inner bark makes a new layer of wood every year. Now the bark makes the wood from some of this aired sap as it goes down in the pipes of the bark.
The airing of the sap.
You remember that I told you in the chapter on leaves, that they have much to do with the growth of a plant. You can now see why this is so. The sap has to go up to the leaves to be made good sap. Just what the air does to it there you are not yet old enough to understand. But after a little time you will be able to understand this, and then you will see that leaves are very properly called the lungs of plants, and that they breathe with them as we do with our lungs, though in a different manner.
The sugar made from the sugar-maple.
I have said that the sap that goes up is not of much use, and that every thing in the plant is made from the sap that goes down. This is not always so. In the sugar-maple it is the sap that goes up in the early spring that has the sugar in it. The sugar-gatherers tap the trees before the leaves are put forth. The leaves, then, have nothing to do with making the sugar. How it is made we can not understand. We suppose that it is done in the root, where the mouths are that drink up the sap from the earth. But though we do not know how it is, in some way every sugar-maple as soon as it begins to be warmed by the air of spring becomes at once a sugar-factory.
Though most of our sugar comes from the sugar-cane of southern climates, a great deal is made from the sap of the sugar-maple in some parts of the northern and western states in this country. A very busy time they have in some places in the early spring in collecting the sap and in boiling it down. The sirup is often sold as maple-sugar molasses. But more often it is made into sugar; and great quantities of it are sold every year. In some places where it is made many of the people use no other sugar.
The sap always in motion except in winter.
The sap is all the time in motion in the trees and plants in all the warmer months of the year. It is always going up and coming down. It does so till the leaves fall and the cold of winter comes. Then all this motion stops. And through the winter the sap is almost as still as if the trees and shrubs were dead. Then when the spring comes, the mouths in the roots begin again to suck up sap from the ground, and it runs up and down in the little pipes as it did the year before.
As you look at all the trees and plants about you, think how much sap there is running up and down in their pipes. Look at a very large tree, and think of this. In multitudes of pipes in that huge trunk the sap goes up to the very end of all the branches to the leaves, and then it comes down in other pipes. How wonderful this is, and yet how few there are that ever think about it!
Questions.—Where are the pipes in which the sap goes up in a tree? Where are the pipes in which it comes down? What is said about the water in the sap? What becomes of a part of this water? Why is it necessary for the sap to go up to the leaves? Are things made from the sap that goes up, or that which comes down? How is it with the sugar in the maple? Where is its sugar made? Is the sap always in motion?
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