Now just think over the various things that are made from the sap in plants. There are wood, bark, leaves, flowers, fruits, thorns, perfumes, colorings, sugar, starch, gum, various medicines, etc. And then there are many other things that I have not mentioned. How strange it is that so many and such different things can be made from what the plants suck up out of the earth! As you look at the ground under your feet, you can hardly believe that so much can be got out of it. It is the busy little mouths in the roots that get from it what is needed to make all these different things.
Questions.—What is said of the sugar-maple? What is said of sugar in some roots and fruits? As there is no sugar in the ground, how does it get into plants? Can any body make sugar from earth? What plants are starch-factories? Mention some medicines made in plants. What is said about plants that are gum-makers? What is said about perfumes being made in plants? What about colors? What is said about indigo? Mention now all the things that you can think of that are made from the sap in plants.
CHAPTER XXXI.
CIRCULATION OF THE SAP.
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.