Take, in your garden or flower-pot, a plant that is dry and drooping. Pour water on it. Soak the earth about it well. In a very little time you will see the plant revive. The leaves will look fresh. The flowers will hold up their drooping heads. I have seen a plant quite changed in this way, in half an hour. So you see, in this little time the roots drank the water from the earth. The water, as sap, ran up all the pipes, through all the partition walls. The flower was fed in every part. When the plant is young and tender, it takes in much of its food through the whole surface or skin. It drinks in air, rain, and dew, through its skin. But as the plant gets older, and harder, it feeds more and more by the new roots and young leaves. At last, it takes almost all its food by the root and new leaves. I shall soon tell you what leaves eat. Why then does any plant or tree look so much better for a good washing? One reason is that the dust is washed off. Then the pores, or little holes in the skin, are free and open. The plant can breathe through all its skin. Do not forget that. Your skin is also full of little pores. They need to be kept open by plenty of washing. Let us go back to those little root-mouths. When they drink water, they get some solid stuff. Every drop of water has a mite of mineral in it. It is this solid matter that makes plant fibre. It is this that is left as ashes when you burn a plant, or bit of wood. The juice of plants is called _sap_. When the root drinks the water from the earth, it travels up through the pipes, through all the cell-wells. So it soon reaches the leaves. What do the leaves do with it? The leaves, having a wide surface, spread out the sap to the light and air. The heat of the sun cooks the sap. That is, it sets free, in a sort of steam, the clear water. The solid parts of the sap are left. You know that things are made thicker by cooking. Does not the molasses become thicker when you cook it for candy? When the sap is made thicker it is like glue. But it is still very thin glue. Also the light and air and heat change the sap in other ways of which I cannot now tell you. But the sap, being changed in the leaf, is now true plant stuff. It must travel through the plant to build it up. It is only in the green parts of the plant that the sap is changed to true plant material. What was at first mineral, held in water and air, is now vegetable, or plant matter. Now you see why a plant must have air and light, or it will not grow. Now that it is fit for plant food, the sap sets out to run through all the plant. It must go to every part to build all up. There are not two sets of tubes, one for raw and one for cooked sap. All must run through the same cells. So you see, the sap will be mixed. The raw sap going up has some food sap in it. The food sap runs about with some raw sap mixed with it.