“The water must have travelled up the long stalk to the flower,” said Frank. [Illustration: The Fourth Experiment.] The fourth experiment was left in the window, and two or three weeks passed before any change was noticed in any of the four twigs which had been placed in the water. Then roots began to grow. In the two whole twigs these roots grew out at the bottom end. But in those twigs from which the belt of outer layers had been removed, it was quite different. Here the roots grew out—not at the bottom, but just where the ring of bare wood began and at the top of it. Frank and Tom were quite puzzled. They could not understand why the roots should come at the bottom in two of the twigs and not in the other two. “First of all, then,” said Uncle George, “these new roots were made from materials which came from inside the plant. These building materials are carried through the plant dissolved in water—just as you dissolve sugar in your tea. Water containing these dissolved stuffs in a plant is called _sap_. “We have seen, by our first three experiments, that water travels up the wood part of the stem. This experiment shows us that sap travels _down_ the stem in the layers outside the wood. For, when I removed the outer layers and left a bare ring of wood, the flow of the sap was stopped and the new roots formed there.” “And where does this sap come from first of all?” Tom asked. “It is really formed in the leaf first of all. I think I told you that plants take most of their food from the air by means of their green leaves. In the great quantities of water which pass up the wood and into the air from the leaves there is always a very little mineral matter dissolved. This small quantity of mineral matter comes from the soil. This, along with water and the large quantity of matter taken from the air, are changed, inside the leaf, into a fluid which we call sap. “Our four experiments show us that water travels up through the wood-layer from soil to leaf; and also that sap travels down through one of the outside layers of the stem.” Questions on Lesson III. 1. How does water travel in a plant? How can you prove this? 2. If we enclose a leafy plant in a glass vessel, we see that water is given off by the leaves. How is it that we do not see this water when the plant is grown in the open air? 3. What is “sap”? Where is it first formed? How does it travel in the twig or stem? 4. If we keep ferns growing under a glass bell—or in a glass case—we never have to water them. Can you explain this? IV.—SNOWDROP, CROCUS, AND TULIP.