  “We have seen how an ordinary twig is built up,” said Uncle George. “Let us now try to find out what goes on inside the twig; and in order to do this we shall have to perform one or two simple experiments.” Uncle George took two wide-mouthed glass jars. They were both perfectly dry, and each could be closed with a large, tight-fitting cork. He placed some fresh green leaves inside one of the jars. The other remained empty. Then both jars were tightly corked up, and both corks covered outside with wax. First Experiment. “That is experiment number one,” he said, as he placed both jars in the window. He next took a small plant which was growing in a pot. He wrapped the pot up in thick, dry paper, so that the paper covered up everything but the plant. The edges of the paper were tied tightly round the lower part of the stem of the plant with a string. The plant was put in the window, and over it Uncle George placed a glass bell-jar. Second Experiment. “The third experiment is much simpler,” he said. “You see I just put one of the willow twigs into a glass half filled with water, and into the water I pour some red ink. Frank, place a white pansy in the coloured water beside the willow twig.” Uncle George’s fourth experiment was as follows:— He placed four willow twigs in a glass of water. But from two of these twigs he first removed a broad ring of the bark and outer layers, leaving about an inch of the wood bare near the lower end of the twig. “Now,” said he, “we will come back in about an “The fourth glass one will have to be left for several weeks; and we must take care to keep water always in the glass containing these four twigs.” About an hour afterwards, Uncle George and the boys came to look at the experiments. “Let us examine experiment one first,” said Uncle George. “The empty jar is just the same as when we put it there,” said Tom. “The jar containing the leaves is all dimmed, and there are tiny drops of water on the inside of it.” “Where did that water come from, Tom?” “It must have come out of the leaves.” “Exactly so! Now look at the second experiment, and you will see that the bell-jar which covered both plant and pot is also dimmed with moisture. Pot and soil were securely covered up, so that this moisture on the glass must have come from the leaves of the plant. Where do you think this water really comes from?” “From the soil in the pot,” said Frank. “If we did not water those plants which we keep in pots, they would die.” “Then we have learnt that water travels up the stems of plants,” said Uncle George, “also that it is drawn from the soil and is given off by the leaves. The third experiment, where we placed a twig in coloured water, will, I think, show us which part of the stem the water travels up.” Uncle George peeled the bark off the lower end of the willow twig which was placed in the mixture of red ink “It travels up through the wood-layer,” said Frank. “And look at the pansy flower. It was white when we put it into the glass, now it is all streaked with red.” “The flower itself,” said Uncle George, “is not near the coloured water.” “The water must have travelled up the long stalk to the flower,” said Frank. 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 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 Questions on Lesson III.
|
|
