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 and water. He removed all the layers until he came to the wood. The wood was stained red. He cut slice after slice off the twig, and it was found that the coloured water had gone quite an inch up through the wood. None of the other layers of the twig were stained red.
“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 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