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 hour, and I think we
shall find that some change has taken place in each of our first three
jars.
“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 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.
[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?