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 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?”