“Now, take your knives,” he said, “and carefully peel off a very small piece of the brown covering. You will have to be very careful, as it is very thin and rather difficult to remove. Ah, Frank, you have done that very nicely. Now, hold it up to the light and tell us what you think it is for.”
“It is the skin or covering of the twig,” said Frank.
“It is; and if you look with your lens you will see that the tiny markings on it are holes. It is really a thin layer of bark or cork. Perhaps you can tell me why the twig is covered with a thin layer of cork?”
“To keep water from getting in,” said Frank.
“Rather to keep water from getting out,” said Uncle George. “You must remember that water is continually passing up stems from the roots. Water cannot pass through cork. If we were to remove the cork layer from the outside of a growing twig, that twig would shrivel up and die. There is also a layer of cork protecting the willow twig, but it is so very thin that we can see through it. Remove as much of the cork layer as you can, and tell me what is underneath.”
“There is a layer of green stuff beneath,” said Frank.
“Just so,” said Uncle George. “Now, if you remember, I once told you that plants took most of their food from the air by means of the green stuff in their leaves. This green layer in the twig does the same thing; but how can the air get in if it is covered up by a layer of cork?”
“Oh, I see now,” said Tom, “what the tiny holes or pits are for—to let air in to the green layer underneath.”
Diagram Sections of (A and B) a One-Year Old and (C and D) Two-Year Old Stem.