“I see some very tiny marks on it,” said Tom. “There are no rings marking the year’s growth; and the buds are not arranged in pairs.” [Illustration: Horse-Chestnut Twig, showing Three Years’ Growth.] “That is quite right, Tom. Willow grows very quickly. Your twig is all one season’s growth. It is smooth and green—not brown like the horse-chestnut twig. The buds are arranged alternately. That is to say, there is only one bud at each node.” “Now, let us see what the horse-chestnut twig is made of.” Uncle George next cut two pieces off the top part of the horse-chestnut twig and handed one to each of the boys. “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.” [Illustration: Diagram Sections of (A and B) a One-Year Old and (C and D) Two-Year Old Stem.] 1. Bark. 2. Green Layer. 3. Bast. 4. Slippery Layer. 5. Wood. 6. Pith. “That is what they are for, Tom,” said Uncle George. “Scrape away this green layer. It is greenest on the outside and is rather thick. There are really two or three layers there, but they cannot be separated with a pen-knife. What do you come to next?” “A white, woody layer with a very slippery surface,” said Frank. “That slippery surface is itself a layer, and a very important one,” said his uncle. “The wood, you can see, is a very thick layer. In the centre you have a mass of dry, spongy stuff. This is called the pith.”