“We are going to have a peep into the inside of these twigs,” said Uncle
George, as he laid some willow and horse-chestnut twigs on the table.
“First,” he said, “let us examine the outside of them.” He handed a
small willow branch to Tom and a horse-chestnut branch to Frank.
“Now, Frank, tell me all you see.”
“I see the rings of scars which mark each year’s growth,” said Frank.
“This year’s growth is at the top, above the first ring of scars. Below
this is last year’s growth. Then comes another ring of marks, and below
this again is the growth of two years ago.”[1]
“That is to say,” said his uncle, “the upper part of your twig is about
one year old, the middle part is two years old, and the lower part is
three years old. Go on, Frank.”
“The upper part is light brown, while the middle and lower parts are
dark brown.”
“Take your lens, Frank, and look at that brown covering carefully.”
“Oh, it is studded all over with little oval marks like pits,” said
Frank.
“Now, what about the buds?”
“The buds,” said Frank, putting down his lens, “are in pairs; and the
stem is swollen where each pair of buds comes off.”
“Very good,” said Uncle George. “That is exactly what I wanted you to
notice. The swollen parts of a stem are called its _nodes_. In every
stem, buds and leaves occur at the nodes. Nodes are very well seen in
grass and corn stems.”
“Then at the top of the twig there is a very large bud and a pair of
small buds, one on each side of it,” said Frank.
“You have described it very well,” said his uncle. “Now, Tom, what about
the willow twig?”
“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.”
Uncle George then cut the twig straight across, and the boys saw that
each layer formed a ring. In the middle there was a round mass of pith.
Around this was a thick ring of wood with the thin slippery layer
outside. Outside this was a fairly thick ring, the outer half of which
was green in colour. And outside the whole lot was the ring of the thin
bark or cork which the boys had first examined.
“We have seen,” said Uncle George, “how a one-year-old stem is built.
Let us now make a clean cut through the two-year-old part of the twig,
and another through the three-year-old part.
“You see there are two rings of wood in the two-year-old part and three
rings of wood in the three-year-old part. What does this show us?”
“That a ring of wood is added every year,” said Frank.
“And so, three years ago, this thick branch was a tiny bud,” said Uncle
George, pointing to the lower part of the twig.
Exercises on Lesson I.
1. An apple will keep sound for months if the skin is unbroken. If we
remove a small piece of the skin, the apple soon shrivels up.
How is this?
2. Can you explain why we ought not to eat the outer skins of the
plum, grape, tomato, pear, etc.?
3. Cut a stout twig of any tree straight through. Make a rough sketch
showing the different layers, and tell how old the twig is.
4. Get stout twigs of different trees, such as ash, elm, holly,
sycamore. Take about an inch length of each. Split these down
the centre, and see if you can make out the different layers
on each side of the pith. Make an enlarged drawing of one of
these.