wood,
everything already inside the trunk
stays
the
same,
And so
a
tree never gets thinner, only thicker.
That's a good thing, because the tree is also
growing taller, and that means it's getting heavier. It
needs a sturdy trunk to support its weight, just like
you need
a
strong skeleton to support your growing
body. That's why adults are stronger than children,
and older trees are stronger than younger ones.
There's something else trees have that makes them
similar to people. Your blood flows through tiny tubes
called arteries that run from your heart to every part
of your body. Trees have to pump water from their
roots
all
the way
up
to their
crowns,
so
they
have an
extensive system of tiny
tubes,
too, called vessels.
You might be able to see these water vessels if
you
look at the rings in
a
piece of wood—they look like
small holes.
But water only flows in the tree's outermost
rings.
That's why in the summer, if everything is
working properly, a tree will be quite wet under
its bark, Farther inside, the wood becomes drier.
Nothing's happening there anymore, and the tree
can't feel anything there, either. That's why it
doesn't really matter if the inner wood begins to
rot. Even if its trunk becomes hollowed out like an
empty pipe, the tree will still
be
just fine,
*
But there's one more question. Why does
a
tree
need
a
trunk at all? Couldn't its crown grow
straight out of its roots?
A
tree needs a trunk so it can grow taller than
all the other plants around it, Otherwise it would
just be
a
bush. (Bushes don't have trunks.) Since
they're the tallest plants in the world, trees
don't need to be afraid of other plants. They can
grow above them all—except for other trees, of
course. [We'll find out later why there are some
anxious oaks out there.)
FIND
A
BIRCH
LOG.
If your family
buys
firewood,
it
will often
have
birch logs
in It—you can
recognize
them by their white
and
black
bark.
If
you
don't
have any firewood at home, ask your friends
who have fireplaces or fire pits.
Spread some dish detergent over one end of
the log. Now press your lips against the other
end
and
blow.
If
the
experiment works properly,
you'll see soap
bubbles
coming
out.
That happens
because the air you blow travels through the
water vessels in the wood until it reaches the
soapy film at the other end.
