T
ake a tree and peel off the
outer “skin” or bark and what
you’ll find is two kinds of wood.
Closest to the edge there’s a moist,
light, living layer called sapwood
packed with tubes called xylem
that help a tree pipe water and
nutrients up from its roots to its
leaves; inside the sapwood there’s
a much darker, harder, part of the
tree called the heartwood, which is
dead, where the xylem tubes have
blocked up with resins or gums
and stopped working. Around the
outer edge of the sapwood (and
the trunk) is a thin active layer
called the cambium where the tree
is actually growing outward by a
little bit each year, forming those
famous annual rings that tell us
how old a tree is.
Slice horizontally through a
tree, running the saw parallel to
the ground (perpendicular to the
trunk), and you’ll see the annual
rings (one new one added each
year) making up the cross-section.
Cut vertically through a tree trunk
and you’ll see lines inside running
parallel to the trunk formed by the
xylem tubes, forming the inner
structure of the wood known as its
grain. You’ll also see occasional
wonky ovals interrupting the grain
called knots, which are the places
where the branches grew out from
the trunk of a tree. Knots can make
wood look attractive, but they can
also weaken its structure.
2 . wood architecture