ger stems, the bark is composed of an
inner, live layer, and an outer or dead portion.

Between the pith at the center and the bark at the outside is the
wood. It will be noted that the portion next to the bark is white or
yellowish in color. This is the _sapwood_. It is principally through
the sapwood that the water taken in by the roots is carried up to
the leaves. In some cases the sapwood is very thin and in others it
is very thick, depending partly on the kind of tree, and partly on
its age and vigor. The more leaves on a tree the more sapwood it
must have to supply them with moisture.

[Illustration: FIG. 144.--Pine Wood. (Magnified 30 times.)]

Very young trees are all sapwood, but, as they get older, part of
the wood is no longer needed to carry sap and it becomes
_heartwood_. Heartwood is darker than the sapwood, sometimes only
slightly, but in other instances it may vary from a light-brown
color to jet black. It tends to fill with gums, resins, pigments and
other substances, but otherwise its structure is the same as that of
the sapwood.

[Illustration: FIG. 145.--Cross-section of Oak.]

The wood of all our common trees is produced by a thin layer of
cells just beneath the bark, the _cambium_. The cambium adds new
wood on the outside of that previously formed and new bark on the
inside of the old bark. A tree grows most rapidly in the spring, and
the wood formed at that time is much lighter, softer and more
porous than that formed later in the season, which is usually quite
hard and dense. These two portions, known as _early wood_ or spring
wood, and _late wood_ or summer wood, together make up one year's
growth and are for that reason called _annual rings_. Trees such as
palms and yucca do not grow in this way, but their wood is not
important enough in this country to warrant a description.

[Illustration: FIG. 146.--White Oak Wood. (Magnified 20 times.)]

If the end of a piece of oak wood is examined, a number of lines
will be seen radiating out toward the bark like the spokes in a
wheel. These are the _medullary rays_. They are present in all
woods, but only in a few species are they very prominent to the
unaided eye. These rays produce the "flakes" or "mirrors" that make
quartersawed (radially cut) wood so beautiful. They are thin plates
or sheets of cells lying in