Our Common Woods: Their Identification, Properties And Uses
Woods have different values for various practical purposes because of
their peculiarities in structure. A knowledge of the structural parts of
wood is therefore necessary as a means of recognizing the wood and of
determining why one piece is stronger, heavier, tougher, or better
adapted for a given service than another.
Structure of wood: If one examines a cross-section of the bole of a
tree, he will note that it is composed of several distinct parts, as
shown in Fig. 145. At the very center is a small core of soft tissue
known as the _pith_. It is of much the same structure as the pith of
cornstalk or elder, with which all are familiar. At the outside is
the _bark_, which forms a protective covering over the entire woody
system. In any but the younger 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.
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.
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.
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 between the other wood cells. They
extend out into the inner bark.
While much may be seen with the unaided eye, better results can be
secured by the use of a good magnifying glass. The end of the wood
should be smoothed off with a very sharp knife; a dull one will
tear and break the cells so that the structure becomes obscured.
With any good hand lens a great many details will then appear which
before were not visible. In the case of some woods like oak, ash,
and chestnut, it will be found that the early wood contains many
comparatively large openings, called _pores_, as shown in Figs. 146
and 147. Pores are cross-sections of vessels which are little
tube-like elements running throughout the tree. The vessels are
water carriers. A wood with its large pores collected into one row
or in a single band is said to be _ring-porous_. Fig. 146 shows such
an arrangement. A wood with its pores scattered throughout the
year's growth instead of collected in a ring is _diffuse-porous_.
Maple, as shown in Fig. 152, is of this character.
All of our broadleaf woods are either ring-porous or diffuse-porous,
though some of them, like the walnut, are nearly half way between
the two groups.
If the wood of hickory, for example, be examined with the magnifying
lens, it will be seen that there are numerous small pores in the
late wood, while running parallel with the annual rings are little
white lines such as are shown in Fig. 149. These are lines of _wood
parenchyma_. Wood parenchyma is found in all woods, arranged
sometimes in tangential lines, sometimes surrounding the pores and
sometimes distributed over the cross-section. The dark, horn-like
portions of hickory and oak are the _woodfibers_. They give the
strength to wood.
In many of the diffuse-porous woods, the pores are too small to be
seen with the unaided eye, and in some cases they are not very
distinct even when viewed with a magnifier. It is necessary to study
such examples closely in order not to confuse them with the woods of
conifers.
The woods of conifers are quite different in structure from
broadleaf woods, though the difference may not always stand out
prominently. Coniferous woods have no pores, their rays are always
narrow and inconspicuous, and wood parenchyma is never prominent.
The woods of the pines, spruces, larches, and Douglas fir differ
from those of the other conifers in having _resin ducts_, Fig. 144.
In pines these are readily visible to the naked eye, appearing as
resinous dots on cross-sections and as pin scratches or dark lines
on longitudinal surfaces. The presence or absence of resin ducts is
a very important feature in identifying woods, hence it is very
important to make a careful search for them when they are not
readily visible.
How to identify a specimen of wood: The first thing to do in identifying
a piece of wood is to cut a smooth section at the end and note
(without the magnifier) the color, the prominence of the rays and
pores, and any other striking features. If the pores are readily
visible, the wood is from a broadleaf tree; if the large pores are
collected in a ring it belongs to the ring-porous division of the
broadleaf woods. If the rays are quite conspicuous and the wood is
hard and heavy, it is oak, as the key given later will show. Close
attention to the details of the key will enable one to decide to
what group of oaks it belongs.
In most cases the structure will not stand out so prominently as in
oak, so that it is necessary to make a careful study with the hand
lens. If pores appear, their arrangement, both in the early wood and
in the late wood, should be carefully noted; also whether the pores
are open or filled with a froth-like substance known as _tyloses_.
Wood parenchyma lines should be looked for, and if present, the
arrangement of the lines should be noted.
If no pores appear under the magnifying lens, look closely for resin
ducts. If these are found, note whether they are large or small,
numerous or scattered, open or closed, lighter or darker than the
wood. Note also whether the late wood is very heavy and hard,
showing a decided contrast to the early wood, or fairly soft and
grading into the early wood without abrupt change. Weigh the piece
in your hand, smell a fresh-cut surface to detect the odor, if any,
and taste a chip to see if anything characteristic is discoverable.
Then turn to the following key:
KEY
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Tree Studies
How To Identify Trees
Group I The Pines
The White Pine (pinus Strobus)
The Pitch Pine (pinus Rigida)
The Scotch Pine (pinus Sylvestris)
Group Ii The Spruce And Hemlock
The Norway Spruce (picea Excelsa)
Hemlock (tsuga Canadensis)
Group Iii The Red Cedar And Arbor-vitae
Red Cedar (juniperus Virginiana)
Arbor-vitae; Northern White Cedar (thuja Occidentalis)
Group Iv The Larch And Cypress
The European Larch (larix Europaea)
Bald Cypress (taxodium Distichum)
Group V The Horsechestnut, Ash And Maple
The Horsechestnut
The White Ash (fraxinus Americana)
Sugar Maple (acer Saccharum)
Silver Maple (acer Saccharinum)
Red Maple (acer Rubrum)
Norway Maple (acer Platanoides)
Box Elder (acer Negundo)
Group Vi Trees Told By Their Form: Elm, Poplar, Gingko And Willow
American Elm (ulmus Americana)
Lombardy Or Italian Poplar (populus Nigra, Var Italica)
Gingko Or Maidenhair Tree (gingko Biloba)
Weeping Willow (salix Babylonica)
Group Vii Trees Told By Their Bark Or Trunk: Sycamore, Birch, Beech,
Blue Beech, Ironwood, And Hackberry
The Sycamore Or Plane Tree (platanus Occidentalis)
Gray Or White Birch (betula Populifolia)
American Beech (fagus Americana)
Blue Beech Or Hornbeam (carpinus Caroliniana)
Hackberry (celtis Occidentalis)
Group Viii The Oaks And Chestnut
White Oak (quercus Alba)
Black Oak (quercus Velutina)
Red Oak (quercus Rubra)
Pin Oak (quercus Palustris)
Chestnut (castanea Dentata)
Group Ix The Hickories, Walnut And Butternut
Shagbark Hickory (hicoria Ovata)
Mockernut Hickory (hicoria Alba)
Black Walnut (juglans Nigra)
Group X Tulip Tree, Sweet Gum, Linden, Magnolia, Locust, Catalpa,
Dogwood, Mulberry And Osage Orange
Tulip Tree (liriodendron Tulipifera)
Sweet Gum (liquidambar Styraciflua)
American Linden (tilia Americana)
The Magnolias
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Informational