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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



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