By A. N. Foulger

U.S. Department of AgricultureForest Service

Forest Products Laboratory

April 1969

PA-900

For sale by the Superintendent of Documents, U.S. Government Printing OfficeWashington, D.C. 20402 - 60 cents.

Contents

A WORD ABOUT WOODSoftwoodHardwood The woody cellExamining wood structure

WOOD IN PLANTSThe growth sheath idea (Exp. 1)Where water moves in plants (Exp. 2)How water moves in plants (Exp. 3)Size of openings between cells (Exp. 4)What happens inside a leaning softwood

tree (Exp. 5)WOOD ANATOMY

The three faces of wood (Exp. 6)Why woods have different properties

(Exp. 7)What a single wood cell looks like (Exp. 8)Compression wood in softwoods (Exp. 9)Tension wood in hardwoods (Exp. 10)

Page11356889

1012

131414

15161718

EFFECTS OF ANATOMYWhich way does the water go (Exp. 11)Not all oaks are alike (Exp. l2)What happens when wood gets wet

(Exp. 13)What is specific gravity? (Exp. 14)How specific gravity affects expansion

(Exp.15)Making paper (Exp. 16)How strong is a single fiber? (Exp. 17)Why use glue? (Exp. 18)Why use plywood? (Exp. 19)When to use a blunt nail (Exp. 20)

GLOSSARYPHOTOGRAPHS OF WOOD STRUC-

TURE

Page191920

2122

23252628293031

34

Preface

Teachers often ask, “Are there no simpledemonstrations I can use to show how a familiarmaterial such as wood behaves? And, betterstill, aren’t there ways of showing why ?” Toanswer these many requests, the demonstrationshere have been selected to show some of theproperties of wood and how these propertiesrelate to the cellular structure of wood. Thismanual starts with some experiments usingwoody plants, follows with illustrations of woodanatomy, and finally illustrates the effect ofanatomy on performance. Most of the demon-strations are quite simple, requiring little or nospecial apparatus, and may be used in lowergrades. A few are more elaborate and needequipment normally f o u n d in a botanylaboratory.

The intent is not to provide a course of study,but rather to present a collection of demonstra-tions which can be used in series, or individually,

to illustrate a particular lesson. Several of thestudies can be expanded to meet the needs ofolder pupils, or where suitable equipment isavailable. Without doubt, many of the demon-strations can be improved, and there is littlereason why new studies in this area should notbe devised by the thoughtful teacher or theimaginative student working on a scienceproject.

The Forest Products Laboratory of the ForestService, U.S. Department of Agriculture, whichprepared this manual, greatly appreciates thewillingness of the following instructors andauthors to permit their work to be included: P.B. Kaufman, Department of Botany, Universityof Michigan, Ann Arbor, Mich.; R. W. Kennedy,Canadian Forest Products Laboratory, Van-couver, B. C.; J. L. Farrer, Department of Bot-any, University of Toronto, Toronto, Ont.; andD. C. McIntosh, The Mead Corp., Chillicothe,Ohio.

iii

A WORD ABOUT WOODWood is so commonplace that we take it for

granted. Yet that solid chunk of wood becomesan intricate arrangement of strong, though tiny,cells when seen under a microscope. If we trimthe surface of a piece of wood with a sharp knifeand look at it under a 10 × hand lens, we canbegin to see this cellular network.

The majority of wood cells are long and thinwith tapered ends, rather like hollow toothpicks.When we say that they are long, however, wemean only that they are much longer than theyare wide, For example, cells from white pinewood are about 4 millimeters long, which is alittle taller than this capital 1. Most of the cellsare this shape, and have their long dimensionnearly parallel to the long direction of the treestem. Whether we see the large or small dimen-sion of the cell depends on how the wood is cut.

Generally, woods are divided into two groups,softwoods and hardwoods, depending on the typeof tree. The softwoods consist of the cone-bear-ing trees, which have needles or scalelike leaves,such as the pines, spruces, and firs. The othergroup, called hardwoods, consists of broadleavedtrees which usually drop their leaves in the fall.These include the oaks, ashes,. birches, maples,and other contributors to the fall colors. The sep-aration into these two groups does not mean thatall the hardwoods have harder wood than thesoftwoods, but it is a convenient way to dividewoods based on their structure.

SoftwoodThe arrangement of cells in a piece of soft-

wood is shown in figure 1. We speak of threefaces of wood, namely the cross-sectional (1),radial (2), and tangential (3). The cross-sectional face is the surface we see if we cutacross a tree stem, or when we look down at atree stump.

Growth RingsIt is on the cross-sectional face that the

growth rings, or annual rings (4), are mosteasily seen. A growth ring is the layer of woodformed around the stem in a single year. Eachyear the tree trunk grows in width by producinga new layer of wood cells just below the bark.At the same time, the covering bark expands to

accommodate the new wood by the addition ofnew bark cells. Both wood and bark are pro-duced from a layer of living cells, called thecambium, between the wood and bark.

Earlywood and Latewood CellsCells formed in the spring and early summer

are wider in the radial direction, thinner walled,and shorter than those produced later in sum-mer. The thin-walled cells comprise the early-wood (5) and the thicker walled ones latewood(6). Each growth ring consists of both early-wood and latewood. Thus, on looking at a crosssection one can see the alternate rows of light-colored earlywood and darker latewood. Bycounting the number of bands of earlywood be-tween the pith, which is in the very center of thestem, and the bark, we can tell the age of the treeat that particular height. Almost all the uprightcells in softwoods are relatively long withpointed ends. These are called tracheids andsupport the tree in addition to providing apassage for liquids to move from the roots to thecrown.

Wood RaysThe bottom left of figure 1 is marked (2), indi-

cating the radial face. This is what we would seeif we cut the stem along a radius from the pithto the bark. On this face the rays (7), wherefood is stored and distributed radially in thestem, are most clearly visible. The ray cells havetheir long dimension lying horizontally in thetree. Rays are several cells deep and may be oneto several cells wide. A ray may consist only ofray cells (7) or it may contain a horizontal resinduct running along the center. This is called afusiform ray (8).

On the third face, the tangential (3), the endview of the rays can be seen, and it is on this facethat it is easiest to tell whether they are simpleor fusiform.

PitsSolutions pass from cell to cell through small

openings in the cell wall called pits. As the cellwalls in two adjacent cells develop, small circu-lar depressions are left in the thickening sec-ondary wall. Usually the depressions occuropposite each other in adjoining cells, so that a

1

Figure 1.—Wood structure of a softwood.

2

pair of pits forms with only the pit membrane,composed of the two primary walls and theinter-cellular layer, between the two pit cavitiesas illustrated in figure 2. The two pits are some-times referred to as a pit pair, but usually theterm pit is applied to the openings in the two cellwalls. The pit membrane acts as a filter and alsoas part of the supporting wall.

The three main types of pits, called bordered,simple, and half-bordered, are shown diagram-matically in cross section (fig. 2). The pit be-tween two ordinary support cells, or tracheids,has an overhanging border, hence the namebordered pit (2A). The opening between two

M–134,593Figure 2.—Cross sections of: A, a bordered pit; B,

simple pit; and C, a half-bordered pit.

ray cells has no border, and is called a simple pit(2B). As one might expect, the pit between atracheid and a ray cell has a border on thetracheid side but none on the ray side. Thus it ishalf-bordered (2 C). In bordered and half-bordered pits of softwoods, the pit membranehas a circular thickening in the center, called thetorus. If this is pushed against the pit border itforms an effective seal against the movement ofliquids through the pit.

In the softwood block diagram, figure 1, abordered and a simple pit are shown, (11) and(12) respectively.

Resin DuctsResin ducts may run up and down, figure 1,

(9), or horizontally (10) in a stem. A resin ductis formed by a space between several cells ex-panding to form a large opening in the wood. Aswould be expected from the name, these ductsare usually full of sticky resin, which is releasedfrom the cells lining the opening, or duct.

Hardwood

When we look at the hardwood block, figure 3,we can see both similarities to, and differencesfrom, the softwood. There are the same threefaces and the general structure is similar, withthe long dimension of most of the cells parallelto the length of the tree. However, we find agreater variety of cells in a hardwood than in asoftwood, and the cells are not arranged in theorderly radial rows found in the softwood, seefigure 1.

In figure 3 the radial face (2) allows us to seethe side view of the ray (7), while on the tan-gential face (3) the ends of the rays are visible.The hardwood rays may be from one to manycells wide, and seldom contain resin ducts as arefound in the softwoods.

Vessels

In the diagram of the hardwood block, figure3, some rather large cells called vessel segments(8) can be seen. Unlike the resin ducts in soft-woods, each vessel segment develops from asingle cell. The segments are joined end to end inthe stem, like a series of drainpipes, the longtubes they form serving as the main passagesfor liquid moving from the roots to the crownof the tree.

3

M-134,265

Figure 3.—Wood structure of a hardwood.

4

At the end of each vessel segment there is asmall grill, called a perforation plate (9), visiblehere on the radial face. The arrangement of theopenings in the grill is different for each speciesof tree. The remainder of the wood is made upmainly of fibers, which are cells similar to soft-wood tracheids, and parenchyma cells contain-ing food material.

Between vessels the pits usually are bordered,as are those between vessels and fibers. Pits be-tween vessels or fibers and ray cells are simpleon the ray cell side and usually bordered on theother.

Ring-Porous and Diffuse-Porous

In the hardwood shown (yellow-poplar) invessels are about the same diameter both in theearlywood (5) and in the latewood (6). Whenthe vessels are almost the same width through-out, the wood is called diffuse-porous becausethe vessels are scattered (diffused) throughoutthe entire annual ring.

Other hardwoods, oaks for example, havemuch wider vessel segments in the earlywoodthan in the latewood. These hardwoods arecalled ring-porous because, on looking at a stemcross section, the wide vessels or pores appearas rings, with intervening bands of wood be-tween them having smaller vessels.

Sapwood and HeartwoodSapwood is made up of the outermost growth

rings in the stem, those lying just under thecambium and bark. One cannot give a precisefigure for the width of this zone as it varies fromspecies to species, from tree to tree, and evenwithin a single tree. In the sapwood many of thecells are still living, containing starch and sug-ars, and it is through this wood that water andnutrients flow from the roots to the leaves. Thewood generally is lighter in color than the heart-wood, and less resistant to decay and insectattack.

While sapwood is present in all trees, not allspecies have a definite heartwood zone. Near thecenter of the stem, as a tree grows older, thewater content of the wood gradually decreases,there is a reduction in the amount of stored foodmaterial, and resins, gums, and tannins accumu-late. This zone, which progressively increases inarea, is referred to as the heartwood, and differsfrom sapwood particularly in its chemical com-

position. Typically the heartwood is darker incolor than the sapwood, as in black walnut.

Physical changes in the cells during heartwoodformation may include the torus being pressedagainst the pit border, blocking the pit aperture.Or tyloses may develop, where a parenchymacell expands into, and fills, the cell cavity of anadjoining, nonparenchyma cell. The chemicaland physical changes do not reduce the strengthof the wood, but make it more resistant to decayand insect attack. Thus while the outer ring ofsapwood both supports the tree and serves as astorage area and passage for nutrients andwater, the heartwood is dead tissue servingsimply as a support. Heartwood may be foundin both softwood and hardwood species.

The Woody Cell

When we talk of cells in wood our main con-cern is with the cell wall. This cell casing deter-mines the size and shape of the cell. Almost allof the cells in the wood we used were no longerliving, that is, did not contain living protoplasm,when the tree was felled. To be sure, there wasthe living cambium layer, and the few cells justinside it which are always alive in the tree stem.In addition, there were storage cells, calledparenchyma, both in the rays and arranged ver-tically in the stem. These retain sugar and starchafter the surrounding, supporting tracheids(the long, pointed cells), and vessels (the wide,conducting cells) have lost their protoplasm andbecome a passage for liquids to pass up the stem.Thus for the most part, we are concerned notwith living cell contents, but rather with thewalls of the cells that make up the wood.

Lignin

If we look at a tracheid under a high-poweredmicroscope, the wall appears as a series of lay-ers. This is shown in the diagram in figure 4.The intercellular layer, which lies between thecells and serves to hold them together, consistslargely of lignin. The composition of lignin is notfully known, but it serves to stiffen the plantstem. As we can see from figures 1 and 3, thecells overlap each other for part of their length,and, bound together, form the solid substance ofwood. In papermaking, the intercellular sub-stance may be dissolved away to free the sepa-rate fibers, which can then be made into a sheet.

5

M-134,271

Figure 4.—Cross section of a woody cell showing theseveral layers in the cell wall.

Arrangement of Cellulose in the Cell WallThe cell wall consists mainly of cellulose

arranged in long threadlike chains, and inter-spersed with lignin, and hemicelluloses. Woodcontains 40 to 55 percent cellulose, 15 to 25 per-cent hemicellulose, 15 to 30 percent lignin, and2 to 15 percent other substances called extrac-tives. The long cellulose molecules are groupedtogether in strands, which in turn form thicker,ropelike structures called fibrils. The arrange-ment of the fibrils is indicated in figure 4. In theprimary wall they form a loose irregular net,then on passing into the first layer of the sec-ondary wall, S1, the network becomes more pre-cise. By the time the S2 layer of the secondarylayer is reached, the fibrils run almost parallelto each other in a spiral around the cells. Thislayer is the thickest of the several in the cell walland has the greatest effect on how the cell be-haves. The smaller the angle which the flbrilsmake with the long direction of the cell, thestronger is the cell. Finally, in the innermostlayer of the cell wall, S3, the fibrils are once againin a netlike arrangement.

While the cellulose gives flexibility to the cellwall, it is the lignin which makes the cell wallsrigid. Lignin does not have a fibrous structurebut, forming round the fibrils, gives a wall con-structed like reinforced concrete.

ExtractivesExtractives form only a small part of the cell

wall, but can be quite important. In many spe-cies the color or smell of the wood is due to ex-tractives, some examples of which are lignans,tannins, terpenes, and polyphenols. Woods re-sistant to decay, such as black walnut or red-wood, usually have a high extractive content,while some species will corrode metal when thetwo are in contact for a long time.

These substances are called extractives be-cause they can be removed, or extracted, fromthe wood by heating it in water, alcohol, orvarious other chemicals.Inorganic Constituents

The various salts comprising the inorganicconstituents of North American woods amountto between 0.2 and 0.9 percent by weight. Themain elements present are calcium, magnesium,potassium, and sodium, together with lesserquantities of other elements, for example, boron,copper, iron, and manganese. These substancesoccur as carbonates, phosphates, silicates, andsulphates, and are distributed throughout alllayers of the cell wall. The inorganic constitu-ents make up the ash when wood is burned.

Examining Wood Structure

Whether it is more desirable to provide thestudent with slide material already prepared orto have the student himself cut the tissue de-pends mainly on the amount of time and thefacilities available. If there is time to teach thestudent how to make at least passable sections,this is desirable. However, if the student is un-likely ever again to require this aptitude, itwould be better to provide prepared slides, asthe making of good microtome sections usuallydoes not come quickly. This takes much less classtime, and the student is more likely to see clearlythe features under discussion. A compromisemight be to demonstrate how the sections aremade, then conduct the studies using preparedslides.

Maceration, in which the wood has been di-vided into single, whole cells by dissolving thebonding middle lamella, are useful to illustratethe different shapes and sizes of individual cells.A simple maceration technique is described onpage 16. Macerated tissue can be kept on hand

6

and students allowed to prepare temporaryslides with glycerine, applying stain themselves.

Instructions on making sections and macera-tion are given in such books as D. A. Johansen’s“Plant Microtechnique,” 1950, McGraw-HillBook Company, Inc., New York; and J. E. Sass’“Botanical Microtechnique,” 1958, Iowa StateCollege Press, Ames. Excellent, concise instruc-tions are to be found in “The Preparation ofWood for Microscopic Examination,” Forest

Products Research Leaflet No. 40, 1968, avail-able from Her Majesty’s Stationery Office, Lon-don, Great Britain.

For general information on wood structureand properties, see Panshin, de Zeeuw, andBrown, “Textbook of Wood Technology,” Vol. 1,McGraw-Hill Book Company, 1964; or “TheStructure of Wood,” by F. W. Jane, A & C BlackLtd., 1956.

7

WOOD IN PLANTSExperiment l—The Growth Sheath Idea

ObjectiveTo show how wood layers are laid down in the

tree.

Material

Small conifer stem, a discarded Christmastree for example. Growth sheaths in a tree stemare diagramed in figure 5.

M-134,269

Figure 5.—Diagram of a tree stem cut away to the center,exposing the growth sheaths. The top of each sheathshows the height of the tree at the end of a givengrowing season.

Method

Carefully saw the tree stem vertically downthe center exposing the pith. It may be desirableto saw to one side of the pith, then trim to thecenter. If desired, one or two side branches canbe treated in the same way. Needles can be fixedon the untrimmed branches by spraying with aclear lacquer.

The exposed stem can be used to demonstratethe pattern of the annual growth sheaths shownin figure 5. A growth sheath is the cone of newwood which the tree produces each year as itincreases in height and width.

ObservationUsing the model and diagram, one can

illustrate:1. How the tree increases each year in height

and diameter.2. How branches form in the stem.3. How the age of a tree can be told from the

number of rings: The true age can be found onlyby counting rings at ground level, because thenumber of rings decreases with height in thestem (fig. 5).Application

1. The shape of the tree and its taper affectsawing. Thus in sawing a tree with a highlytapered stem, if the saw is parallel to the bark,straight-grained lumber results, because the sawfollows the growth sheath. If the saw is parallelto the pith, the lumber has sloping grain becausethe saw moves obliquely across the growthsheaths.

2. The growth of branches results in knotsin the lumber. For clear lumber it is desirable tohave as few branches as possible while still get-ting good growth.

Experiment 2—Where Water Moves in Plants

ObjectiveTo show the principal path of water trans-

port in actively growing woody plants.Material

Woody plants, sharp knives, razor blades,scalpels, two-hole rubber stoppers, medium-sized bottles, ring stands with burette clamps,capillary tubing, rubber bands, pails of water(one per table).Methods

The shoots to be used are plunged into waterimmediately after cutting, and 6 to 7 centime-ters of stem removed from the base of each shootwhile it is submerged. All cutting operationsmust be carried out under water. Prepare thestem bases by the two methods described belowusing sharp knives and razor blades. Two shoots

Figure 6.—The shoot at left was prepared bywhile that at right by method 2.

3 2 5 - 6 5 3 0 - 4 3 9 - 2

M–134,256method 1,

are used for each method.Method 1.— Make longitudinal slits, about 3

centimeters long and 1 centimeter wide, at thebase of shoots in the bark only. Peel back thesebark strips carefully, without tearing them fromthe stem at the top of the slits (fig. 6). Now acylinder of wood is exposed. Remove this cylin-der carefully by cutting out with a sharp knifeor scalpel. The strips of bark must remain intact.

Method 2.— Do the same as in Method 1 exceptleave the wood cylinder intact and tie back thebark strips to the portion of the stem not cut,just above the slits. A rubber band is a conven-ient way to hold the bark in place (fig. 6).

General.— Fit two-hole stoppers into fourmedium-sized bottles, with a glass tube in onehole of each stopper to allow free entry of air.The other side of the stopper should be cut longi-tudinally into the hole, so that a stem can beinserted (fig. 7). Add 500 milliliters of water toeach bottle, and weigh the bottle plus water, butwithout the stopper, to the nearest 1.0 gram.Experiment 2.—Continued on p. 30

M–134,261Figure 7.—A shoot in place and a capillary tube to allow

air entry.

9

Experiment 3—How Water Moves in Plants

Objective

To show water tension and movement of dyein woody plant shoots.

Material

Woody plants in pots, light source, fan, eosin(bluish) solution at 1.0 and 0.1 percent concen-trations, large glass jars, graduated cylinder,sharp knife, ring stands, clamps, and clay.

Demonstration of Water TensionMethod.— Take two plants and place them

about 2 feet in front of the fan and lamp. About5 centimeters from the base of each shoot, care-fully cut out an area of bark about 1 centimeterwide and 3 centimeters long, avoiding any in-

M-134,592

Figure 8.—Demonstration of water tension showing theposition of the clay cup on the stem and, enlarged, thecuts in the stem.

jury to the wood (fig. 8). In the center of thedebarked area make a cuplike dam with plasticclay. Into this cup pour 0.1 percent eosin solutionfrom a graduated cylinder being careful not tospill any over the dam. Record the amountpoured into the cup and mark the level on theclay.

Now with a sharp knife or scalpel, make ahorizontal cut into the wood below the surface ofthe eosin solution in the cup. Keep the plant infront of the fan and light.

Observation.— Observe the results carefully.Note the drop in the level of the eosin solutionand record the total volume remaining in the cupat the end of 2 hours. Cut the stem transverselyand note the pathway of the eosin above andbelow the point of entry.

If convenient, this may be tried on an intacttree in the field in addition to the laboratoryexperiment.Movement of Dyes in Woody Plant Shoots

Method.— Take two plants of the same genusas used above, remove them from the pots, washthe root systems carefully, and place the plantsin a jar of tap water until needed. Place a beakercontaining 500 milliliters of 1.0 percent eosin so-lution on the base of a ring stand fitted with twoburette clamps (fig. 9). Cut halfway throughone stem about midway up the shoot. Do not cutthe other. Place the plant root systems in theeosin solution and suspend the plants by clamp-ing the stems halfway up the shoots. Place thelamp about 1 foot over the setup and turn on thefan about 2 feet from the apparatus.

Observation.— After 2 hours, dismantle theapparatus. Trace the path of the eosin in theroots and stems by cutting them transverselyand looking at the surfaces under a hand lensor dissecting microscope. It is assumed that thexylem sap in transpiring trees is in a state oftension. This can be illustrated by the upwardmovement of the dye in the plant.Note: This study is based on material in a Botany Lab-oratory Manual by permission of P. B. Kaufman, De-partment of Botany, University of Michigan, Ann Arbor,Mich.

10

M-134,263

Figure 9.—The plants suspended with roots in the beakerare used to trace the movement of the dye absorbed bythe roots. The cut to be made in the stem is seenthrough the magnifying glass.

11

Experiment 4—Size of

ObjectiveTo show the relative size of conducting ele-

ments in woody plants.Material

Three small-size funnels, two ring stands andsix clamps, rubber tubing to fit plant stems, cop-per wire, 1 percent aqueous eosin solution, Indiaink, mercury, scalpel, 125-milliliter beaker, andstraight, unbranched plant stems from a varietyof species.

MethodTake an unbranched stem section about 3/8

inch thick and 8 inches long. Make a transversecut at the base of the stem and fit it snugly intoa 6-inch piece of rubber tubing. Tie the tubingin place with copper wire, but not too tightly.Fix the other end of the rubber tubing to thestem of a small pyrex funnel. Tie the tubing tothe funnel stem as mentioned above. Suspend theapparatus in a small ring or burette clamp on aring stand as in figure 10.

Prepare three stem sections in this way. Allmay be attached to the same ring stand. Place a125-milliliter beaker under each stem. Fill eachtube and funnel with a different solution: mer-cury, India ink, and 1 percent eosin. Be espe-cially careful not to spill any mercury. Add theliquid slowly when filling the funnels. To dis-place the air trapped in the tubing, alternatelysqueeze and release the tubing with the fingers.Observation

After the liquid is put in the funnel, note howmuch time elapses until it first appears at thebottom of the stems. Measure the amount ofeach liquid in the beakers at the end of 1, 2, and3 hours. Compare the results for the variousspecies used by the different groups in the class.The large vessels in the hardwoods allow pas-sage of larger particles than can pass throughthe pits in softwood cells.

Openings Between Cells

M-134.257

Figure 10.—Setup for demonstration on conducting ele-ments in woody plants. Three such setups are neces-sary; one each for eosin India ink, and mercury.

Most of the water movement in plants takesplace through the vessels and tracheids. Thevessel segments are arranged end to end, withpits on the side walls, but with relatively largeopenings at the top and bottom of each segment.In contrast, the fibers overlap at the ends andsolutions must pass through the pits, which aremuch smaller than the openings in the perfora-tion plates of the vessels.Note: This study is based on material in a Botany Lab-oratory Manual by permission of P. B. Kaufman, De-partment of Botany, University of Michigan, AnnArbor, Mich.

12

Experiment 5—What Happens Inside a Leaning Softwood Tree

ObjectiveTo illustrate formation of compression wood

in conifer stems.Material

Individually potted, actively growing, conif-erous seedlings, about 3 to 6 months old, whichhave not set buds; for example, Douglas-fir, redpine, or slash pine. Greenhouse space, sectioningand staining material.Method

Separate the potted trees into two groups. Al-low one group to stand in the normal uprightposition for the whole experiment.

For the remainder, tilt plants and pots atabout 60° to the vertical. After 5 days, standthem upright. When they have been upright foranother 5 days, tilt again for another 5 days,making sure that the same side of the plant isdown. Then place in an upright position for 2weeks before harvesting.

Take l-inch-long portions of both groups of

stems, make cross sections, stain with safranin-fast green, and view under a microscope.Observation

The untilted plants will show regular growth,with little difference between the cells. The tiltedplants will show two bands of atypical cells onthe lower side of the stems, representing cellswhich completed part of their differentiationwhile the plants were tilted. Compare this woodwith that seen in “Compression Wood in Soft-woods,” experiment 9.

Application

Compression wood is commonly found on thelower side of branches and leaning softwoodstems, or on the lee side of stems exposed tostrong prevailing winds. It has differentstrength properties than normal wood.Note: This study is based on work reported by R. W.Kennedy and J. L. Farrer in the section, “Tracheid De-velopment in Tilted Seedlings,” of the book, “Cellular Ul-trastructure of Woody Plants,” pages 419 to 453, Syra-cuse University Press, 1965.

13

WOOD ANATOMYExperiment 6—The Three Faces of Wood

ObjectiveTo show those differences which are visible

without a microscope among the cross-sectional,radial, and tangential faces of wood.Material

Section of oak stem with bark attached, about8 inches in diameter and the same in length.While other species can be used, oak is particu-larly desirable because it has definite growthrings and very large, readily visible rays.Method

Saw and sand the section to expose the cross-sectional, radial, and tangential faces, as shownin figure 11. Brush off the surface to clear thepores after sanding, and finish with clearlacquer. This accentuates the wood character-istics.

Observation

On the block cross section:1. Note the arrangement of the growth rings.2. Note the distribution of cells in the growth

rings, with larger vessels in early part of thegrowth rings.

On the block radial section:1. The large rays running out from the pith

to the bark transport food material radially inthe stem and create a distinctive figure in sawnlumber.

2. Note the large tubes of vessels arrangedvertically in the stem.

On the block tangential section:1. The rays are seen in end view, therefore

the cross sections of ray cells and the long di-mension of other cells are exposed.Application

If a block is available with wide and narrowgrowth rings, this can be used to show how nar-row growth rings have a greater proportion ofvessels than wide rings. In machining oak, slow-grown wood with narrow growth rings is easierto machine, as much of ring is air. Fast-grownoak is stronger and heavier, as each growth ringhas a greater proportion of small, thicker-walledcells.

14

M–134,590

Figure 11.—Block of oak cut to show: A, the cross-secticmal, B, radial, and C, tangential faces.

Additional Species

Similar blocks may be prepared for contrast-ing woods:

aspen—has a variety of cells, but vesselsare smaller than oak, rays muchsmaller than oak, and the vessels arespread relatively uniformly through-out the rings.

California red fir-has large rays, no resinducts, relatively uniform cell structure,and very little latewood.

loblolly pine—has small rays, resin ducts,and uniform wood structure with pro-nounced latewood.

Alternative Species

Depending on location it may be easier to ob-tain wood specimens of species other than thosereferred to above. The following alternate spe-cies have characteristics similar to, though notidentical with, those woods already mentioned:

white oak: honey locust, Oregon white oak.aspen: cottonwood, willow.California red fir: balsam fir, eastern

hemlock.loblolly pine: Douglas-fir, red pine.

Experiment 7—Why Woods Have Different Properties

ObjectiveTo show differences in wood anatomy among

species.

Material

Small blocks of oak, aspen, California red fir,and loblolly pine. Sectioning and stainingmaterial.

Method

Prepare slides of transverse, radial, and tan-gential sections according to instructions intexts such as those suggested on page 7. Viewthe slides under the microscope.

ObservationsIn this study the differences in cell size and

arrangement can be illustrated at the micro-scopic level. If slides are not readily available,the photomicrographs at the end of the manualcan be used to show the differences in anatomy.

ApplicationCell variability in oak gives it variety in grain.

Aspen, on the other hand, is a relatively uniformwood, and is easier to work than oak. Similardifferences exist between loblolly pine and Cali-fornia red fir. The greater number of thick-walled cells in the loblolly pine growth ring giveit strength, hence it is often used in construc-tion. Conversely, the relatively few latewoodcells in red fir make it ideal for millwork, as it iscomposed almost entirely of thin-walled cellswhich can be machined easily.

Some examples of differences to look for are given below:

15

Experiment 8–What a Single Wood Cell Looks Like

ObjectiveTo show differences in wood cell shapes and

sizes.Material

Small specimens of softwood and hardwood.Method

Split off small toothpick-size slivers of woodfrom a ½-inch-long block. If pieces are split offalong the grain, more cells will remain wholethan by cutting. Place several pieces in a testtube and cover with strong household bleach.Cork the tube and place in an oven at 50° C. for24 hours.

Remove the tube from oven and stir the con-tents with a glass rod; then the wood shouldbreak into individual cells. If separation is notcomplete, allow it to cook a little longer.

When separation is satisfactory, allow thecells to settle to bottom of test tube, pour off thebleach or remove with an eye dropper, and adddistilled water to wash the cells. Repeat washingtwo or three times.

It may be desirable for the instructor to dothe maceration prior to the class and issue mac-erated material to the students.

The cells may be stained for better viewing byadding a few drops of safranin to the last wash.

Small amounts of the macerated material areput on glass slides with an eye dropper, a dropof glycerin is added, and a cover slip placed ontop. This temporary mount can be viewed underthe microscope.Observation

1. In softwoods:a. The cells are relatively uniform in type

and shape.b. The ray cells are much different from the

upright cells.2. In hardwoods:

a. There is considerable variety in cell typeand size; for example, vessel segments,fibers.

b. Note the brick-shaped cells, parenchyma,which are rather similar in shape toray cells.

c. Note the differences among hardwoods,especially in vessel segment size and inthe perforation plate.

ApplicationLong fibers in conifers make these species de-

sirable for paper, especially where strength isneeded. Shorter, flatter cells from hardwoodsare good for writing papers where a smooth sur-face is more necessary than strength.

16

Experiment 9—Compression Wood in Softwoods

ObjectiveTo show the difference in anatomy between

compression wood and normal wood in conifers.

MaterialA stem disk of any softwood with pronounced

compression wood. Softwood samples, some withcompression wood and some without.

Method

The disk is used to illustrate the occurrenceof compression wood in the stem and the differ-ence in gross appearance between it and normalwood (fig. 12). Make cross section slides of bothnormal and compression wood. Stain both typesof slides with safranin-fast green for the sameperiod of time, examine them under the micro-scope, and compare their appearance.

ObservationNote that in compression wood the growth

rings are wider than normal, while the cells havethicker walls and are more rounded in outlinethan normal cells. There are spaces between thecorners of the compression wood cells, but notbetween normal cells. It maybe possible to showthe difference in lignin content with different in-tensity of staining if safranin alone is used, butthis is rather doubtful. Compression wood is al-ways formed on the downward side of leaningsoftwood stems. It does not occur in hardwoods.

M–28,425F

Figure 12.—Stem cross section of softwood with com-pression wood where the growth rings are wider anddarker, within the rectangle.

Application

Compression wood cells are thicker walled andharder to machine than normal cells. The greateramount of lignin present reduces the suitabilityof compression wood for pulping. Compressionwood shrinks more along the grain than doesnormal wood, causing problems when both areused in the same item.

17

Experiment 10—Tension Wood in Hardwoods

ObjectiveTo show the difference in anatomy between

normal and tension wood in hardwoods.

MaterialA hardwood disk or wood specimen with pro-

nounced tension wood. Samples of wood, somewith tension wood and some without.Method

If possible use a disk to illustrate the occur-rence of tension wood in a stem, and from sam-ples show the difference in gross appearance oftension wood, as in figure 13. Make cross-sectionslides of both normal and tension wood. Stainwith chlorozinc iodide for the same period oftime. (The chlorozinc iodide is a temporarystain. For permanent slides use phloxine and fastgreen.) Examine both types of slides under themicroscope and contrast appearances.

Observation

Note the additional, highly reflective layer onthe inside of tension wood cells. There is a lowerlignin, and higher cellulose, concentration intension wood than in normal wood. Tensionwood is found on the upper side of leaning hard-wood tree stems. It does not occur in softwoods.Compare this with the compression wood in theprevious experiment, “Compression Wood inSoftwoods,” experiment 9.

Application

Tension wood in hardwoods reacts in muchthe same way as compression wood in conifers.The cells are thicker walled than normal, shrink-age along the grain is greater, but lignin contentis lower, and cellulose content is higher. Whenthe wood is machined the fibers tend to pull out,giving a fuzzy surface.

Stains for Tension Wood

Phloxine.— Staining solutionphloxine 1 gram90 percent alcohol 100 milliliters

Fast green.— Stock solutionfast green 0.5 grammethyl cellosolve 50 millilitersabsolute alcohol 50 milliliters

18

M-90,345F

Figure 13.—Top of a berry crate; the two outside piecescontain tension wood but the center does not. Comparethe uneven shrinkage and fuzzy appearance of slatswith tension wood with the smooth surface of thecenter slat.

For staining solution, dilute the fast greenstock solution with 25 milliliters absolute alcoholand 75 milliliters clove oil.

Staining procedure with phloxine and fastgreen.—

1. Wash the sections in 95 percent alcohol.2. Place in phloxine for at least 1 hour.3. Wash in 95 percent alcohol.4. Place in fast green for 35 seconds.5. Wash in 95 percent alcohol.6. Wash in absolute alcohol.7. Clear in clove oil for 5 minutes.8. Wash in toluene.

EFFECTS OF ANATOMYExperiment 11-Which Way Does the Water Go?

ObjectiveTo show differences in liquid penetration with

differences in grain direction.

MaterialBlocks of wood, 2 by 2 by ½ inches, with grain

parallel to the ½-inch dimension in some, and atright angles to the ½-inch dimension in others,figtire 14. A beaker or pan containing coloredwater (a few drops of dye per quart of water)in which two blocks can float side by side.

Method

Saw the blocks to the required dimensionsand brush free of sawdust, or blow clean withan air blast. Dry the blocks in an oven at 100° to105° C. for several hours, so that they will ab-sorb moisture readily when placed in the liquid.Place one of each kind of block on the surface ofthe water.

ObservationThe dye will appear first on the upper surface

of the block with grain direction parallel to theshort dimension. This shows that moisture trav-els more easily along the grain, that is along thecells, than across it.

Application

When wood is in contact with moisture, ex-posed end-grain surfaces will absorb moisturemost rapidly. Also, when wood is being driedthe reverse is true, moisture being lost more

M-134,253

Figure 14.—The two types of blocks required in theliquid penetration test. The upper block has the grainat right angles to the small dimension, while the lowerblock has the grain parallel to the small dimension.

rapidly through end-grain surfaces. Thus theends of a board may split as this part shrinksfaster than the rest of the board; therefore, dry-ing must be carefully controlled to reduce split-ting, or checking, to a minimum.Note: The dimensions of the blocks can be varied de-pending on how much time is available. Differences be-tween species can be shown by using blocks of differentwoods.

19

Objective

Experiment 12—Not All Oaks Are Alike

To show difference in rate of liquid penetra-tion between heartwood of red and white oak.

MaterialBlocks of red and white oak heartwood (not

sapwood), 2 by 2 by ½ inches. Cross-sectionslides of red and white oak. Beakers largeenough to float two blocks and containing col-ored water.

MethodPrepare the blocks as described in the pre-

ceding exercise, but with the grain parallel tothe short length in both blocks. Float these blocksin the beaker of water. The water will appearfirst on the upper surface of the red oak speci-men. Examine the slides under the microscopeand look for differences in the wood structurebetween the two species.

ObservationIn the microscopical examination, the student

should see tyloses blocking vessels in the whiteoak, but no tyloses in the red oak. Tyloses areformed in vessels when the protoplasm in an ad-joining parenchyma cell expands into the vesseland blocks it.Application

The presence of tyloses makes white oak suit-able for tight cooperage, whiskey barrels forexample, because the vessels are blocked andleakage is minimized. Red oak, having no vesselblockage, cannot be used for this type of prod-uct; it can be used for slack cooperage, such asnail kegs or fish barrels.Note: The difference in permeability can also be shownby taking pieces of red and white oak, 2 by 2 by 4 incheslong, and attempting to blow smoke through them. Smokecan be blown through the red oak but not through thewhite oak.

20

Experiment 13—What Happens When Wood Gets Wet ?

ObjectiveTo show the difference between tangential

and longitudinal shrinkage.

MaterialTwo dry wooden slats, 1/8 by 2 by 12 inches,

cut so that one has the grain running parallel tothe long dimension and the other with the grainperpendicular to the long dimension (fig. 15).Waterproof glue, clamps, and spraying device,e.g., two wash bottles.

MethodDry the two pieces of wood in an oven at

100° C. overnight. As soon as they are removedfrom the oven, glue the two pieces of wood to-gether on the 2-inch face, using waterproofglue, clamp them and put them back into theoven until the glue sets. Remove from the ovenand fix the lower end of the piece in a clamp sothat the wood stands upright, figure 16. Soakboth sides of the assembly simultaneously. If de-

M-134,591Figure 15.—The slats for the shrinkage test should be cut

from a block so that: A, the grain runs parallel to thelong dimension, and B, the grain is perpendicular tothe long dimension.

sired, a semicircular scale can be set behind thewood.

ObservationThe assembly will bend away from the side

having the tangentially cut slat, as this side ex-pands much more than the portion with thegrain parallel to the long axis.

ApplicationOne must understand that different amounts

of shrinkage are to be expected along differentdirections in the same piece of wood. Also, dif-ferent woods may have different shrinkagecapacities. If these points are forgotten theshrinkage force may be sufficient to cause severewarping or split the assembly.

Note: A possible variation is to repeat the experimentwith tangentially cut slats of markedly different species,to show shrinkage differences between species. Relatethis to the anatomy of the wood seen in “The ThreeFaces of Wood” and the cell dimensions seen in “WhatA Single Wood Cell Looks Like.”

M–134,254

Figure 16.—After the glue between the slats has set, thewood specimen should be fixed in a clamp and soaked.(Experiment 13).

21

Experiment 14—What is Specific Gravity?

ObjectiveTO define specific gravity and to show how it

varies among species.Material

Wood specimens 1 by 1 by 10 inches of whitepine, southern pine, oak, and aspen. A 500-cc.graduated cylinder, drying oven, balance, andruler.Method

The method used here to determine specificgravity is called the flotation method, becausethe value is found from the amount of the speci-men below the surface when it is floating in aliquid.

First, place the wood samples in an oven at105° C. overnight to remove excess moisture.Remove each specimen from the oven, weigh it,replace in the oven for about 2 hours and re-weigh. Repeat until each specimen reaches aconstant weight, indicating that as much mois-ture as possible has been removed. The samplethen is ready to be placed in the cylinder(fig. 17).

A graduated cylinder is suggested, but anyvessel in which the sample is forced to floatupright can be used. The cylinder should containenough water to float a specimen with its topabove the cylinder. Gently lower the specimeninto the water, taking care not to allow it to sinktoo deeply. When it has reached its floating level,withdraw the specimen from the water andmeasure the length of the sample which wasunder water.

Observation

Specific gravity of an object is obtained bydividing the weight of the object by the weightof an equal volume of liquid.

weight of objectspecific gravity =

weight of equal volume of water

Any liquid may be used, but water is the mostcommon.

The concept of specific gravity comes fromArchimedes’ principle which showed than anobject floating in water was being held up by aforce equal to the weight of water displaced. Ifa sample sank to where the surface of the watercoincided with the top of the sample, the specific

22

M–134,259

Figure 17.—A tall glass cylinder is necessary for thespecific gravity test, so that the block will float almostupright, giving a water mark at approximately a rightangle to the length of the specimen.

gravity would be 1.0. Thus for the sample by ourformula,

Let us assume that the specimen sinks only 4inches into the water, that is, the weight of thespecimen is supported by the force of 1 by 1 by4=4 cubic inches of water. Thus, by the formula,

This is the specific gravity of the entire wood,that is, the cell walls and the spaces in the cells.This value varies for different woods becausesome woods have more air space or more cell

wall material than others, as noted in “WhyWoods Have Different Properties,” experiment 7.The weight, or density, of the cell wall is thesame for all woods, about 1.5. While this value isconstant, the amount of cell wall material in adefinite volume of wood varies considerably.

Application

Because specific gravity is a measure of theamount of cell wall material in wood, it is a use-ful indicator of strength properties and of suita-bility for various uses. Heavy woods, such asDouglas-fir, southern pine, or oak, are used forheavy construction, while lighter ones, whitepine or aspen for example, are desirable whereload bearing is not the prime consideration.

23

Experiment

ObjectiveTo show the relationship

gravity and shrinkage.

Material

15—How Specific

between specific

A series of 2- by 2- by 1-inch wood blocks, cutwith the l-inch length parallel to the grain andthe other two directions being true radial andtangential directions, as shown in figure 18. Dialgage or accurate calipers.

MethodThoroughly soak the specimens in water.

Wipe with damp cloth, weigh, and measure di-mensions in radial and tangential direction withdial gage, figure 18. Place specimens in oven anddry at 105° C. until they reach constant weight.Then remeasure. Percentage of radial and tan-gential shrinkage is calculated as:

Gravity Affects ExpansionPercent shrinkage

ObservationIf a variety of ring widths and species are

used throughout the class, results from thegroup can be compared to show the variationamong blocks of the same species, and betweenblocks of different species.Application

Wood with higher specific gravity tends toshrink and swell more than wood of lower spe-cific gravity. Therefore, when combining differ-ent species, or pieces with different specificgravity, in a single product, one must be awarethat differences in shrinkage and swelling po-tential may cause splitting or warping of thepiece.

M-134,251, 134,260

Figure 18.—For the specific gravity and shrinkage experiment, the wood blocks must be cut so that thegrowth rings appear as in specimen at top right, and not as in top left. Lower illustration shows a dial gage with a specimen in place.

24

Experiment

ObjectiveTo show how a sheet of paper is formed.

MaterialBlocks of softwood, knife or scalpel, bleach,

small heat-resistant jar, and fine wire screen tocover the jar mouth.Method

Split the blocks of softwood along the graininto toothpick-sized pieces. Place the pieces inthe jar, add bleach, and leave in an oven at 50° C.for about 24 hours. Remove the jar from theoven and stir the mixture; if maceration of thewood is not complete, return the jar and con-tents to oven for a short time. When macerationis complete, allow the fibers to settle, pour offbleach, and wash with water three times.

Fill the jar one-quarter full of water and fitthe screen over the jar mouth. Swirl water aboutto place the fibers in suspension, and invert thejar over a sink. The water will run through the

16—Making Paper

screen leaving the fibers in a mat. When thewater has drained off, remove the screen andfibers from the jar and dry them in an oven orover a radiator. When dry, the fiber sheet can belifted off the screen as a sheet of paper.

ObservationNote that the fibers will interlock and form a

woven mat as the water drains off the woodpulp.

Application

In making paper, the wood maybe reduced toindividual cells by chemically separating thecells, as done here, or grinding them apart me-chanically. The fibers are mixed with water andvarious chemicals, then spread onto a movingscreen where the water is removed. The result-ing sheet is passed over warm rollers to completethe drying, and may be run through a series ofpolished rollers to finish the sheet surface.

25

Experiment 17—How Strong Is a Single Fiber?

Objective

To demonstrate the strength of single fiber.

Material

Macerated cell material from southern pine,as these species have relatively long fibers. Bal-ante, burette, beakers, jewelers chain, dissectingmicroscope, forceps, glue, heavy paper or cardstock.Method

Using the technique described in the preced-ing study, macerate the wood. Fibers to be testedare taken with forceps from the macerationwhile it is under a dissecting microscope.

The fiber, handled at its ends with jewelers’forceps, is placed across the narrow ends of two-wedge-shaped paper tabs held in a bulldogspring paper clamp, figure 19. Each end ofthe fiber is glued to the tabs by applying a dropof household glue. The fiber assembly is left over-night to harden.

The fiber assembly is attached to two fine jew-elers’ chains by small hooks inserted throughholes in each tab. These chains are then fastenedto the tension testing device, as in figure 19, andthe spring clamp released. With this arrange-ment, the fiber is automatically alined in the di-rection of the applied load.

The testing device consists of a modified an-alytical balance. One chain of the assembly isattached to one end of the balance arm and theother chain to the balance platform. A 100-mil-liliter aluminum beaker is attached to the otherend of the balance arm and counterbalanced, sothat there is no tension on the fiber assembly.Load is applied to the fiber by admitting waterto the beaker at a rate of 60 milliliters per min-ute. The quantity of water admitted at time ofbreak is recorded.

ObservationNote the amount of water required to break

the fiber. Thin-walled earlywood and thick-walled latewood fibers can be tested and theirrelative strengths compared.

Application

The strength of the individual wood cell isimportant in lumber and in the manufacture ofpulp and paper products.

Note: This study is based on work by D. C. McIntosh,The Mead Corporation, Chillicothe, Ohio, and reportedin “Chemical Composition and Physical Properties ofWood Fibers. III: Tensile Strength of Individual Fibersfrom Alkali Extracted Loblolly Pine Holocellulose,” byB. Leopold and D. C. McIntosh, Tappi, Vol. 44(3):235–240.

26

Figure 19.—ApparatusM-134,264

for testing the strength of a single fiber. In preparation, the fiber is glued to the tabs heldin the clamp, as at top of figure.

27

Experiment 18-Why Use Glue?

ObjectiveTo show differences between nailed and glued

wood beams.

MaterialNine strips of softwood (¼ by 1 by 18

inches), hammer, nails, glue, clamps, two sup-ports, corrugated fiberboard backdrop, and5000-gram weight.

MethodTake three of the strips and nail them, one on

top of the other. Take three others, glue themtogether, and allow the glue to set.

Place the remaining three strips, one on topof the other, across the supports, put the weightin the middle and mark the level of the bottom ofthe beam on the backboard (fig. 20).

Replace the three strips with the three nailed

together, apply the weight, and mark the bottomof the beam. Do the same with the glued beam.

Observation

The three loose strips bend most, the nailedbeam does not bend as far, and the glued beambends least. In the first case there is little sup-port because the loose strips act as individualpieces. When nailed, there is greater unity, andwhen glued it is as though a solid piece of woodwere used.

Application

Glued beams, or laminated beams as they arecalled, are strong and can be made very large.Because they are made of many pieces of woodglued together, they can be made in almost anyshape or size the engineer may want, while re-taining the strength of solid wood.

M-134,258

Figure 20.—The weight is shown on the glued beam. The marks showing the limit of bending for the separate piecesand for the nailed beam are seen below the beam. (Experiment 18).

28

ObjectiveTo show some

and solid wood.

Material

Experiment 19—Why Use Plywood ?

differences between plywood

Five pieces of oak ( 1/8 by 3 by 3 inches), twopieces of oak (3 by 3 by 5/8 inches), waterproofglue, clamp, hammer, sharp-pointed nails, basinof water, and calipers or dial gage.

Method

Glue the five thin sheets of oak together withwaterproof glue, making sure that the graindirections run at right angles to each other inadjoining sheets, as shown in figure 21. Clampand allow the plywood to set completely.

1. Hammer nails near the edge of the pieceof plywood. Do the same with one of the solidoak pieces.

2. Measure with calipers on a dial gage andnote the size of the dry plywood and the solidoak block. Now thoroughly soak both and re-measure. Compare the dimensions before andafter wetting for both specimens.

Observation1. The plywood does not split on nailing, but

the solid block does. In the plywood the cellscross each other at right angles in the differentlayers; when a crack begins, it is stopped by thecells which run across it in the next layer. Thereis less to stop the cracks developing in the solidblock, however.

M-134,255

Figure 21.—Diagram showing alternation of grain direc-tion with each layer in plywood. (Experiment 19.)

2. When soaked, the plywood increases insize less than the solid wood block. Again thecells running at right angles to each other limitthe amount of expansion, while there is no suchcounteracting effect in the solid block.

ApplicationPlywood is another example of taking wood

apart and putting it back together to give a moreuseful product. The oak plywood holds regularnails better than the solid block, and does notshrink and swell as much. For some purposesthis makes it easier to use than solid boards, andalso provides larger flat surfaces.

29

Experiment 20—When to Use a Blunt Nail

ObjectiveTo show effect of wood structure on nailing.

MaterialBlocks of white pine and oak (1 by 1 by 7

inches), hammer, and 4-penny common nails,pointed and with points filed dull.Method

Hammer both types of nails into both typesof blocks about 1½ inch from the end of theblocks.Observation

The softwood, white pine, will not split witheither nail. The oak will split with the pointednail but not with the dulled one.

The softwood cells are relatively long and thinwalled. Thus when the pointed nail goes through,

the walls break, and the length of the cells pre-vents a split going far along the grain.

In the hardwood, the cells are shorter, thickerwalled, and less flexible than in the softwood.When the pointed nail enters and pushes themapart, they split away from each other, and thesplit then continues to increase as the nail pene-trates. When the blunted nail is hammered in, itcrushes the fibers and there is very little pushingapart of the cells; therefore, the wood does notsplit.

Application

When nailing hardwood, for example, inhouse trim, it is desirable to use blunt nails, orto bore holes in the wood, to prevent splitting.Since the softwood does not split and is easy towork, it is preferred for much joinery work.

Experiment 2.—Continued from p. 9

Fit the shoots into the stoppers, under waterin the pail, then transfer to the bottles as quicklyas possible. It is best to hold the bottle over thepail. Support the shoots on ring stands withburette clamps. Leave the apparatus on thebench area in the greenhouse, or in front of awindow.

After 3 days, reweigh the bottles, again with-out stopper, and measure the volume of water ineach bottle. Record any changes in weight orvolume of water for each bottle.

ObservationNote the reduction in the amount of water in

the bottles and the difference in uptake betweenthe stems without wood and those without bark.The exercise shows, in a general way, the prin-cipal pathway of water movement in woodystems.

Note: This study is based on material in a BotanyLaboratory Manual by permission of P. B. Kaufman,Department of Botany, University of Michigan, AnnArbor, Mich.

30

annual ringThe layer of wood, consisting of manycells, formed around a tree stem duringa single growing season. It is seen mosteasily in cross section.

barkAll the material on the outside of thecambium, and formed by the cambium.

cambiumThe layer of actively dividing cellswhich produces wood cells on the sidetoward the pith, and bark cells on theside away from the pith.

cell cavityThe space enclosed by the cell wall andcontaining the protoplasm of the livingcell.

cell wallThe membrane forming the cell bound-ary and which consists of:

primary wall—The first cell wallof the enlarging anddifferentiating cell.

secondary wall—The part of thecell wall laid down insidethe primary wall afterc e l l enlargement hasstopped.

celluloseA complex carbohydrate composed oflong, unbranched molecules, whichmakes up 40 to 55 percent by weight ofthe cell wall substance. It is soluble inacid, but not in alkali.

compression woodWood formed typically on the lowerside of branches and stems of leaningor crooked softwood trees. It is char-acterized by tracheids which arerounded in cross section, have spiralchecks in the walls, and a higher thannormal amount of lignin in the cellwall. Compression wood is denser anddarker than the surrounding wood.

cross sectionThe wood surface exposed when a treestem is cut horizontally and the ma-jority of the cells are cut transversely.

diffuse porousWood of hardwood species in whichthe vessel diameter remains approxi-mately constant throughout the an-nual ring.

earlywoodThe less dense part of the growth ring.It is made up of cells having thinnerwalls, a greater radial diameter, andshorter length than those formed laterin the year.

extractives

fiber

fibril

figure

Any of several low molecular-weightsubstances, such as resins, tannins, oralkaloids, which can be removed fromwood by extracting with a suitablesolvent. They amount to 2 to 15 percentby weight of the wood substance.

A general term used for any long, nar-row cell of wood or bark, other thanvessels or parenchyma. The wood fi-bers include both the tracheids of soft-woods and the fibers of hardwoods.

A threadlike strand made up of groupsof long-chain cellulose molecules in thecell wall and visible with the micro-scope.

Any design or distinctive markingsthat appear on the surface of a piece ofwood.

gelatinous fiberA fiber with a relatively undignifiedinner wall which is found only in thetension wood of hardwoods.

grain directionDirection parallel to the long axis ofthe majority of cells in a piece of wood.

31

growth sheathThe layer of wood laid down over anentire stem and branches during asingle growing season, and consistingof both earlywood and latewood.

hardwoodWood from a broadleaved tree andcharacterized by the presence of ves-sels, e.g., oak, ash, or birch.

heartwoodThe inner part of the stem which,in the growing tree, no longer containsliving cells. It is generally darker thansapwood, though the boundary is notalways distinct.

hemicellulosesLow molecular-weight polysaccharidesfound in association with cellulose inplant cell walls. They amount to 15 to25 percent by weight of the wood sub-stance and, unlike cellulose, are solublein alkali.

latewoodThe denser part of the growth ring. Itis made up of cells having thickerwalls, smaller radial diameter, andgenerally longer than those formedearlier in the growing season.

ligninAn amorphous substance which infil-trates and surrounds the cellulosestrands in wood, binding them togetherto give a strong mechanical structure.It amounts to 15 to 30 percent byweight of the wood substance.

macerateTo dissolve out the bonding materialbetween plant cells to obtain separate,entire cells.

parenchymaTissue composed of cells which areusually brick-shaped, have simple pits,and frequently, only a primary wall.They are mainly concerned with stor-age and distribution of food material.

perforation plateThe wall area with a series of openingsbetween the ends of two vesselelements.

pitA recess in the secondary wall of a cell.

pit, borderedA pit in which the secondary cell wallpartially overhangs the pit membrane.

pit, half-borderedA pair of pits in adjacent cells, one pitbeing simple and the other bordered.

pit, simpleA pit in which the secondary cell walldoes not overhang the pit membrane.

pit apertureThe opening or entrance leading fromthe cell cavity into the pit chamber.

pit chamberThe space between the pit membraneand the overhanging pit border.

pit membraneThe intercellular layer and primarywall of two adjoining cells forming theexternal limit of the pit.

pithThe central core of a stem, consistingmainly of parenchyma or soft tissue.

plywoodA series of thin layers of wood gluedtogether to form a solid sheet. Thegrain direction runs at right angles inadjacent layers.

polysaccharidesCarbohydrates, such as starch or cel-lulose, decomposable into two or moremolecules of monosaccharides (simplesugars like glucose), or theirderivatives.

radial faceThe wood surface exposed when a stemis cut along a radius from pith to bark,and the cut is parallel to the long axisof the majority of the cells.

rayA ribbonlike group of cells, usuallyparenchyma, extending radially in thewood and bark.

32

ray, fursiformA ray which is spindle-shaped whenseen on the tangential wood face, andespecially rays containing resin ductsin conifers.

resin ductA space in which resin has accumu-lated between the cells.

ring porousWood of a hardwood species in whichthe vessel diameter is considerablylarger in the earlywood than in thelatewood.

sapwoodWood immediately inside the cam-bium of the living tree, containingliving cells and reserve materials,and in which most of the upward watermovement takes place.

softwoodWood from a coniferous, or cone-bearing tree, and characterized bythe absence of vessels, e.g., fir,pine, spruce.

specific gravityThe ratio of the weight of a piece ofwood in air to the weight of an equalvolume of water. As wood shrinksand swells depending on the moisturecontent, the moisture content of thewood must be specified to deter-mine the volume.

tangential faceThe wood surface exposed when acut is made at right angles to therays and parallel to the long axisof the majority of cells.

tension wood

torus

tracheid

tyloses

veneer

vessel

Wood formed typically on the upperside of branches and stems ofleaning or crooked hardwood trees.It is characterized by reduced ligninin the cell wall and often by thepresence of an internal gelatinouslayer in the fibers.

Thickened central portion of the pitmembrane in the bordered or half-bordered pit of a softwood.

A long narrow wood cell, the onlyopenings in which are pits. Comparewith a vessel element.

An expansion of a parenchyma cellthrough a pit into a neighboringvessel; the tyloses may fill theinvaded cell cavity partially or com-pletely, reducing or preventing pas-sage of liquids or gases.

A thin slice of wood cut with a knifeor saw from a log.

A series of cells extending longitu-dinally in the stem, the ends of thecells having fused together to forma long tube.

vessel elementOne of the cells forming a vessel,having pits in the walls and a per-foration plate where it joins anothervessel element.

xylemThat portion of the stem, branches,and roots between the pith and thecambium; the wood.

33

PHOTOGRAPHS OFTo obtain a three-dimensional impression of

the structure of a wood block, the photographson the following pages, fig. 22 (M–134249),fig. 23 (M–134247), fig. 24 (M–134248), fig. 25

WOOD STRUCTURE(M–134250), may be cut out and glued to threesides of a 31/8- by 31/8- by 37/8-inch wood blockas shown in the following sketch, fig. 26 (M–134266).

M-134,266

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M-134.249

35

M-134,247

37

M-134,248

39

M-134,250

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U.S. GOVERNMENT PRINTING OFFICE:1969 O-325-653