Biermann Ch. Handbook of Pulping and Papermaking

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524 25. WOOD AND FTOER-GROWTH AND ANATOMY

Fig. 25-4. Onion root tip magnified

150

x and

600

x. The individual chromosomes can be seen in

cells undergoing division.

a ring of bundles. The morphology of the vascu-

lar tissue of various of these plant categories is

detailed in Chapter 29. The scattering of the

xylem remains in those monocots that undergo

secondary growth.

The vascular cambium (Plate 44) of dicots

and coniferous species will form between the

xylem and phloem and result in

secondary

growth;

it increases the diameter of the stem but not the

length and is called a lateral meristem. The

cambium has two types of

cells:

fusiform initials,

which are vertically elongated and divide into the

longitudinal cells, and ray initials, which are more

cubical in shape and divide into the ray cells.

These cells divide by forming tangential walls

(periclinal division) and give rise to daughter cells.

One of the daughter cells, called the mother cell,

retains the ability to divide. The daughter cells

differentiate into the xylem (if interior to the

mother cell) or phloem (if exterior to the mother

cell).

Some hardwoods have cell division in the

radial direction (anticlinal) for an increase diame-

ter. Softwoods and other hardwoods have a di-

agonal division and differential growth forms two

side—by—side cells for an increase in diameter.

Parenchyma cells of the xylem will usually

live until heartwood formation; the prosenchyma

cells of the xylem differentiate into their final form

within a few days or weeks and die. The anatomy

of the xylem (wood) will be considered in detail.

Phloem

Phloem typically grows at only one—sixth to

one—tenth of the rate of xylem. Phloem consists

INTRODUCTION 525

Fig. 25-5. Shoot apices with apical meristems

of sieve tubes, companion cells, parenchyma, and

bast fibers (sclerenchyma). Bast fibers are formed

directly from cambial initials of modified paren-

chyma. The latter are called sclereids and may

have highly branched, unusual shapes. Compan-

ion cells are so named because each sieve tube has

at least one companion cell associated with it.

Bast fibers are lignified and do not easily crush.

The walls of sieve tubes and companion cells

do not become lignified. The sieve tubes usually

function for one year and then die and collapse.

The collapse provides some additional space for

new growing tissue to the interior. Tangential

growth in the bark is sometimes provided by

flaring of the rays.

Bark

Initially a twig is protected from the environ-

ment (against desiccation and mechanical injury)

by the thin epidermis. When the cambium be-

comes active, the increase in diameter will cause

(marked AM) of maple Oeft) and pine (SSx).

the epidermis to rupture. By this time, however,

the periderm (also called rhytidome or outer bark)

has begun to form. It consists of the phellogen or

cork cambium, the tissue outward of

the

phellogen

that is called the phellum and is composed cork

cells with much suberin, and the tissue inner to the

phellogen that is called the phelloderm and is

composed of a thin layer of thin—walled tissue.

Thick—walled fibers (sclerenchyma) may also be

present in the periderm. Sclerenchyma cells from

the bark are called stone cells by the pulp and

paper industry, but the botanical use of the term

stone cells is much more limited.

The periderm is eventually cut off as dead

phloem underneath is shed. Secondary periderms

form in arcs from phloem parenchyma cells to

continue the growth of the outer bark (page 13).

This accounts for the layered nature of bark and

the deep fissures that may form. Detailed bark

anatomy is considered by Chang (1954).

526 25. WOOD AND FIBER-GROWTH AND ANATOMY

Rays

Most cells of wood are oriented vertically to

conduct nutrients and water up and down the bole.

Some cells provide conduction from the pith

outward to the bark. They vary in height from a

few cells in some species to several inches in the

oaks;

in the oaks they cause the ray flecks or

silver grain observed on the radial surfaces. The

rays of softwoods generally account for less than

5 %

of the volume of the wood. The rays serve to

store food and transport it horizontally in

the

stem.

General types of

cells

in wood

(The term cell is correctly used for living

cells only, but its use to previously living cells of

wood is in common practice.) There are two

broad categories of cells in mature wood:

prosenchyma and parenchyma. They may each be

oriented longitudinally and, in the case of ray

cells,

horizontally. The term prosenchyma is not

used commonly in the pulp and paper literature,

but it includes fibers (tracheids of softwoods and

hardwoods), libriform fibers, and vessel elements.

The latter two occur in hardwoods only.

Prosenchyma are dead in the mature xylem.

Parenchyma cells are rectangular with an

aspect (length to width ratio) of about 2—4 com-

pared with 50—100 that is typical of elongated

fibers. They are small, thin—walled cells and

ftmction in the wood by storage, secretion, and

Fig. 25-6. Crystals (top) of the marginal ray

cells of Abies concolor.

wound healing. They may occur longitudinally or

radially with the latter outnumbering the former in

most species. They may store calcium oxalate as

crystals, tannins, starch, fats, or proteins. Paren-

chyma cells are often living in sapwood. Calcium

oxalate crystals are observed in some true firs,

sitka spruce, incense cedar, and black walnut.

Rhomboidal and rectangular crystals are found m

marginal parenchyma cells of

some

Abies (Fig. 25-

6).

Specific crystal types occur in certain tropical

species (Brown and Panshin, 1940, p. 214).

Softwood cell types

The volume of parenchyma cells in softwoods

is typically 5% in spruce and pine to 10% in larch

and fir; however, their mass is only

1—2.5%

the total mass. They may occur as strand or

epithelial (around resin canals) cells in the

longitudinal or horizontal (as part of the rays)

directions. Likewise tracheids may be longitudinal

or horizontal. All softwoods have at least longitu-

dinal tracheids and ray parenchyma. The non-

parenchyma cells of softwoods are all tracheids

and fibers. Fig. 25-7 shows the structure of a

representative softwood.

Hardwood cell types

The amount of parenchyma cells in hard-

woods is much higher and more variable with a

volume percentage of 10—35% and mass percent-

age of 5% or more. They may occur as strand,

fusiform, or epithelial cells (such as those found in

longitudinal gum canals in injured sweetgum) if

they are oriented longitudinally (named axial) and

as ray

{upright

ox procumbent) or epithelial paren-

chyma if oriented horizontally. (Some of these

parenchyma types are rare in U.S. species.)

Fusiform parenchyma occurs as marginal paren-

chyma of red maple.

Fibers include

vessel elements

and fiber cells.

The fiber cells include the ubiquitous libriform

fibers. The other fiber cells include fiber

tracheids;

vasicentric tracheids

found in the genus

Fraxinus and the family Fagaceae; and vascular

tracheids found in the Ulmaceae family of elms

and hackberry. Fig. 25-8 shows the structure of

a representative hardwood.

The cell composition of hardwoods and other

properties such as fiber length are very important

due to their higher complexity compared to soft-

INTRODUCTION

527

Key: Cross section: 1-la, ray; B, dentate ray tracheid; 2, resin canal; C, thin-walled longitudinal

parenchyma; D, thick-walled longitudinal parenchyma; E, epithelial cells; F, radial bordered pit pair cut

through torus and pit apertures; G, pit pair cut below apertures; H, tangential pit pair; 4-4a, latewood.

Radial view: 5-5a, sectioned fusiform ray; J, dentate ray tracheid; K, thin-walled ray parenchyma; L,

epithelial cells; M, unsectioned ray tracheid; N, thick-walled parenchyma; 0, latewood radial pit (inner

aperture); 0\ earlywood radial pit (inner aperture); P, tangential bordered pit; Q, callitroid-like

thickenings; R, spiral thickenings; S, radial bordered pits (middle lamella removed); 6-6a, sectioned

uniseriate, heterocellular ray.

Tangential view: 7-7a, strand tracheids; 8-8a, longitudinal parenchyma (thin walled); T, thick-walled

parenchyma; 9-9a, longitudinal resin canal; 10, fusiform ray; U, ray tracheids; V, ray parenchyma;

W, horizontal epithelial cells; X, horizontal resin canal; Y, opening between horizontal and vertical resin

canals; 11, uniseriate, heterocellular rays; 12, uniseriate, homocellular ray; Z, small tangential pits in

latewood; Z', large tangential pits in earlywood. Reprinted from Howard and Manwiller (1969).

Fig. 25-7. The three—dimensional structure of southern pine, a softwood.

woods. The high amount of parenchyma cells in In hardwood pulp the parenchyma cells are

many species is detrimental to papermaking. often selectively removed with fines during pro-

Table 25-6 gives some values for U.S. southern

hardwoods growing on softwood sites. This table

also shows that average fiber length is quite

variable among species, position in the tree, and

other factors.

cessing. Because of their small size they do not

contribute to paper strength but they contribute

appreciably to low freeness in pulp.

The vessel element content of hardwoods is

about 10% in birches, 40% in oaks, and over 55%

528 25. WOOD AND FIBER-GROWTH AND ANATOMY

Key: Surfaces are given as TT for the cross section, RR for the radial surface, and TC for the tangential

surface. Structures include WR for wood ray and AR for annual growth ring with S for earlywood and

SM for latewood. P is a vessel with SC for sclariform plate. F denotes libriform fiber. K is cell pit.

Approx.

125

Reprinted from USDA For. Ser. Tech. Note No. 210, 1952.

Fig. 25-8. The three—dimensional structure of yellow poplar, a hardwood.

in red gum by volume. In the latter two cases, the volume is empty space. The length of vessel

amount by mass is much smaller since most of the elements ranges from 0.1 to 2 mm (but 0.5 to 1

INTRODUCTION 529

Table 25-6. Proportions of cells in 6 in. diameter hardwoods growing on southern pine sites.^

Species

Ash, green

Ash, white

Elm, American

Elm, winged

Hackberry

Hickory, true, Carya

Maple, red

Oak, black

Oak, northern red

Oak, white

Sweetbay (Magnolia)

Sweetgum

Tupelo, black

Yellow poplar

Average, stem wood

Aver., branch wood

composition by volume of stems-

Vessels

14.5

31.1

28.2

20.1

14.7

23.2

14.7

15.3

14.7

30.1

46.2

38.4

43.3

20.5

19.7

Fiber

57.3

57.0

42.2

40.0

50.9

53.2

50.6

37.1

41.3

41.2

50.3

37.6

39.8

42.6

44.4

45.6

Rays

14.2

14.0

12.0

15.5

13.6

18.1

12.8

20.8

19.0

20.7

15.2

14.0

17.2

11.4

17.0

15.9

Longitud.

Parenchy.

14.0

14.4

14.8

16.2

15.4

14.0

13.4

27.4

24.5

23.5

4.4

2.1

4.6

2.7

18.1

18.8

-Mean cell length.

Fibers

Stem

1.16

1.22

1.30

1.25

1.12

1.29

0.83

1.30

1.27

1.22

1.24

1.54

1.76

1.39^

1.27

Branch

0.84

0.87

0.99

0.93

0.86

0.98

0.66

0.98

0.90

0.88

0.97

1.20

1.40

0.97

0.96

millimeter-

Vessel^

Elements

0.26

0.29

0.22

0.26

0.44

0.42

0.43

0.42

0.40

1.32

1.33

0.89

Mean

tree age

(Years)

From Koch (1985). Averages of woods of 10 trees.

From Panshin and de Zeeuw as listed in Koch

(1985).

Other works show values on the order of 1.7 to 1.8 mm in mature wood.

nun is common) depending on the species; some

values are given in Table 25-6. Libriform fibers

have small, simple pits. Fiber tracheids have

bordered pits smaller in size than the intervessel

pits.

Tracheids have numerous pits of about the

same size as the intervessel

pits.

All of these fiber

types will be discussed in more detail in the

appropriate chapters.

Cell

pits

Pits (page 17) are openings in the fiber wall

that allow conduction of materials between cells.

A pit of one cell is almost always associated with

a pit of a second wall so the term pit pair is often

used. There are three types of pit pairs as shown

in Fig. 25-9. Each half of a pit pair may be

simple (with a uniform hole, as in parenchyma

cells) or bordered (with a thickening in the wall

around it, as in most tracheids and vessels). Two

simple pits form a simple pit pair, one simple pit

with a bordered pit form a half—bordered pit pair,

and two bordered pits form a full bordered pit

(Fig. 25-9). The pits of hardwoods lack the

thickening in the middle, the torus (tori is the

plural form), and are much more variable than

those of softwoods.

Miscellaneous considerations

The wood first laid down in an annual growth

ring of temperate woods is called the earlywood.

It is especially suited for conduction. The remain-

der of the growth ring, the latewood, is especially

suited for strength. These purposes will become

more clear as the anatomy of wood is considered.

25.2 SAMPLE PREPARATION FOR

IDENTIFICATION OR MICROSCOPY

Preparation

wood

samples

Details on sample preparation and microsco-

py of a wide variety of plant tissues are found in

any of the several classic microtechnique books

such as Johansen (1940) or Sass (1940). Details

on the preparation of wood samples are given in

many of

the

wood anatomy resources listed below.

If you are serious about wood identification with

530

25. WOOD AND FIBER-GROWTH AND ANATOMY

Side

Views

Simple

pit pair

HQIF-bordered

pit pair

Bordered

pit polr

Fig. 25-9. The three types of

pits:

simple,

half-

bordered, and bordered.

a hand lens and knife, see Hoadley, 1990; this is

an excellent and reasonably priced resource.

For routine analysis of large numbers of

wood samples in a mill, where often only a few

species must be distinguished, freehand cutting

with a very sharp knife or single—edged razor

blade may be suitable. A lOx hand lens may

provide the necessary resolution of the transverse

section (and others, if necessary) for identification.

Moistening the freshly cut surface often helps see

detail. In most cases, one will probably want to

use a single—edged razor blade to prepare samples

for examination under the microscope. Double-

edged platinum—chrome blades will give very

good results (for safety, the edge not being used

can be wrapped in cardboard and taped); also,

they may be cut in half lengthwise with two pairs

of pliers and mounted in a pencil—type hobby

knife holder with split collet.

The preparation of wood and fiber samples

for microscopy is in many regards easier than for

other plant tissues since wood is relatively inert

and strong. It is, however, difficult to cut on the

microtome. Wood sections for high—quality

photomicrographs (i.e., those presented in this

chapter) are prepared on a microtome to give

sections about 10—25 fim thick. The magnifica-

tion of a microscope is about

35—600

x, although

an oil immersion lens brings this to

1000

x. This

corresponds to a resolution of about 0.3 /xm for

routine work. Dissection microscopes have a

range in magnification from about

x to

For high—quality sectioning (photomicrosco-

py),

small cubes (about 1 cm per side) are pre-

pared. Soft species of wood (white pine, aspen,

and spruce) are merely boiled until they sink prior

to cutting. Other species are softened with 2—4%

KOH in refluxing ethanol for one hour or longer,

followed by washing. Acetic acid/hydrogen

peroxide or any of a multitude of reported meth-

ods have also been used. Tropical species with

high levels of SiOj (several percent) may require

treatment with HF (special precautions must be

used with this material) to remove it as SiF4. Fig.

25-10 shows the standard arrangement of wood

sections on a microscope slide. An alternate

arrangement is to have the cross and radial sec-

tions on top and the tangential section beneath the

radial section.

Wood and fiber samples are often stained

with safranin O

(0.5—1%

in water or 50% etha-

nol),

a red stain selective for cellulose. Fast green

is used with safranin on many other botanical sam-

ples as a counter stain (for example, to show

phloem tissue).

Analysis of

wood

chips

Many mills purchase wood chips from a wide

variety of suppliers. Often they wish to confirm

the identity of the wood species occurring from a

supplier. A random sample should be obtained

that represents the population of interest. In the

laboratory highly skilled anatomists can identify

about one wood chip each minute (60/hr). About

Plnus strobuB 1

P1nac*a« 1

Sofronln 1

J B

9/8/78

OP-

Fig. 25-10. One arrangement of wood samples

on a slide (t is longitudinal direction).

SAMPLE PREPARATION FOR IDENTIFICATION OR MICROSCOPY 531

50 or 100 chips might be identified from the

representative sample depending on the require-

ments of the study. Since the chips have already

darkened, microscopic examination is usually re-

quired for confidence in

the

species determination.

It may be difficult to determine the orienta-

tion of the wood chip, but snapping it in half

usually presents a fresh radial surface. Decay in

wood chip piles occurs faster with hardwoods than

with softwoods. Hardwoods have more parenchy-

ma (that initially heat the pile by respiration of

food) and the heartwood of species used

pulping

is generally not as decay resistant as that of the

softwoods. Hemlocks, lodgepole pine, and most

spruces have only moderate decay resistance

the

heartwood. True firs are the only U.S. softwoods

with minimal decay resistance in the heartwood.

Macerations

Wood samples are macerated to study the

liberated fibers, such as for fiber length studies.

Maceration is a highly selective pulping operation.

Wood is first cut into pieces about 2 mm square

by 25 mm in the longitudinal direction. Air is

removed by soaking (perhaps with vacuum) or

boiling in water.

The most gentle maceration technique is the

use of sodium chlorite acidified with acetic acid

[Spearin and Isenberg, Science, 105(2721):214

(1947)].

Caution: impurities can make sodium

chlorite explosive. In one method, 35 mL of

water is used with 0.6 g sodium chlorite and 5

drops of glacial acetic acid for 1 hour at 90°C in

a hood. Alternately 2 grams of sodium chlorite

with 12 drops of acetic acid can be used for 3

hours at 85 °C in a hood.

The method of

Franklin

is also gentle. Wood

pieces are treated with a solution consisting of

equal parts of glacial acetic acid and

hydrogen

peroxide at 60°C for 48 hours. Wilson {Pulp

Paper Mag, Can.

55(7):

127-129(1954)]

has re-

viewed maceration techniques.

After the chemical treatment the wood chips

are washed in gently flowing water overnight so

the fibers do not separate. The pieces are then

teased apart and stained as appropriate. Tempo-

rary slides are made with Karo. Permanent slides

are made by dehydration using

ethanol series of

15%-30%-50%-70%-85%-95%-abs-

abs—50%

abs + 50% xylene—xylene.

Preparation

of paper samples

A small sample of

paper

can be boiled in

1 %

NaOH to hydrolyze the sizing and help disperse

the fibers. The sample is then washed and agitat-

ed to disperse the fibers.

Mounting

of sections and macerated fibers

Mounting media have an index of refraction

similar to that of

the

glass microscope slide. Corn

syrup (Karo) is usefiil for temporary slides; it is

often diluted with water (about 80:20 syrup:water)

to give a good working solution where bubbles do

not interfere. Tappi T 263 suggests the use of

50:50 solution of glycerin and 95% ethanol with

heating to the boiling point (after the cover glass

is applied) to drive off air. These do not require

dehydration of

the

sample. (Dehydration involves

replacing water with ethanol and ethanol with

xylene so that nonpolar materials can be used.)

Canadian balsam (from the pitch pockets of

the bark of balsam fir) is the traditional mounting

medium for permanent slides. Synthetic resins are

also available. A coverslip can be held in place

with two clothespins when waiting for the mount-

ing medium to dry when making permanent slides.

The

use of keys

Dichotomous keys involve a series of separa-

tions of possible wood species based

identifying

characteristics. Each selection is a choice of two

possibilities (such as the presence or absence of a

feature). A key using hand lens identification is

included to demonstrate their use. For example,

a wood that has vessels must be a hardwood so all

softwood species are eliminated. If it is ring

porous then many of the hardwood species are

eliminated. Identification of additional features

eventually results in its species determination.

An alternate to the key is the use of end-

punched cards where each number corresponds to

(the presence or absence of) a single feature

(hardwoods: Brazier and Franklin, 1961; soft-

woods: Phillips, 1948). A hole away from the

edge is used if the feature is absent and a notch

(where the hole is enlarged to include the edge) is

used if the feature is present. By using a wire in

a location corresponding to a given observed

feature in the unknown, appropriate cards can be

made to fall from the deck. The use of a second

feature narrows the determination to a smaller

532 25. WOOD AND FffiER-GROWTH AND ANATOMY

group. Eventually the correct species is deter-

mined. The observation of a few key features

(such as spiral thickening in tracheids) allow the

correct determination very quickly. This system

has been computerized for faster use (North

Carolina State University). The use of keys for

softwoods and hardwoods is considered in their

respective chapters.

25.3 WOOD VARIATION

Reaction wood consists of compression wood

in softwoods and tension wood in hardwoods. A

variety of grain distortions are also considered in

this section.

Compression

wood

Pillow and Luxford (1937) summarize com-

pression wood as an abnormal type of wood

occurring as a rule on the lower side of

nonvertical trunks and branches among all conifer-

ous species of

trees.

An increase in the amount of

deviation of trunks from a vertical position or in

the rate of diameter increment of individual trees

increases the formation of compression wood.

Compression wood is identified by markedly

eccentric annual growth rings (Fig. 25-11). In the

wide portion of the growth ring there is a higher

than normal level of latewood that has a more

gradual transition from the earlywood to latewood

than in normal wood.

Under

microscope the latewood tracheids of

compression wood appear to be nearly circular in

cross section whereas those of normal wood are

more or less rectangular (Fig. 25-12). The fibrils

of the secondary cell walls in compression wood

make a higher angle in relation to the longest axis

of the cells than do the fibrils of normal wood and

these walls contain microscopic checks. The

spiral thickenings that normally occur in Douglas—

fir and a few other coniferous species are fewer in

number and less distinct in compression wood than

in normal wood and may be confined to earlywood

fibers in pronounced compression wood.

The length of tracheids in compression wood

is generally 20—30% less than that in normal

wood. The fibril angle of the secondary cell wall

layer of softwood tracheids is typically about 20—

24° in earlywood and below 10° in latewood; in

pronounced compression wood the angles are

about 35—38° and 24—30°, respectively.

The lignin content of compression wood is

about 5% higher and the cellulose content is about

8% lower than normal wood in spruce and red-

wood. The density of pronounced compression

wood is 15—40% greater than normal wood. The

longitudinal shrinkage of compression wood from

Fig.

25-11.

Cross section of southern pine log

with conspicuous compression wood. From

PiUow and Luxford (1937).

Fig. 25-12. Normal wood of Douglas-fir with

rectangular cells (left) and compression wood

with rounded cells (center), and checks in fibers

with low S-2 fibril orientation (right) (200x).

WOOD VARIATION 533

green to oven—dry condition varies from 0.3 to

2.5%,

whereas normal wood has a shrinkage of

0.1 to 0.2%. The transverse shrinkage of com-

pression wood is less than that of normal wood.

When adjustments are made for differences in

density, compression wood is lower in practically

all strength properties as compared to normal

wood (although it is quite hard) and is highly

correlated to the differences in the slope of fibrils

in compression wood. It is brittle and fails in

clean pieces without the splintering normal wood

has during failure. Compression wood accounts

for much bowing and twisting in manufactured

lumber. Pulp from compression wood tends to

fragment rather than fibrillate and generally leads

to very weak pulps with poor bonding potential.

Tension wood

Tension wood (Perem, 1964) occurs on the

underside of branches and leaning stems of angio-

sperms (hardwoods) and is not easily observed in

freshlyfelled timber. Upon aging (by weathering

or exposure to strong UV light) it becomes visible

by its light color or higher luster compared to

normal wood (Fig. 25-13). Tension wood often

occurs without the eccentric growth ring pattern of

compression wood. It is less obvious and consid-

ered to be much less of a problem than compres-

sion wood and, therefore, is not as well studied.

Tension wood has much higher longitudinal

shrinkage (0.3 to 0.7% compared to 0.0 to 0.25%)

and toughness and lower modulus of rupture than

normal wood. It has a strong tendency to give a

fuzzy surface on green wood and to cause warping

and other distortions in lumber products during

drying. It may be slightly higher, slightly lower,

or of no significantly different specific gravity than

normal wood depending on the species.

Most species (basswood is an exception) have

gelatinous fibers in the tension wood (Fig. 25-14).

It is the layer inside the S—2 layer that is gelati-

nous and this often separates from the rest of the

fiber cell wall during sample cutting for microsco-

py, especially in thin—walled fibers. This layer is

almost entirely free of lignin. On average, these

fibers are about 5% lower in lignin and corre-

Fig. 25-13. Tension wood (top) of weathered

trembling aspen. From Perem (1964).

Fig. 25-14. Fibers with gelatinous layers

(separated from the rest of the cell wall) of

tension wood from trembling aspen at

500

From Perem (1964).