Wood .,.., Fa.r. 9(3). 1977. w. 17S-183. 1978 by the Society of Wood ~ aDd TecbaoiOIY

BARK STRUCfURE OF SOUTHERN UPLAND OAKS!

Elaine T. H awardAssociate Research Chemist

Southern Forest Experiment Station, USDA, Pineville, La. 71300

(Received 4 February 1977)

ABSTRAcrBark structure of eleven oak species commonly found on southern pine sites was exam-

ined and described. In inner bark (phloem), groups of thick-walled lignified fibers andIclereids are interspersed among thin-walled cellulosic elements (parenchyma, sieve tube~bers, and companjon oelIs). 'nJeIe fibers and sclereids greatly influence the bark'sdensity, hardDeSl, and ~ physical and mechanical characteristics. The innennostperiderm is the boundary between inner and outer bark. In outer bark (rhytidome), areasof collapsed, dead phloem are enclosed by periderm layers. Peridenn shape and spacingvary greatly within species. Great differences in exterior roughness and bark thickness also~ within species.

Kev1DOrds: QuefCUS spp., anatomy, bark, oaks, phloem, peridenn, rhytidome.

The objective of the present study wasto observe, compare, and describe the bark~'tructure of eleven upland oaks growingon southern pine sites. Species sampledare listed below:

Common name

Black oakBlackjack oak~-

Scientific name

Que1'CUS oelutina Lam.Q. mariltmdiCG MuenchhQ. falCtJt4 vaT.

pagod4efoUG Ell.Q. laurlfoUG MichLQ. rob1'a L.Q. stellato Wangenh.Q. cocc1nea Muencbh.Q. shuma1'da Buckl.Q. falCtJt4 Michx.Q. nig1'a L.Q. alba L.

Laurel oakNorthern red oakPost oakScarlet oakShumard oakSouthern red oakWater oakWhite oak

INTRODUcnON

Small hardwood trees growing on sitesbetter suited to southern pine present amajor forest utilization problem in theSouth today. Oaks comprise about 48% ofthe hardwood volume on such sites (Chris-topher et al. 1976), and their bark accountsfor a considerable part of total volume.For example, on oaks 6 inches in dbh,bark represents about 17% of stem volume.Removing the bark usually presents adisposal problem and results in the wasteof large quantities of material that shouldbe utilized. A possible solution is utiliza-tion of these small hardwoods as whole-tree chips.

When wood with bark is processed, thebark characteristics peculiar to the speciesoften determine the nature and magnitudeof the problems encountered. Because barkproperties are influenced by their structure,behavior of a bark under certain conditionsmay sometimes be predicted from a knowl-edge of bark anatomy. Thus, the types ofcells present, their arrangement, relativeamounts, and physical dimensions andproportions are all of major importance inutilization.

Of these, post and white oaks belong tothe white oak group; all others are con-sidered red oaks. Species comparisonsinvolving quantitative data and statisticaldifferences in bark cell morphology arecurrently under investigation at the Soutn-em Forest Experiment Station.

PAST WORK

Although phloem has long been a favor-ite topic for research by numerous botanists(notable among them are Huber 1939;Holdheide 1951; Srivastava 1964; Esau

172 FALL 1977, V. 9(3)

. The author thanks Dr. Floyd Manwiller, South-ern Forest Experiment Station. who supplied thebark samples.WOOD AND FIBER

173Oil BARK STRUcruRE

1005, 1009), few references are found thatspecifically describe the particular speciesincluded in this study. Descriptions of theouter bark are particularly sparse.

Chang (1954) d~bed white oak andnorthern red oak barks and suggested atable of diagnostic features of oak barksaccording to two groups-subgenera Ery-throbalanus Spach. (red oaks) and Lepi-dobalanus Endl. ( white oaks) . Martin( 1963) included photos and brief descrip-tions of black and northern red oak barksin his dissertation on bark thermal proper-ties and fire injury. No information wasfound on the structure of the other oakbarks examined in the present study.

slide, press section down with coatingside up. Flood with absolute alcohol,and blot firmly.)

7. Soak slide in acetone to remove Par-ladion.

8. Proceed through the alcohol series towater, then stain with safranin andfast green.

ANATOMY

The vascular cambium surrounds thestem at the boundary of the wood andbark. It produces wood (xylem) to theinterior and phloem (conducting and stor-age cells of the bark) to the exterior. Thelayer of phloem produced each year isonly a fraction as thick as the annual layerof wood. Each new layer of phloempushes the older phloem layers outwardfrom the enlarging wood stem.

A new tissue-the phellogen or corkcambium-appears within various areas ofthe older phloem at some distance outsidethe vascular cambium. The phellogen isa layer of dividing cells that producetangentially oriented layers of periderm.The impervious periderms protect the deli-cate phloem tissue from harmful exttirpalinfluences. Portions of older phloem aresealed off from supplies of nutrients andmoisture by the periderm layers, and cellsof these isolated areas of phloem sub-sequently die. Each periderm dies whena newer one is formed further inward. Thetissues are pushed outward by each year'sgrowth; when outer layers do not flake offas rapidly as interior ones are formed, athick, rough bark eventually accumulates.Longitudinal cracks form to accommodatetangential stresses caused by growth in thetree's girth.

The term "bark" as used in this paperwill refer to all tissues produced outside thevascular cambium. It consists of twoportions-the light colored inner bark (liv-ing phloem) and the dark outer bark(rhytidome). The innermost peridermseparates the two zones ( Fig. 1 ) . Oakbark tissues and cells are listed below:

PROCEDURE

One tree of each species ( 5.5 to 6.5inches in dbh, outside bark) was cutfrom each of ten locations throughout aneleven-state area from Virginia to Texas.Whole bark, microtome sections, andmacerated bark (including phloem) werestudied to determine cell types present,their arrangement, and possible speciesdifferences. Samples were generally ex-tremely brittle whether embedded or not;therefore, the following procedure wasdeveloped to keep sections intact during

handling:1. Make preliminary microtome cut to

smooth block surface.2. Press cellophane tape finnly onto dry

block surface; then make cut.

3. Coat the section with Parlodion2 inacetone.

4. Soak tape in xylene to release tapefrom coated section.

5. Bleach section in ammoniated hydro-gen peroxide solution (10 ml of 20volume H2O2 and four drops concen-trated NH.OH).

6. Mount. (Spread albumen thinly on

. Mention of trade names is solely to identifymaterials used and does not imply endorsementby the U.S. Deparbfient of Agriculture.

174 ELAINE T. HOWARD

Flc. 1. Southern red oak bark, cross-sectional (top), radial ( left ) , and tangential ( foregroUlld )views. Sclereid groups appear as white areas on cut surfaces. Rays protrude on surface next tocambium.

band (about 200 to 300 pm, according toHuber 1958) next to the cambium is activein conduction. When phloem ceases tofunction as conducting tissue, its structurebecomes greatly modified. Thin-walledcells readily collapse and become distorted,their arrangement becomes disorganized,and the original tissue arrangement be-comes indiscernible not far from thecambium. Most of the early collapse in-volves sieve-tube members and companioncells; parenchyma distortion occurs mainlyafter separation from the inner bark by aperiderm. Only fibers and sclereids haverigid walls that resist distortion.

Sieve tubes.-Organic solutes are con-ducted primarily by the sieve tubes, whichare comprised of individual sieve tubemembers joined end to end in longitudinalseries. Only those in a narrow zone nextto the cambium actively conduct solutes,

Inner bark (phloem)Sieve tube elementsFibersSclereidsVertical parenchymaRay parenchymaCompanion cells

Outer bark (rhytidome)Old phloemPeriderm

PhellogenPhellem (cork)Phelloderm

Phloem

The principal food-conducting tissue ofthe tree is the phloem, or inner bark, whichtransports substances manufactured in thecrown downward to other parts of the tree.Oaks have a fairly thick inner bark,generally 3 to 7 mm, but only a narrow

176 ELAINE T. HOW AM

~

~

~

.1

1111~ '1

.1);l~

~'( 11~ -'.

4.,.., ~

~...&,.

Flc. 3. Schematic drawing of outer bark from southern upland cab. Peridenn is comprised of I, 2,and 3. Arrow points toward tree exterior. TranBVerle oiew. 1.-Pbellem: A, typical cork cells: B,thick-walled rork band. 2.-A1ellogeo (rork cambiUID). 3.-Pbelloderm. 4.-Old phloem tissue: c.Eiben; D, 1cleIads; E, ooIlapsed thiD-walled elements (sieve tubes, (X)Inpanion cells, parenchyma); F,crystal-bearing puendlyma along margins of fiber groUP'; G, ray p&reOO1yma. Tangenl#Dl oiew. H,broad ray; J, sclerified ray cells; K, narrow ray; L, fiber pits; M, polygonal pbellem arrangement.

cellulosic walls. They communicate withother cells by means of specialized portionsof the wall called sieve areas. Sieve areashave clusters of perforations or poresthrough which connecting strands join ad-jacent sieve elements. Plasmodesmata con-nect the sieve areas with parenchyma cells(Esau 1005, 1~). During the functioninglife of the sieve element, a deposit calledcallose builds up around each connectingstrand and eventually over the whole sievearea. Callose usually disappears after con-

duction ceases; sieve areas then appear asthin areas with numerous tiny perforations.On the end walls, sieve areas are groupedinto compound sieve plates. structures com-parable to perforations of vessels in wood.The number of sieve areas in each platevaries. Chang (1954) describes white andnorthern red oab as usually having threeto eight sieve areas per plate, rarely overtwelve. Sieve areas on side walls usuallyare less highly specialized and do not formsieve plates. Pores of these sieve areas

177OAK: BARK: STRucroRE

FIc. 4. Radial sectiml of ~ oak bark. Arrow indicates ezterior of tree. Ffben (F) 8aXXD-panied by crystal-rontaining parend1yma, periderm (PD), old phloem tiIIue (PH), narrow ray (R)seen in end view after tissue collapse and distortion, sclereid aroups (S).

are usually crushed outside the narrow con-ducting zone; the delicate and extremelythin walls appear almost transparent inmacerations (Fig. 2), and in cross section

generally are smaller than those of the ends(Evert et aI. 1971).

Oalc sieve tube members sometimes canbe difficult to study in detail because they

178 ELAINE T. HOW ARD

they can be distinguished from parenchymaonly if the sieve areas at the end are inview.

Fibe1's.- The mechanical strength of oakbark is provided by two cell types-fibersand sclereids. Oaks have typical phloemfibers. These are oriented vertically andare the only greatly elongated cells of thebark (Figs. 2 and 3). In Quercus, fibersdevelop secondary walls and differentiateas mechanical cells close to the cambium.They often mature in the current growthseason (Esau et al. 1953). The phloemfibers are somewhat shorter than corre-sponding wood fibers but are otherwisesimilar in appearance. They are long andslender with tapered overlapping ends,thick walls, and narrow lumens. Fibers ofall species examined had lignified walls.A tendency of the broad inner portion ofthe cell wall to separate from the outerportion during microtoming was noted.' Afew narrow simple pits are found in thewalls; occasional bordered pits may occur.

The fibers are in small groups arrangedas widely spaced, discontinuous tangentialbands, which are usually only about twoto five cells wide. Strands of crystal-bear-ing parenchyma are found along the mar-gins of the fiber band. In older, non-functioning phloem, groups of sclereidsdevelop adjacent to the fibers, usually onthe side away from the cambium (Fig. 4).Occasionally the fibers may be found com-pletely enclosed within sclereid groups.

SclereidY.-Sclereids form a high propor-tion of oak bark and greatly influence itsphysical and mechanical characteristics.These dense cells lend rigidity, hardness,and brittleness to the bark and are respon-sible for some processing problems inproducts such as fine papers. These hard,irregular cells (frequently called -stonecells") are grouped into compact massesthat are readily visible on cut surfaces ofboth inner and outer baric (Figs. 1 and 4).

FIG. S. Sclereids and cryIta1s of southern redoak bark. Sclereid walls are birefringent and havenumerous tiny pits that give a "granular" appear-~ in polarized light.

On cross sections they appear as shinyspots that often form short tangential bands.

Sclereids originate from ordinary paren-chyma cells in the phloem (usually in oldernonconducting phloem) and thus areformed later than fibers. Hardwoodsclereids often undergo some changes inshape and size during their transformationfrom parenchymatous cells, but theyusually do not become as twisted orbranched as those in conifers. Sclereidwalls are thick and heavily lignified andhave distinct lamellate layers with nu-merous simple pits (Fig. 5).

Parenchyma.-Parenchyma of phloem isarranged in two systems: longitudinal( primarily in strands) and horizontal( rays ). Longitudinal parenchyma is ratherabundant in oaks; the amount varies withina species and with the environment (Zahur1959). These cells are usually irregularlydistributed among sieve tubes but some-times are in tangential bands if the tissueshave not yet become greatly distorted.

. R. F. Evert (personal communication) Dotesthat these layers tend to separate because phloemfibers in oak are gelatinous. Only the outer part,which consists of middJe lamella, primary wall.and some secondary wall, is not gelatinous.

179OAK: BARE STRUcruRE

accompanying sieve tube member. Thetwo types of cells die and collapse simul-taneously; they often remain attached inmacerated material. In the oaks studied,companion cells were often difficult todistinguish from ordinary parenchyma bylig'-t microscopy.

Crystals.-Crystals of various types areabundant throughout the bark of all oaksexamined. As in phloem of many otherplants, they are probably composed ofcalcium oxalate deposited as a byproductof metabolism (Esau 1005). Crystals occurclustered within a cell or are solitary.Parenchyma strands with one crystal fillin~each cell are quite common in oaks andare usually associated with fibers andsclereids. A variety of shapes are foundamong oak crystals. Spiculate clusters(druses) are especially frequent. and vari-ous polygonal crystals also abound (Fig.5). Most are fairly isodiametric (about12-,'35 pm), and no greatly elongated formswere found.

RhytidomeThe rhytidome, or outer bark, is the

dark-colored, dead tissue outside the newestperidenn. It insulates the tree and protectsit from mechanical injury and desiccation.

Several marked changes occur in thetransfonnation of phloem tissue into rhyt-idome: appearance of the cork cambium(phellogen) and production of the peri-denn, accumulation of considerable de.posits of dark-colored substances withinthe cells, and subsequent death of all tis-sues outside the new peridenn.

Periderms.-The peridenns protect theinner bark from moisture loss. They arereadily visible on cut surfaces as discon-tinuous lines of variable pattern more orless parallel to the circumference. Onlythe innennost peridenn is alive. Its fonna-tion seals off vital food and water suppl1esfrom inner bark to the previous peridenn.

A periderm is composed of three tissues-phellogen, phellem, and phelloderm. Thephellogen, or cork cambium, is the layerof cells that forms cork (phellem) toward

Phloem parenchyma cells are somewhatcylindrical in cross section and usuallyhave only thin cellulosic walls with nu-merous primary pit fields by which theycommunicate with each other and raycells. They contain stored products suchas tannins and other phenolic compounds,starches, oils, fats, and various types ofcrystals. Parenchyma remain functionallong after the sieve elements die, and someof the parenchyma cells acquire thicksecondary walls and become modified assclereids.

Rays.-Phloem rays provide horizontalconduction within the inner bark. Theytransport nutrients from the actively con-ducting zone to living parenchyma and theinnermost phellogen. The outer portions ofrays die when eventually sealed off by for-mation of a new and deeper periderm.

When oak bark is split from the wood ator near the cambium, rays are readilyvisible on the inner surface of the phloemand usually protrude from the bark (Fig.1). They are the outward continuation ofthe wood rays, and like them, occur in twodistinct sizes: narrow rays (uniseriate orpartially biseriate) and broad, multiseriaterays. They are homocellular, i.e., comprisedof only procumbent ray cells that areusually fairly short and thin-walled.

Many ray cells undergo modification tobecome sclereids. They develop thick,lignified secondary walls with numeroussimple pits and sometimes change in shape.The sclerification process begins near thecambium and increases as the cells arepushed outward.

Because of the functional and structuralchanges occurring in the bark and theresulting physical stresses, the rays maybecome distorted in nonfunctioning phloemand the outer bark. Some rays dilate bymultiplication of cells within the ray,producing an archlike pattern in the outerportion of the inner bark.

Companion ceUs.-Companion cells are

narrow, highly specialized parenchymatouscells that appear to be intimately associatedwith sieve elements. They are producedby the same cambial cell that produced the

18:> ELAINE T. HOW A1U)

FE. 6. WaDs of pheDem celIs are oomiderably thicker in red oab ()eft) than in white oeb( right). In both, narrow bands of lignified cells (dark lines) alternate with ordinary cork tissue.

a secondary wall of suberin. composed ofalternating phenolic and wax lamellae, thatrenders them practically impervious tomoisture and gases (Sitte 1957; Esau 1005).In the oaks studied. typical cork cells com-prise most of the; phellem. Narrow bandsof lignified cells, often only one cell wide.frequently alternate with wider bands ofthe typical cells (Fig. 6) . The lignifiedcells appear to have become somewhatsclerotic; the walls are thickened andpitted, but the cells retain their originalshape. As Chang (1954) observed. wallsof phellem cells are usually noticeablythicker in red oaks than in white oaks.often twice as thick ( Fig. 6) . In thisstudy, this distinction was found to begenerally true; but in red oaks some areasof phellem had quite thin walls, and agradual increase in wall thickness acrossthe periderm was frequently observed.

Phelloderm. to the inside of the phello-gen, is inconspicuous and resembles ad-jacent parenchyma. Only a few layers arepresent, generally six to eight ceDs or less.and these often cannot be distinguished

the outside and parenchyma (phelloderm)on the inside. Early phellogens originatefrom cortical cells, but in older bark theyarise from parenchyma of the nonfunctionalphloem. The derivative cells retain thepolygonal shape of the mother phellogencell for the most part but differ in wallstructure. They are in distinct radial align-ment but are not aligned tangentially (Fig.3) . Periderm width depends on length oflife of its phellogen; if it is replaced by anew phellogen deeper in the bark after ithas been active for a short duration, onlya few layers of cells would have beenproduced and the periderm would benarrow.

The phellem, or cork tissue, is primarilyresponsible for preventing moisture lossfrom the stem. Oaks of the species studieddo not produce extensive cork layers asdoes cork oak (Quercus suber L. ). Phellemis a compact tissue of small cells that ap-pear as flat rectangles in radial and crosssections, and are polygonal in tangentialview (Fig. 3). Typical hardwood corkcells possess unpitted cellulose walls with

181OAK: BARK STRUCl'URE

FIc. 7. Great differences in surface texture areevident among these three northern red oak slabs,all from 2 to 6 feet above ground.

from neighboring parenchyma unless theirradial alignment is evident (Fig. 4; Fig. 6,right); consequently, this inner portion ofthe periderm is often overlooked by manyobservers. The function of this tissue isuncertain.

Deposited MateriaU.-Rbytidome con-tains an abundance of deposited materialsthat provide its dark coloraton. These darkreddish-brown deposits-mainly phenolicsubstances such as tannins and phlob-apbenes-are found primarily in paren-chyma and crushed sieve tubes. It isthought that these materials act as anti-oxidants and inhibitors of fungal attack(Srivastava 1964; Somers and Harrisonl~ ) . Their value in tanning leather haslong been recognized, and they are con-sidered a possible source of phenolics foradhesives.

Most oaks have rough bark with longitu-dinal furrows and ridges; but laurel, water,and white oaks usually have relativelysmooth bark. Smooth bark is also found onyoung growth, on upper stems andbranches, and on some individual trees ofhigh growth rate. Of the small-diametertrees examined, blackjack, post, and south-ern red oaks had thick bark; the laurel,cherry bark, water, and white oak sampleswere usually thin-barked. Other specieswere either intermediate in thickness orshowed considerable variation. Southernupland oaks of small diameter generallyhave fibrous, stringy bark. Bark from mostred oaks is hard and friable when cutacross the grain, but the outer bark ofwhite and post oaks tends to be soft andflaky. This difference in hardness may bedue mostly to the amounts of sclereidspresent, but such a conclusion awaits quan-titative comparisons of cell types for thevarious species.

Bark surface characteristics, however,are not a totally reliable indicator fo,l:species identification since they can begreatly modified by environmental in-fluences. Thus, a high degree of variabilityis found within species (Fig. 7). Grossfeatures such as bark thickness, proportionof inner and outer balk, scaliness, anddepth of fissures depend to a great degreeon tree vigor and growth rate, age, andheight on the tree. Guttenberg's ( 1951 )photographs demonstrate the marked in-crease in oak bark roughness with declinein tree vigor.

Anatomical differences in oak bark offersome additional help in identifying species.As noted by Chang (1954), cork cells ofwhite oaks generally have thinner wallsthan those of red oaks. White oaks retaintheir original phloem structure to a greaterdegree than red oaks. White oak rays aregenerally evident in a straight course pastperiderrns ( Fig. 8). Chang ( 1954) re-ported that an archIike pattern in olderphloem was a characteristic distinguishingred oaks from white oaks, but in the presentStudy such patterns were frequently foundin the white oaks as well.

Spg:(ES COMPARISON

In oaks, as in other trees, bark surfacecharacteristics can be of supplementaryvalue in the identification of species. Theexterior surface of white oak bark is usuallylighter in color than that of other species.

l~OAK BARI a-mUCI1JRE

Other characteristics given by Chang( 1954) as general differences between thetwo groups were not confirmed by thepresent study. These included his compari-son of inner to outer bark thickness, andthe description of peridenns and scleren-chyma as being in distinct tangentialalignment in white oaks (in conb"ast to aloose irregular arrangement in red oaks).Both types of arrangements were observedin both groups of oaks; bark thickness wasfar too variable in these samples to serveas a distinguishing characteristic betweenthe two groups .

Inner bark of black oak is bright goldwhen freshly cut, but the color may notbe found in dry samples. No other charac-teristic was found that would, with cer-tainty. identify an individual tree as amember of a particular species.

trends of specialization of the phloem. Am. J.Bot. 40:9-19.

EVERT, R. F., B. P. DsSHPANDE, AND S. E. EICH-HORN. 1971. Laten! sieve-area pores inwoody dicotyledom. Can. J. Bot. 49:1509-1515.

C1nTENBBIIC, S. 1951. Listen to the bark.South. Lumbennan 183(2297):220-222.

HoLDBEIIE. W. 1951. Anatomie mftteleuro-paisd1er CebolzriDdeD In H. Freund's Hand-budl der MiknBkopie in der Technik, Vol. 5,Part 1, pp. 193-367. Umschau Verlag:Frank-fort-am-Main.

HUBER, B. 1939. Du SiebriSbensystem unsererBaume oDd seine Jahrezeitlid1en Verinde-nmgen. Jahrb. will. Bot. 88:176-242.

-. 1958. Anatomical and physiological in-vestigations on food translocation in trees.Pages 367-379 in K. V. Thimann, ed., Thephysiology of forest trees. Ronald Press, NewYork.

MAIn"IN, R. E. 1963. Thermal and «her pr0p-erties of bark and their relation to fire injuryof tree stems. At.D. thesis, Univ. of Mich.,Ann Arbor. 256 pp.

SnTE, VON P. l'iE7. Der Einbau der Kork-Zellwande. In E. Treiber, ed., Die 0leIIIieder pflanzenzellwaod. Springer- Verlag, Berlin.511 pp.

SoMERS, T. C., AND A. F. HA1UU8ON. 1967.Wood tanDins-iIOlatioD and significance inhost resistance to VertkiUium wik disease.Aust. J. BioI. Sci.20:475-479.

SRIVASTAVA, L. M. 1964. Anatomy, chemistry,and physiology of bark. In J. A. Rombergerand P. Mikol&, edt., IDterDaticmal review offorestry reseatdI. Vol. 1. Academic Press,New York. 240 pp.

~ M. S. 1959. Comparative study of sec-ondary phloem of 423 species of woodydialtyledons belooging to ~ famdJes. CorDellAgric. Ezp. StD. MSDOir 358. Ithaca. N.Y.100 pp.

flr;yERENCES

CHANG. Y.-P. 1954. Anatomy of common NorthAmerican pulpwood barks. Tapp! MODOgr.Ser. 14, pp. 127-146.

CHRISTOPBBR, J. F.. H. S. STBRNITZD, R. C.BBL1'%, J. M. EAP.uI, AND M. S. HEDLUND.1976. Hardwood distribution on pine litesin the South. USDA For. Servo Resour. Bull.50-59, 27 pp. South. For. Exp. St.., NewOrlMDI, LA.

ESAu. I:. 1965. Plant anatomy. 2nd ed. J<iIuWiley & Sons, Inc.. New York. 767 pp.

-. 1969. Encyclopedia of plant anatomy:The phloem. Gebriider Bomtraeler, Berlin.

~pp.-, V. I. CImADLB, AND E. M. GU'POBD, Jft.

ItS3. Compatative structure and possible