Wood.,.., Fa.r. 9(3). 1977.w. 17S-183 . 1978by the Societyof Wood~ aDdTecbaoiOIY BARK STRUCfURE OF SOUTHERN UPLAND OAKS!
Elaine T. H award AssociateResearch Chemist SouthernForest ExperimentStation, USDA, Pineville, La. 71300 (Received 4 February 1977)
ABSTRAcr Bark structure of eleven oak speciescommonly found on southern pine sites was exam- ined and described. In inner bark (phloem), groups of thick-walled lignified fibers and Iclereids are interspersedamong thin-walled cellulosic elements (parenchyma, sieve tube ~bers, and companjon oelIs). 'nJeIe fibers and sclereids greatly influence the bark's density, hardDeSl, and ~ physical and mechanical characteristics. The innennost periderm is the boundary between inner and outer bark. In outer bark (rhytidome), areas of collapsed,dead phloem are enclosedby periderm layers. Peridenn shape and spacing vary greatly within species. Great differencesin exterior roughnessand bark thicknessalso ~ within species. Kev1DOrds:QuefCUS spp., anatomy,bark, oaks,phloem, peridenn, rhytidome.
INTRODUcnON The objective of the present study was to observe,compare, and describethe bark Small hardwood trees growing on sites ~'tructureof eleven upland oaks growing better suited to southern pine present a on southern pine sites. Species sampled major forest utilization problem in the are listed below: South today. Oaks comprise about 48% of the hardwood volume on such sites (Chris- Common name Scientific name topher et al. 1976), and their bark accounts for a considerable part of total volume. Black oak Que1'CUSoelutina Lam. For example, on oaks 6 inches in dbh, Blackjack oak Q. mariltmdiCG Muenchh ~- Q. falCtJt4 vaT. bark represents about 17% of stem volume. pagod4efoUG Ell. Removing the bark usually presents a Laurel oak Q. laurlfoUG MichL disposal problem and results in the waste Northern red oak Q. rob1'a L. of large quantities of material that should Post oak Q. stellato Wangenh. Scarlet oak Q. cocc1nea Muencbh. be utilized. A possible solution is utiliza- Shumard oak Q. shuma1'da Buckl. tion of these small hardwoods as whole- Southern red oak Q. falCtJt4 Michx. tree chips. Water oak Q. nig1'a L. When wood with bark is processed, the White oak Q. albaL. bark characteristics peculiar to the species often determine the nature and magnitude Of these, post and white oaks belong to of the problems encountered. Becausebark the white oak group; all others are con- properties are influenced by their structure, sidered red oaks. Species comparisons behavior of a bark under certain conditions involving quantitative data and statistical may sometimesbe predicted from a knowl- differences in bark cell morphology are edge of bark anatomy. Thus, the types of currently under investigation at the Soutn- cells present, their arrangement, relative em Forest Experiment Station. amounts, and physical dimensions and proportions are all of major importance in PAST WORK utilization. Although phloem has long been a favor- . The author thanks Dr. Floyd Manwiller, South- ite topic for researchby numerousbotanists ern Forest Experiment Station. who supplied the (notable among them are Huber 1939; bark samples. Holdheide 1951; Srivastava 1964; Esau WOODAND FIBER 172 FALL 1977,V. 9(3) Oil BARK STRUcruRE 173
1005,1009), few referencesare found that slide, presssection down with coating specifically describethe particular species side up. Flood with absolutealcohol, included in this study. Descriptionsof the and blot firmly.) outer bark are particularly sparse. 7. Soakslide in acetoneto remove Par- Chang (1954) d~bed white oak and ladion. northern red oak barks and suggesteda table of diagnostic features of oak barks 8. Proceedthrough the alcohol seriesto water, then stain with safranin and according to two groups-subgeneraEry- throbalanusSpach. (red oaks) and Lepi- fast green. dobalanus Endl. ( white oaks). Martin ( 1963) included photos and brief descrip- ANATOMY tions of black and northern red oak barks The vascular cambium surrounds the in his dissertationon bark thermal proper- stem at the boundary of the wood and ties and fire injury. No information was bark. It produces wood (xylem) to the found on the structure of the other oak interior and phloem (conducting and stor- barks examinedin the present study. age cells of the bark) to the exterior. The layer of phloem produced each year is PROCEDURE only a fraction as thick as the annual layer of wood. Each new layer of phloem One tree of each species ( 5.5 to 6.5 pushes the older phloem layers outward inches in dbh, outside bark) was cut from each of ten locations throughout an from the enlarging wood stem. A new tissue-the phellogen or cork eleven-state area from Virginia to Texas. cambium-appears within various areas of Whole bark, microtome sections, and the older phloem at some distance outside macerated bark (including phloem) were the vascular cambium. The phellogen is studied to determine cell types present, a layer of dividing cells that produce their arrangement, and possible species tangentially oriented layers of periderm. differences. Samples were generally ex- The impervious periderms protect the deli- tremely brittle whether embedded or not; cate phloem tissue from harmful exttirpal therefore, the following procedure was influences. Portions of older phloem are developed to keep sections intact during sealed off from supplies of nutrients and moisture by the periderm layers, and cells handling: of these isolated areas of phloem sub- 1. Make preliminary microtome cut to sequently die. Each periderm dies when smooth block surface. a newer one is formed further inward. The 2. Press cellophane tape finnly onto dry tissues are pushed outward by each year's block surface; then make cut. growth; when outer layers do not flake off 3. Coat the section with Parlodion2 in as rapidly as interior ones are formed, a thick, rough bark eventually accumulates. acetone. Longitudinal cracks form to accommodate 4. Soak tape in xylene to release tape tangential stressescaused by growth in the from coated section. tree's girth. 5. Bleach section in ammoniated hydro- The term "bark" as used in this paper gen peroxide solution (10 ml of 20 will refer to all tissuesproduced outside the volume H2O2 and four drops concen- vascular cambium. It consists of two trated NH.OH). portions-the light colored inner bark (liv- 6. Mount. (Spread albumen thinly on ing phloem) and the dark outer bark (rhytidome). The innermost periderm . Mention of trade names is solely to identify separates the two zones ( Fig. 1 ) . Oak materials used and does not imply endorsement by the U.S. Deparbfient of Agriculture. bark tissues and cells are listed below: 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 to cambium.
Inner bark (phloem) band (about 200 to 300 pm, according to Sieve tube elements Huber 1958) next to the cambium is active Fibers in conduction. When phloem ceases to Sclereids function as conducting tissue, its structure Vertical parenchyma becomes greatly modified. Thin-walled Ray parenchyma cells readily collapse and become distorted, Companion cells their arrangement becomes disorganized, Outer bark (rhytidome) and the original tissue arrangement be- Old phloem comes indiscernible not far from the Periderm cambium. Most of the early collapse in- Phellogen volves sieve-tube members and companion Phellem (cork) cells; parenchyma distortion occurs mainly Phelloderm after separation from the inner bark by a periderm. Only fibers and sclereids have Phloem rigid walls that resist distortion. Sieve tubes.-Organic solutes are con- The principal food-conducting tissue of ducted primarily by the sieve tubes, which the tree is the phloem, or inner bark, which are comprised of individual sieve tube transports substances manufactured in the crown downward to other parts of the tree. members joined end to end in longitudinal Oaks have a fairly thick inner bark, series. Only those in a narrow zone next generally 3 to 7 mm, but only a narrow to the cambium actively conduct solutes,
176 ELAINE T. HOWAM
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...&,. Flc. 3. Schematicdrawing of outer bark from southern upland cab. Peridenn is comprisedof 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, ooIlapsedthiD-walled elements (sieve tubes, (X)Inpanioncells, 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 with duction ceases; sieve areas then appear as other cells by means of specialized portions thin areas with numerous tiny perforations. of the wall called sieve areas. Sieve areas On the end walls, sieve areas are grouped have clusters of perforations or pores into compound sieve plates. structures com- through which connecting strands join ad- parable to perforations of vessels in wood. jacent sieve elements. Plasmodesmatacon- The number of sieve areas in each plate nect the sieve areas with parenchyma cells varies. Chang (1954) describes white and (Esau 1005, 1~). During the functioning northern red oab as usually having three life of the sieve element, a deposit called to eight sieve areas per plate, rarely over callose builds up around each connecting twelve. Sieve areas on side walls usually strand and eventually over the whole sieve are less highly specialized and do not form area. Callose usually disappears after con- sieve plates. Pores of these sieve areas
~ OAK: BARK: STRucroRE 177
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). generally are smaller than those of the ends are usually crushed outside the narrow con- (Evert et aI. 1971). ducting zone; the delicate and extremely Oalc sieve tube members sometimes can thin walls appear almost transparent in be difficult to study in detail because they macerations (Fig. 2), and in cross section 178 ELAINE T. HOW ARD they can be distinguishedfrom parenchyma only if the sieve areas at the end are in view. Fibe1's.-The mechanicalstrength of oak bark is provided by two cell types-fibers and sclereids. Oaks have typical phloem fibers. These are oriented vertically and are the only greatly elongatedcells of the bark (Figs. 2 and 3). In Quercus,fibers develop secondarywalls and differentiate as mechanicalcells close to the cambium. They often mature in the current growth season(Esau et al. 1953). The phloem fibers are somewhat shorter than corre- sponding wood fibers but are otherwise similar in appearance.They are long and slender with tapered overlapping ends, thick walls, and narrow lumens. Fibers of all speciesexamined had lignified walls. A tendency of the broad inner portion of the cell wall to separatefrom the outer portion during microtomingwas noted.' A few narrow simple pits are found in the FIG. S. Sclereidsand cryIta1s of southern red walls; occasionalbordered pits may occur. oak bark. Sclereid walls are birefringent and have numeroustiny pits that give a "granular" appear- The fibers are in small groups arranged ~ in polarized light. as widely spaced,discontinuous tangential bands, which are usually only about two to five cells wide. Strandsof crystal-bear- On cross sections they appear as shiny ing parenchymaare found along the mar- spots that often form short tangential bands. gins of the fiber band. In older, non- Sclereids originate from ordinary paren- functioning phloem, groups of sclereids chyma cells in the phloem (usually in older develop adjacent to the fibers, usually on nonconducting phloem) and thus are the side away from the cambium (Fig. 4). formed later than fibers. Hardwood Occasionallythe fibers may be found com- sclereids often undergo some changes in pletely enclosedwithin sclereid groups. shape and size during their transformation SclereidY.-Sclereidsform a high propor- from parenchymatous cells, but they tion of oak bark and greatly influence its usually do not become as twisted or physical and mechanical characteristics. branched as those in conifers. Sclereid These dense cells lend rigidity, hardness, walls are thick and heavily lignified and and brittlenessto the bark and are respon- have distinct lamellate layers with nu- sible for some processing problems in merous simple pits (Fig. 5). products such as fine papers. Thesehard, Parenchyma.-Parenchyma of phloem is irregular cells (frequently called -stone arranged in two systems: longitudinal cells") are grouped into compact masses ( primarily in strands) and horizontal that are readily visible on cut surfacesof ( rays ). Longitudinal parenchyma is rather both inner and outer baric (Figs. 1 and 4). abundant in oaks; the amount varies within a species and with the environment (Zahur . R. F. Evert (personal communication) Dotes 1959). These cells are usually irregularly that these layers tend to separatebecause phloem distributed among sieve tubes but some- fibers in oak are gelatinous. Only the outer part, which consists of middJe lamella, primary wall. times are in tangential bands if the tissues and somesecondary wall, is not gelatinous. have not yet become greatly distorted. OAK: BARE STRUcruRE 179
Phloem parenchyma cells are somewhat accompanying sieve tube member. The cylindrical in cross section and usually two types of cells die and collapse simul- have only thin cellulosic walls with nu- taneously; they often remain attached in merous primary pit fields by which they macerated material. In the oaks studied, communicate with each other and ray companion cells were often difficult to cells. They contain stored products such distinguish from ordinary parenchyma by as tannins and other phenolic compounds, lig'-t microscopy. starches, oils, fats, and various types of Crystals.-Crystals of various types are crystals. Parenchyma remain functional abundant throughout the bark of all oaks long after the sieve elements die, and some examined. As in phloem of many other of the parenchyma cells acquire thick plants, they are probably composed of secondary walls and become modified as calcium oxalate deposited as a byproduct sclereids. of metabolism (Esau 1005). Crystals occur Rays.-Phloem rays provide horizontal clustered within a cell or are solitary. conduction within the inner bark. They Parenchyma strands with one crystal fillin~ transport nutrients from the actively con- each cell are quite common in oaks and ducting zone to living parenchyma and the are usually associated with fibers and innermost phellogen. The outer portions of sclereids. A variety of shapes are found rays die when eventually sealed off by for- among oak crystals. Spiculate clusters mation of a new and deeper periderm. (druses) are especially frequent. and vari- When oak bark is split from the wood at ous polygonal crystals also abound (Fig. or near the cambium, rays are readily 5). Most are fairly isodiametric (about visible on the inner surface of the phloem 12-,'35pm), and no greatly elongated forms and usually protrude from the bark (Fig. were found. 1). They are the outward continuation of the wood rays, and like them, occur in two Rhytidome distinct sizes: narrow rays (uniseriate or The rhytidome, or outer bark, is the partially biseriate) and broad, multiseriate dark-colored, dead tissue outside the newest rays. They are homocellular, i.e., comprised peridenn. It insulates the tree and protects of only procumbent ray cells that are it from mechanical injury and desiccation. usually fairly short and thin-walled. Several marked changes occur in the Many ray cells undergo modification to transfonnation of phloem tissue into rhyt- become sclereids. They develop thick, idome: appearance of the cork cambium lignified secondary walls with numerous (phellogen) and production of the peri- simple pits and sometimeschange in shape. denn, accumulation of considerable de. The sclerification process begins near the posits of dark-colored substances within cambium and increases as the cells are the cells, and subsequent death of all tis- pushed outward. sues outside the new peridenn. Because of the functional and structural Periderms.-The peridenns protect the changes occurring in the bark and the inner bark from moisture loss. They are resulting physical stresses, the rays may readily visible on cut surfaces as discon- become distorted in nonfunctioning phloem tinuous lines of variable pattern more or and the outer bark. Some rays dilate by less parallel to the circumference. Only multiplication of cells within the ray, the innennost peridenn is alive. Its fonna- producing an archlike pattern in the outer tion seals off vital food and water suppl1es portion of the inner bark. Companion ceUs.-Companion cells are from inner bark to the previous peridenn. A periderm is composed of three tissues- narrow, highly specialized parenchymatous cells that appear to be intimately associated phellogen, phellem, and phelloderm. The with sieve elements. They are produced phellogen, or cork cambium, is the layer by the same cambial cell that produced the of cells that forms cork (phellem) toward 18:> ELAINE T. HOW A1U)
FE. 6. WaDs of pheDemcelIs 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. the outside and parenchyma (phelloderm) a secondary wall of suberin. composed of on the inside. Early phellogens originate alternating phenolic and wax lamellae, that from cortical cells, but in older bark they renders them practically impervious to arise from parenchyma of the nonfunctional moisture and gases (Sitte 1957; Esau 1005). phloem. The derivative cells retain the In the oaks studied. typical cork cells com- polygonal shape of the mother phellogen prise most of the; phellem. Narrow bands cell for the most part but differ in wall of lignified cells, often only one cell wide. structure. They are in distinct radial align- frequently alternate with wider bands of ment but are not aligned tangentially (Fig. the typical cells (Fig. 6) . The lignified 3) . Periderm width depends on length of cells appear to have become somewhat life of its phellogen; if it is replaced by a sclerotic; the walls are thickened and new phellogen deeper in the bark after it pitted, but the cells retain their original has been active for a short duration, only shape. As Chang (1954) observed. walls a few layers of cells would have been of phellem cells are usually noticeably produced and the periderm would be thicker in red oaks than in white oaks. narrow. often twice as thick ( Fig. 6) . In this The phellem, or cork tissue, is primarily study, this distinction was found to be responsible for preventing moisture loss generally true; but in red oaks some areas from the stem. Oaks of the speciesstudied of phellem had quite thin walls, and a do not produce extensive cork layers as gradual increase in wall thickness across does cork oak (Quercus suber L. ). Phellem the periderm was frequently observed. is a compact tissue of small cells that ap- Phelloderm. to the inside of the phello- pear as flat rectangles in radial and cross gen, is inconspicuous and resembles ad- sections, and are polygonal in tangential jacent parenchyma. Only a few layers are view (Fig. 3). Typical hardwood cork present, generally six to eight ceDs or less. cells possessunpitted cellulose walls with and these often cannot be distinguished OAK: BARK STRUCl'URE 181
Most oaks have rough bark with longitu- dinal furrows and ridges;but laurel, water, and white oaks usually have relatively smoothbark. Smoothbark is alsofound on young growth, on upper stems and branches,and on some individual trees of high growth rate. Of the small-diameter trees examined,blackjack, post, and south- ern red oaks had thick bark; the laurel, cherrybark, water, and white oak samples were usually thin-barked. Other species were either intermediate in thickness or showed considerable variation. Southern upland oaks of small diameter generally have fibrous, stringy bark. Bark from most red oaks is hard and friable when cut across the grain, but the outer bark of white and post oaks tends to be soft and flaky. This difference in hardnessmay be due mostly to the amounts of sclereids FIc. 7. Great differencesin surfacetexture are present,but such a conclusionawaits quan- evident among these three northern red oak slabs, all from 2 to 6 feet above ground. titative comparisonsof cell types for the various species. Bark surface characteristics, however, from neighboring parenchyma unless their are not a totally reliable indicator fo,l: radial alignment is evident (Fig. 4; Fig. 6, species identification since they can be right); consequently, this inner portion of greatly modified by environmental in- the periderm is often overlooked by many fluences. Thus, a high degreeof variability observers. The function of this tissue is is found within species (Fig. 7). Gross uncertain. featuressuch as bark thickness,proportion Deposited MateriaU.-Rbytidome con- of inner and outer balk, scaliness,and tains an abundance of deposited materials depth of fissuresdepend to a great degree that provide its dark coloraton. These dark on tree vigor and growth rate, age, and reddish-brown deposits-mainly phenolic height on the tree. Guttenberg's ( 1951) substances such as tannins and phlob- photographs demonstratethe marked in- apbenes-are found primarily in paren- creasein oak bark roughnesswith decline chyma and crushed sieve tubes. It is in tree vigor. thought that these materials act as anti- Anatomical differencesin oak bark offer oxidants and inhibitors of fungal attack someadditional help in identifying species. (Srivastava 1964; Somers and Harrison As noted by Chang (1954), cork cells of l~ ) . Their value in tanning leather has white oaks generally have thinner walls long been recognized, and they are con- than those of red oaks. White oaks retain sidered a possible source of phenolics for their original phloem structureto a greater adhesives. degree than red oaks. White oak rays are generally evident in a straight coursepast Spg:(ES COMPARISON periderrns ( Fig. 8). Chang ( 1954) re- In oaks, as in other trees, bark surface ported that an archIike pattern in older characteristics can be of supplementary phloem was a characteristicdistinguishing value in the identification of species. The red oaksfrom white oaks,but in the present exterior surface of white oak bark is usually Study such patterns were frequently found lighter in color than that of other species. in the white oaks as well.
OAK BARI a-mUCI1JRE l~
Other characteristics given by Chang trends of specializationof the phloem. Am. J. ( 1954) as general differences between the Bot. 40:9-19. two groups were not confirmed by the EVERT, R. F., B. P. DsSHPANDE,AND S. E. EICH- HORN. 1971. Laten! sieve-area pores in present study. These included his compari- woody dicotyledom. Can. J. Bot. 49:1509- son of inner to outer bark thickness, and 1515. the description of peridenns and scleren- C1nTENBBIIC,S. 1951. Listen to the bark. chyma as being in distinct tangential South. Lumbennan 183(2297):220-222. alignment in white oaks (in conb"ast to a HoLDBEIIE. W. 1951. Anatomie mftteleuro- paisd1erCebolzriDdeD In H. Freund's Hand- loose irregular arrangement in red oaks). budl der MiknBkopie in der Technik, Vol. 5, Both types of arrangements were observed Part 1, pp. 193-367. UmschauVerlag:Frank- in both groups of oaks; bark thickness was fort-am-Main. far too variable in these samples to serve HUBER,B. 1939. Du SiebriSbensystemunserer Baume oDd seine Jahrezeitlid1en Verinde- as a distinguishing characteristic between nmgen. Jahrb. will. Bot. 88:176-242. the two groups. -. 1958. Anatomical and physiological in- Inner bark of black oak is bright gold vestigations on food translocation in trees. when freshly cut, but the color may not Pages367-379 in K. V. Thimann, ed., The be found in dry samples. No other charac- physiology of forest trees. Ronald Press,New York. teristic was found that would, with cer- MAIn"IN, R. E. 1963. Thermal and «her pr0p- tainty. identify an individual tree as a erties of bark and their relation to fire injury member of a particular species. of tree stems. At.D. thesis, Univ. of Mich., Ann Arbor. 256 pp. SnTE, VON P. l'iE7. Der Einbau der Kork- flr;yERENCES Zellwande. In E. Treiber, ed., Die 0leIIIie CHANG. Y.-P. 1954. Anatomy of common North der pflanzenzellwaod. Springer- Verlag, Berlin. American pulpwood barks. Tapp! MODOgr. 511 pp. Ser. 14, pp. 127-146. SoMERS,T. C., AND A. F. HA1UU8ON.1967. CHRISTOPBBR,J. F.. H. S. STBRNITZD, R. C. Wood tanDins-iIOlatioD and significance in BBL1'%,J. M. EAP.uI, AND M. S. HEDLUND. host resistance to VertkiUium wik disease. 1976. Hardwood distribution on pine lites Aust. J. BioI. Sci.20:475-479. in the South. USDA For. Servo Resour. Bull. SRIVASTAVA,L. M. 1964. Anatomy, chemistry, 50-59, 27 pp. South. For. Exp. St.., New and physiology of bark. In J. A. Romberger OrlMDI, LA. and P. Mikol&, edt., IDterDaticmalreview of ESAu. I:. 1965. Plant anatomy. 2nd ed. J