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 ~ ~ .1 1111~ '1 . 1); l~ ~'( 11 ~ -'. 4.,.., ~ ~ ...&,. 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.