Bark Advanced article Simcha Lev-Yadun, University of Haifa-Oranim, Tivon, Israel Article Contents . Introduction . Structure and Development . Regulation of Cork Development . Dilatation Online posting date: 16th May 2011 Bark comprises all the tissues outside the vascular cam- Structure and Development bium of a vascular plant. The majority of the bark of woody plants develops from three meristems: the vascular The bark includes primary and secondary phloem, cortex, cambium that gives rise to the secondary phloem, the first periderm, sequent periderms (rhytidome) and tissues phellogen that gives rise to the cork and the dilatation formed by dilatation growth (Esau, 1965, 1969; Roth, 1981; meristem that produces parenchyma cells to prevent Fahn, 1990; Junikka, 1994). The structure of the bark in cracking when the axis increases in diameter. Bark tissues roots of a given species is usually somewhat less compli- have a critical role in defending plants from pathogens cated than in the shoot, and the description here refers to the shoot rather than to the root. The wealth of known struc- and herbivores through their physical and chemical tures and functions of the bark is never found in a single properties. They also defend from environmental hazards species, and there are also many variations at different ages such as sun irradiation, desiccation, wind, flooding, hail, and with changing growth conditions in the same individual snow and even fire by forming a thick cork layer. The bark (Borger, 1973; Roth, 1981; Lev-Yadun and Aloni, 1990). has a critical role in storage and transport of organic Additional variation is expressed following wounding or molecules and in many plants the bark also contributes to pathogen attacks (Borger, 1973). See also: Phloem Struc- photosynthesis. Many of the various defensive and toxic ture and Function substances found in barks are used by humans as medi- In woody plants, two regions are distinguished within cines, spices and for various industries. Gene exploring in the bark: the inner bark, which is alive and where certain barks is expected to result in many beneficial molecules cells may redifferentiate and become meristematic or for agriculture, medicine, food and industry. change their fate (e.g., parenchyma cells that turn into sclereids); and the outer, dead bark cut off from live tissues by dead isolating cork layers – the rhytidome. We distinguish between primary bark (originating from primary meristems – protoderm, ground meristem and procambium) and secondary bark (originating from sec- Introduction ondary meristems – the vascular cambium, the phellogen and dilatation meristem) (Esau, 1969; Fahn, 1990). Bark, which includes all tissues formed outside the vascular The contribution of the primary meristems to the pri- cambium, is structurally, physiologically and functionally a mary bark is as follows. The protoderm gives rise to the very complex part of the plant. The major functions of the epidermis, which may exist for many years or may be bark are translocation and storage of organic materials, replaced by cork (periderm). The ground meristem gives water storage and wound healing, protection from herbi- rise to the cortex, made of parenchyma, collenchyma, vores and pathogens, protection from environmental haz- fibres, sclereids, idioblasts of various types, resin ducts, ards, and in the shoot, photosynthesis. In many leafless or gum ducts or laticifers. The procambium gives rise to the almost leafless plants, all or most of a plant’s photosynthesis primary phloem, including the primary phloem fibres. In a is performed by the bark. An ecologically important func- number of plants, the border between the cortex and the tion of bark is to protect trees from fires. Thick barks, being primary phloem is marked by the starch sheath, a layer rich poor conductors of heat, isolate the sensitive live tissues of in starch grains, considered to be homologous with the many tree species from fires. See also: Epidermis: Outer Cell endodermis of the root. An endodermis, with its typical Layer of the Plant casparian strip, is known from the shoot of only a small number of plants. In addition to these components, in ELS subject area: Plant Science shoots, leaf traces composed of xylem and phloem cross the cortex before they fuse with the central vascular cylinder How to cite: (Esau, 1965; Fahn, 1990). See also: Cork; Starch and Starch Lev-Yadun, Simcha (May 2011) Bark. In: Encyclopedia of Life Sciences Granules (ELS). John Wiley & Sons, Ltd: Chichester. The vascular cambium (a secondary lateral meristem) is DOI: 10.1002/9780470015902.a0002078.pub2 the origin of the secondary phloem. The distinction ENCYCLOPEDIA OF LIFE SCIENCES & 2011, John Wiley & Sons, Ltd. www.els.net 1 Bark between the primary and secondary phloem is easy in most gymnosperms and dicotyledons since the secondary phloem has a radial component (the vascular rays) in addition to the axial component, whereas the primary phloem does not include rays. However, hundreds of spe- cies produce secondary phloem with no rays as a special adaptation (Larson, 1994; Lev-Yadun and Aloni, 1995). The secondary phloem of many species has many bands of fibres (bast fibres) (Esau, 1965, 1969). When cambial activity produces large amounts of xylem, the phloem is pushed outward, and the old, nonconducting and soft sieve cells collapse and flatten, but the bands of fibres, axial parenchyma, sclereides and rays remain intact (Esau, 1965, 1969; Fahn, 1990). See also: Lateral Meristems; Meristems In many conifers, resin ducts develop in both the primary and secondary bark. They produce resin with species or Figure 1 Cross-section of the stem of a small tree of Calotropis procera showing a microscopic view of the secondary xylem with the pores of the even genotype-specific chemical composition. In many water conducting vessels (red bottom part); live part of the bark with the angiosperms, laticifers or gum ducts develop and produce band of latex-forming ducts in the middle and the outer layers of cork with special resins and latexes. Wounding or pathogen attacks a typical lenticel in the centre (the green stained). result in the differentiation of additional (traumatic) resin ducts in conifers and gum ducts in many dicotyledons old phloem (Fahn, 1990). Although for each organ and (Fahn, 1979; Fink, 1999). See also: Conifers; Gymno- species there is a typical cell layer in which phellogen is sperms; Latex and Laticifers; Plant Gums initiated, there are many exceptions. Genetic, physiological At a certain stage of shoot development, a periderm and environmental factors largely influence the timing and (cork tissue) may appear. The periderm is a secondary location of phellogen initiation. In some plants, phellogen tissue formed from a secondary meristem – the phellogen, initiation occurs within a short time (only a few days) after also known as cork cambium. The function of the periderm an organ is formed, but in others it can be delayed for is to isolate and thus protect the live tissues from both biotic decades. In some plants, only a first periderm is formed, and and abiotic damaging factors (Fahn, 1990; Sandved et al., this periderm may continue its activity for the rest of the 1993). In certain trees, the periderm may reach a thickness plant’s life. The best-known case of such periderm is the of dozens of centimetres, but usually it is only several cork oak of the Iberian peninsula, from which most of millimetres to several centimetres thick. A well-known case the global commercial cork originates (Fahn, 1990). is the giant sequoia of the western USA, in which bark In many woody plants, there is a second stage of cork thickness of mature trees ranges from 25 to 80 cm. The formation – the rhytidome or subsequent periderms. In periderm is composed mostly of cork (phellem) cells, which plants that form subsequent periderms, live parenchyma die after differentiation, and their secondary cell walls cells of the secondary phloem redifferentiate and form an are rich with suberin. The periderm also includes a certain internal zone of phellogen. The activity of this phellogen amount of parenchyma (phelloderm) cells, and some- produces a layer of cork cells that isolate all tissues outward times, for instance in pines, layers of hard, heavily lignified to them from nutrients and so cause them to die. All tissues sclereids within the phellem zone (Fahn, 1990). As with (cork and other tissues) found outward from the innermost resin and gum ducts, wounding and pathogenesis induces subsequent periderm compose the rhytidome (Fahn, 1990). the formation of additional cork tissues (wound periderm) See also: Parenchyma (Borger, 1973). See also: Cork In several tropical tree species, especially trees of the In many plant species, there are many lenticels, which savannah, spines develop on the surface of the trunks (e.g., serve as gas exchange shafts through the almost imper- Hura crepitans). Usually these spines are made of a special meable cork. Lenticels are made of many loosely arranged type of cork that develops from islands of phellogen (Roth, cells that form a continuity of intercellular spaces with the 1981). inner tissues and appear as dots and stripes on the bark In trees, bark tissues usually comprise a much smaller surface (Fahn, 1990; Figure 1). fraction of the trunk volume than wood. Similarly, the Initials of the phellogen mostly divide outward to pro- effort to study bark development, anatomy and physiology duce phellem (cork) cells, but in many species a small is only a small fraction of that given to wood production. fraction of its cell divisions are inward, to form the par- Thus, we do not know much concerning bark biology. The enchymatic phelloderm. Derivatives of the phellogen are best model plant, Arabidopsis thaliana, has both primary usually arranged in radial files. Initiation of periderm starts and secondary bark, although it is a rather small annual at different distances from the shoot apex and from different and most of its secondary bark tissues are formed in the tissues.