Types of Bark and Functions

Most naturalists are aware that the type of bark can be used as a diagnostic feature to identify species. In temperate regions, where trees are winter deciduous, students learn to identify species based on bark (and winter bud) characteristics, and bark types are useful taxonomic characters in other habitats as well.

Often more than one term can be used to describe the bark of a tree or shrub. The young stems often have a different appearance than larger branches, and branches do not resemble the bark found at the base of a trunk. The following are some of the terms used to describe bark.

• Ring bark. Where a stem has concentric successive periderms (entirely cylindrical), a tight or smooth bark can develop.
• Scaly bark. Where a stem has discontinuous, overlapping successive periderms, patches of bark will form and can be shed. This is generally termed a scaly bark.
• Fissured bark. Fissured bark has a net-like pattern from splitting of the outer and inner bark. Many trees with fissured bark have well-developed secondary phloem fibers in the inner bark (bast fibers). Bast fibers, which formerly were widely used for making cordage and paper, give the trunk a very strong cover. Fibers would, on the one hand, give added strength for bending and limit stress on sieve tubes while also protecting the sugar-conducting sieve tubes of phloem, blocking sucking or burrowing insects. Fibrous barks are not necessarily fissured, as in many species of figs (Ficus).
• Smooth bark. Smooth barks can be smooth and very glossy to dull. Generally these have a very thin outer bark.
• Peeling bark (exfoliating). Especially for ring bark species, occurrence of thick-walled cells alternating with thin-walled cells causes barks to separate like thin wrappers, peeling or exfoliating from where a layer breaks (Bursera, examples one type and another view and another view; Prunus, Betula, certain Jatropha). Peeling barks of eucalyptus species (examples: E. maculata, E. citriodora, E. deglupta, versus a nonshedding ironbark) occur because layers of bark separate where there is thin-walled phloem parenchyma. Shagbark hickory (Carya ovata) has peeling strips of fibrous bark. Species of Melaleuca (Examples: one species and another view and M. styphelioides) and Callistemon (family Myrtaceae) often have exfoliating layers like paper. Some species of legumes also have peeling bark.
• Cracked bark. As a tree increases in girth, great tension on the bark can cause vertical and horizontal cracking. Cracks may develop along planes of stress or more simply outline boundaries of scaly bark.
• Furrowed cork. The cork oak (Quercus suber) and a number of other trees (Cussonia spicata, Erythrina latissima, and Phellodendron) have deeply furrowed bark with thick accumulation of cork cells. These cork cells tend to have thin-walls and are filled with air. Corky outer bark may also appear in longitudinal arrangements or wings (Casuarina, Aristolochia, and Zanthoxylum).
• Green stems. Species with photosynthetic stems almost always are able to capture sufficient sunlight when development of the opaque initial periderm is delayed or totally suppressed. A stem can contribute significantly to total carbon uptake of the plant only if stomates are present at a substantial density, and can remain functional over several years.
  • Stem succulents, such as cacti and euphorbs, have a long delay in the initiation of periderm. These stems do not tear or split because the epidermis undergoes extra cell divisions during its formation for increasing the surface area. Instead, periderm formation occurs primarily in response to stress and injury, e.g., sun damage along rib margins. When periderm forms, it typically arises from the outermost layer of cortex and requires a long time to encircle the axis. In many arborescent cacti, for example, only the trunk is fully covered by scaly bark.
  • Photosynthetic old stems, such as palo verdes (Cercidium and Parkinsonia aculeata), over many years experience cell divisions of the epidermis, cortex, and phloem, thereby increasing stem surface area while stem circumference increases without disturbing the operations of the stomates. Even a fully mature palo verde may have very little bark formation on the trunk. Many shrubs of desert and semiarid habitats have photosynthetic stems that remain green for several years before developing the initial periderm.
  • Many families with both herbs and shrubs have woody species with thick stems that are partially green for more than a year. Especially herbaceous plant groups tend to show a long delay in full acquisition of bark (example: Tithonia diversifolia).
  • Certain bottle trees form green trunks that very slowly develop stripes or patches of periderm (Ceiba and other Bombacaceae, examples Pseudobombax and another view and Fouquieria columnaris, Brachychiton rupestris). There may, in fact, be no carbon uptake by these green stems, because they lack functional stomates, but green tissues can obtain energy via the light reaction and recycle internal carbon dioxide, which was generated by cell respiration of the bark cells. Even a fully mature tree may still have patches of green on the trunk.
  • Some species possess smooth, tight or peeling, thin, nearly transparent outer bark hiding a deeper layer of green, photosynthetic tissue. These species do not appear to have photosynthetic stems, because the surface bark is tan to red (examples: Bursera, Pachycormus, Jatropha spp.).
• Corky wings. One diagnostic character is the presence of corky wings on young stems. In some cases, wings are due to stimulation of localized phellogens along a stem angle, as in winged euonymus (Euonymus alatus). Wings also occur on young stems of sweet gum (Liquidambar). Longitudinal splitting is the cause of stem wings in certain species of elm (Ulmus).
• Monocotyledonous periderm. Certain arborescent species, e.g., Cordyline terminalis, Beaucarnea stricta, Dracaena draco, and Yucca, segments of suberized cells appear but without having a special cambium. This is termed storied cork. Aloe bainesii and other arborescent aloes have a scaly to exfoliating type of surface layers. In palms, the outer cells of the trunks become heavily lignified, and a few palm species (Livistona) also have suberin deposited in surface cell walls. No monocotyledon has a true periderm as found in dicotyledons and gymnosperms.
• Bark armature. Bark on old stems and trunks may have formidable armature in the form of heavy, persistent bark prickles (also called bark emergences). Examples are found in Bombacaceae (Chorisia speciosa; Ceiba aesculifolia and another view), Zanthoxylum (one example and another view), and certain Sapotaceae. Stipular spines may also persist on old stems.

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