Bark (botany)

• 1. Introduction

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  Bark makes up between 10 and 20 percent of the weight of a woody vascular plant, yet most of what you touch on a tree trunk is already dead. The rough, corky surface that wraps an oak or a pine is a graveyard of cells that no longer receive water or nutrients. Beneath it, hidden, a thin living layer keeps the whole plant alive. Bark is the outermost layer of the stems and roots of woody plants, and it is a nontechnical term covering every tissue outside the vascular cambium. Trees, woody vines, and shrubs all carry it. Herbaceous plants and the stems of young plants do not. So what exactly counts as bark, and what is merely sitting on top of it? Why does a dead outer shell matter so much that removing a ring of it can kill a tree? And how did this layer end up as the source of cork, cinnamon, quinine, aspirin, canoes, and rope for Viking longships?

• 2. Inner And Outer

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  The word bark hides a split. It overlays the wood and consists of two parts, the inner bark and the outer bark. The inner bark, in older stems, is living tissue, and it includes the innermost layer of the periderm. The outer bark is where the dead tissue gathers, covering the surface of the stem along with parts of the outermost periderm and everything lying outside it. This dead outer shell on trees, the part external to the living periderm, has its own name, the rhytidome. Inside a mature woody stem, the layers run in order from the outside in. After the bark and periderm come the cork, the cork cambium, the phelloderm, the cortex, and the phloem. Then comes the vascular cambium, and only then the wood itself, the xylem, divided into sapwood and heartwood, with the pith at the center. The cork cambium and the vascular cambium are the only parts of a woody stem where cell division happens. Undifferentiated cells in the vascular cambium divide rapidly, pushing secondary xylem inward and secondary phloem outward, a quiet engine driving everything the rest of this story depends on.

• 3. Becoming Bark

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  A young stem has no bark at all. From the outside in it carries an epidermis, which may be replaced by periderm, then cortex, then primary and secondary phloem, the vascular cambium, and the xylem. The epidermis is a layer of cells covering the plant body, including stems, leaves, flowers, and fruits, shielding the plant from the outside world. As the stem ages and grows, that arrangement transforms. In old stems the epidermal layer, the cortex, and the primary phloem become separated from the inner tissues by thicker formations of cork. Because the thickening cork layer cuts these cells off from water and nutrients, they die. That dead layer is the rough corky bark that forms around tree trunks. Cork itself, often confused with bark in everyday speech, is the outermost layer of a woody stem, derived from the cork cambium. It guards against parasites, herbivorous animals, and diseases, and against dehydration and fire. Its protective power comes from chemistry. Cork cell walls contain suberin, a waxy substance that protects the stem from water loss, blocks insects from invading, and prevents infections by bacteria and fungal spores.

• 4. The Periderm

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  A secondary covering called the periderm forms on small woody stems and on many non-woody plants. It is built from three tissues, the cork or phellem, the cork cambium or phellogen, and the phelloderm. The periderm arises from the phellogen, which acts as a lateral meristem, and it takes over the protective job once done by the epidermis. In woody plants the epidermis of newly grown stems is replaced by the periderm later in the same year. A layer of cells forms beneath the epidermis, the cork cambium, and these cells produce cork cells that turn into cork, while a limited number of layers may form interior to them as the phelloderm. The new cork is impermeable to gases and water, so the epidermis, cortex, and older secondary phloem outside the periderm die off. That seal creates a problem, because living cells in the cambium layers still need to exchange gases during metabolism. The answer is the lenticel. Lenticels form during the production of the first periderm layer, and their numerous intercellular spaces allow gaseous exchange with the outside atmosphere. As the bark develops, new lenticels appear within the cracks of the cork layers. The most familiar example of cork periderm is humble. The skin on a potato tuber, which is itself an underground stem, is the cork of the periderm.

• 5. Chemistry Of Defense

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  Up to 40 percent of bark tissue is made of lignin, a biopolymer that provides structural support by crosslinking between polysaccharides such as cellulose. Bark also holds tannins, suberin, and other polysaccharides, and that mix turns out to be remarkably resistant to rot. Condensed tannin sits in fairly high concentration in bark tissue and is thought to inhibit decomposition. Partly because of it, the degradation of lignin is far less pronounced in bark than in wood. Suberin plays its own defensive role; in the cork layer it has been proposed to act as a barrier to microbial degradation, protecting the plant's internal structure. The lignin itself is built differently in bark. Analysis during decay by the white-rot fungus Lentinula edodes, the Shiitake mushroom, using 13C NMR, showed that bark lignin contained more Guaiacyl units than Syringyl units compared with the plant's interior. Guaiacyl units resist degradation because they carry fewer aryl-aryl bonds, can form a condensed lignin structure, and have a lower redox potential. The concentration and type of lignin units may give plants protected by bark extra resistance to fungal decay.

• 6. Wounds And Healers

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  Frost crack and sun scald split bark from the environment, while woodpeckers and boring beetles attack it from the living world. During the rutting season, male deer and other male members of the Cervidae rub their antlers against trees to remove their velvet, stripping bark in the process. Bark is also damaged by being bound to stakes or wrapped with wires. That injury was once called bark-galling, treated by laying clay on the galled place and binding it with hay, though in modern usage galling usually means an abnormal growth caused by insects or pathogens. The stakes are high because bark is a physical barrier to disease, especially from fungi, so stripping it leaves the plant exposed. Damage to the phloem blocks the transport of photosynthetic products through the plant. When a band of phloem all the way around the stem is removed, the plant usually dies quickly, a method called girdling that people use in forestry and horticulture despite its ugly results. Recovery varies widely. Some species grow a callus that heals over the wound quickly but leaves a clear scar, while others such as oaks do not produce extensive callus repair, and sap is sometimes produced to seal the area. Bark is also a habitat. Insects, fungi, mosses, algae, and other vascular plants live in or on it, many as pathogens or parasites, some in symbiotic relationships.

• 7. What Bark Becomes

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  In Scandinavia, bark bread is made from rye with the toasted and ground innermost layer of bark from scots pine or birch added in. The inner bark, the phloem, is edible in some trees, harvested in hunter-gatherer societies and in times of famine. The Sami people of far northern Europe remove large sheets of Pinus sylvestris bark in the spring, prepare and store it, and eat the inner bark fresh, dried, or roasted. As a construction material, bark was used widely in pre-industrial societies, and birch bark in particular peels off in long, mechanically cohesive sheets. Those sheets built canoes, drainage layers in roofs, shoes, and backpacks, and in settler colonial Australia bark served as exterior wall cladding and roofing. In the cork oak, Quercus suber, the bark grows thick enough to harvest as cork without killing the tree, and more than 20 centimeters of thickness has been reported. Bark holds strong fibers called bast, and in northern Europe there is a long tradition of using bark from coppiced young branches of the small-leaved lime, Tilia cordata, to make cordage and rope, used in the rigging of Viking Age longships. The commercial list runs long. Cork, cinnamon, quinine from the bark of Cinchona, and aspirin from the bark of willow trees all come from this layer, while oak, Quercus robur, supplies the tannic acid used in tanning. Even the leftovers find use; bark chips from lumber production become bark mulch, and shredded bark grows epiphytes that cannot thrive in ordinary soil. Wood bark also contains lignin that, when pyrolyzed, yields a bio-oil rich in natural phenol derivatives, used to replace fossil-based phenols in the phenol-formaldehyde resins that bind oriented strand board and plywood.