Bark is every tissue outside the cambium
On woody plants, bark is not one skin but a stack of living and dead layers beyond the vascular cambium. The living inner bark moves sugars; the outer bark, or rhytidome, is mostly dead cork. By weight, bark can make up ten to twenty percent of a woody plant.
Botanists treat bark as a nontechnical label for everything outside the vascular cambium on stems and roots of trees, shrubs, and woody vines. Herbaceous plants and young stems lack it. Inside that outer zone sit living phloem tissues that conduct nutrients, while older stems add dead surface layers. The outermost corky zone on mature trunks is often called rhytidome. Cork cell walls hold suberin, a waxy barrier that limits water loss and blocks many insects, bacteria, and fungal spores from invading the stem.
As stems thicken, a cork cambium forms under the epidermis and produces impermeable cork. Cells stranded outside the new periderm—epidermis, cortex, older phloem—die from starvation of water and nutrients, yielding the familiar rough bark. Within the periderm, lenticels keep air exchanging with living cells. Only the cork cambium and vascular cambium keep dividing in a woody stem. That division builds secondary xylem inward as wood and secondary phloem outward into the bark layers.
Bark tissue is rich in biopolymers: tannins, lignin, suberin, and polysaccharides. Up to forty percent may be lignin, and condensed tannins appear to slow decay, so bark lignin breaks down less readily than wood lignin. Products drawn from bark include cork from Quercus suber—sometimes more than twenty centimeters thick—plus cinnamon, quinine from Cinchona, willow-derived aspirin precursors, and tannins from oak. Strip a complete ring of living phloem and the tree usually dies; that girdling cuts the sugar highway that feeds the plant.
Source: Bark (botany)
