Bark is the outermost layer of stems and roots of woody plants, such as trees and shrubs). It refers to all the tissues outside the vascular cambium and is a multifunctional structure that carries out several crucial functions for plants. The tissues included in bark depend on how broadly the term is defined and the age of the plant. Broadly defined, bark refers to all those tissues outside the vascular cambium, or all tissues from the living phloem outward. However, in popular use, the term bark is often used in reference to only the cork or only to the periderm (cork, cork cambium, and phellederm) in plants with secondary growth.

The layers of a mature woody stem from the outside to the inside include the following):

• Bark
• Periderm
• Cork (phellem or suber), includes the rhytidome
• Cork cambium (phellogen)
• Phelloderm
• Cortex
• Phloem
• Periderm
• Vascular cambium
• Wood (xylem)

Bark tissues make up by weight between 10 and 20% of woody vascular plants and consist of various biopolymers, tannins, lignin, suberin, and polysaccharides). Bark carries out several crucial functions for plants, including translocation of photosynthates, storage of starch, soluble sugars, water, and other compounds, protection from herbivores, pathogens, and high temperatures, wound closure, as well as mechanical support, photosynthesis, and likely being involved in xylem embolism repair.

Bark also acts as a reservoir for rainwater and absorbs water like a sponge during storms. The amount of water that can be absorbed by bark depends on the tree species and its physical structure. The outer bark is the trees protection from the outside world, while the inner bark, or “phloem,” is the pipeline through which food is passed to the rest of the tree.

In addition to its functional roles, bark has been used for various purposes, such as making cloth, canoes, and ropes, and used as a surface for paintings and map making). The bark of some trees is edible, and diverse teas are produced from bark.