Trunks, also called boles, are the stems of woody plants and the main structural element of trees. The woody part of the trunk consists of dead but structurally significant heartwood and living sapwood, which is used for nutrient storage and transport. Separating the wood from the bark is the cambium, from which trunks grow in diameter. Bark is divided between the living inner bark (the phloem), which transports sugars, and the outer bark, which is a dead protective layer.

The precise cellular makeup of these components differs between non-flowering plants (gymnosperms) and flowering plants (angiosperms). A variety of specialised cells facilitate the storage of carbohydrates, water, minerals, and transport of water, minerals, and hormones around the plant. Growth is achieved by division of these cells. Vertical growth is generated from the apical meristems (stem tips), and horizontal (radial) growth, from the cambium. Growth is controlled by hormones, which send chemical signals for how and when to grow.

Plants have evolved to both manage and prevent damage from occurring to trunks. Trunks are structured to resist wind forces, through characteristics such as high strength and stiffness, as well as oscillation damping, which involves taking energy, and therefore damage (by extension), out of the trunk and into the branches and leaves. If damaged, trunks employ a complex and slow defence mechanism, which starts by creating a barrier to the incoming disease. Eventually, diseased cells are replaced by new, healthy cells, once the threat is contained.

Trunks not only support the extensive ecological function of living trees, but also play a large ecological role when the trees eventually die. Dead trunk matter, termed coarse woody debris, serves many roles including: plant and animal habitat, nutrient cycling, and the transport and control of soil and sediment. Most trees grown outside the tropics can be dated (have their age estimated) by counting their annual rings. Variations in these rings can provide insights into climate, a field of study called dendroclimatology. Trunks have been in continuous use by humans for thousands of years including in construction, medicine, and a myriad of wood-related products. Culturally, trunks are the subject of symbolism, folk belief, ritual, and feature in art of many mediums.

All vascular plants (those that have xylem and phloem tissues) have both roots and stems. But only gymnosperms, and angiosperms that are both woody and sprout two initial leaves (dicots), have trunks. The rest of the angiosperms can be categorised as either herbaceous plants with one initial leaf (monocots), like bamboo, or herbaceous plants with two initial leaves (dicots), like flax. Neither grow trunks.

Trunks, the stems of woody plants, connect the roots to the upper branches, canopy, and leaves. In general, the trunk of woody plants, which is their most easily identifiable feature, consists of: heartwood, sapwood, cambium, inner bark, outer bark, and the pith. In this way, the xylem, or wood, of the tree is separated from the bark by the cambium, which functions as a lateral meristem. The cambium promotes growth radially. The younger part of the xylem (the sapwood) conducts water up from the roots to the leaves. It also acts as storage for food, through the parenchyma, which is made up of ray cells. While only 10% of the sapwood cells are alive, the heartwood, the darker part of the xylem, is completely dead. It proves structural value to the plant. The pith is the most minor feature of the trunk, being a remnant from when the stem was not yet woody. The purpose of producing a trunk is to enable a taller plant, with greater stability.

Earlywood and latewood describe the difference in density between wood grown early (low density) and later (high density) in the growing season. Tree rings, seen when the trunk is viewed in cross-section, are the result of the difference in cambial growth rates during the year. The difference in thickness of the cells of earlywood and latewood is generally responsible for the presence of growth rings. They are most pronounced in conifers and are mostly not annual in equatorial regions. In angiosperms, annual rings are also influenced by the proportion of different cells present in the different regions. This varies between genera, however. The outer annual ring or rings are generally responsible for most of the water transport in trees, to differing degrees.

Up to 90% of the xylem of gymnosperms is made up of vertically oriented tracheids, a type of conductive cell, which often overlap one another. To facilitate liquid transfer, the cell walls of tracheids contain pits, and are around 100 times as long as they are wide. They also provide structural strength through their thick cell wall. In the horizontal (or radial) direction, the most significant component in gymnosperms are wood rays, formed by the cambium. They consist of groups of cells which both store carbohydrates and minerals, but also move water, minerals, and other compounds in the horizontal direction. Ray tracheids and parenchyma, in different combinations, make up the structure of wood rays. Parenchyma chiefly function as nutrient storage, but can also assist in liquid transport to a limited degree. They also supply mechanical strength to the tree.