A tree trunk is living plumbing wrapped in dead armour
Trunks, or boles, are the main stems of woody plants and the structural core of trees. Dead heartwood and living sapwood form the wood; cambium adds girth between wood and bark; living inner bark (phloem) ships sugars while outer bark is sacrificial armour.
Gymnosperms and woody dicot angiosperms build true trunks; other vascular plants have stems and roots without that architecture. Apical meristems lengthen the axis; cambium thickens it under hormonal control, with specialised cells storing water, carbs, minerals, and hormones while moving them around the plant.
Mechanical design resists wind through stiffness and by dumping oscillatory energy into branches and leaves. When wounded, trunks wall off infection slowly and later replace diseased cells. After death, coarse woody debris still cycles nutrients, shelters organisms, and shapes soil and sediment.
Inside the wood, the two big seed-plant lineages use different plumbing. In gymnosperms up to 90 percent of the xylem is vertical tracheids, pitted conducting cells about a hundred times longer than wide that also lend strength, crossed radially by rays laid down by the cambium. Angiosperms rely instead on stacked vessel elements for most water transport, backed by thick-walled fibres, and those vessels are what tell hardwoods from softwoods. The cambial zone itself is only 1 to 10 cells thick, and auxin, the hormone that also keeps side buds dormant, dominates its divisions. A leaning tree lays down dense reaction wood, called tension wood in angiosperms and compression wood in gymnosperms.
Outside the tropics, annual rings date ages and feed dendroclimatology. Humans have long used trunks for building, medicine, and craft, and cultures load them with ritual and art. The bole is therefore both column and archive—structure in life, habitat and history in death.
Source: Trunk (botany)
