Fibers are strands of cells that are characterized by an elongate shape and a thickened secondary cell wall composed of cellulose and hemicellulose. Dead at maturity, fiber cells possess tapered, overlapping ends that form long, multicellular fibers. These fibers impart elastic strength to stems, leaves, roots, fruits, and seeds of flowering plants. Most fiber cells arise from vascular tissues and are commonly found in association with phloem tissue, although fibers may also be found in xylem or independent of vascular tissue. Fiber cells typically incorporate lignin in their secondary wall, a substance that creates additional stiffness in fiber cells.

In commerce, plant fibers are broadly defined to include materials that can be spun or twined to make fabrics and cordage, used directly as filling materials, or included in paper production. Plant fibers of commerce are