The hard structural and water-conducting tissue that is found in many perennial plants and forms the bulk of trees and shrubs. It is composed of tough-walled secondary xylem and associated cells, such as fibres. The wood of angiosperms is termed hardwood, e.g. oak and mahogany, and that of gymnosperms softwood, e.g. pine and fir. New wood is added to the outside of the old wood each growing season by divisions of the vascular cambium (see growth ring). Only the outermost new wood (sapwood) functions in water conduction; the inner wood (heartwood) provides only structural support.