Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs. Wood has been used for thousands of years for fuel, as a construction material, for making tools and weapons, furniture and paper. More recently it emerged as a feedstock for the production of purified cellulose and its derivatives, such as cellophane and cellulose acetate.