In plants, lignin found within the cell wall, along with cellulose, provides the plant with its woody structure and durability. It is not used in paper-making but is a by-product of paper production. It is abundant in second and third-class papers (such as carbon copy papers, yellow papers like straw paper). Lignin decomposes over time, shortening the life of the paper. Many plant tissues contain carbohydrates and, in addition to foreign components, an amorphous and polymeric substance called lignin. In mature tree tissues, the amount of lignin varies between 18% and 38%. Lignin is also found in various proportions in grasses and forage plants. In living plants, the biological role of lignin is to create a cell wall that is extremely resistant and durable, along with cellulose and other carbohydrates. The chemical properties of lignin, carbohydrates, and proteins - which are the other two groups of natural polymers - are quite different from each other. For example, tree lignins are primarily aromatic in nature and are not easily or completely soluble in known solvents. Hydrolysis of monomer units is not possible, and they lack the uniform structure which is a key feature of other natural polymers.