Cellulose is a linear polymer composed of glucose. The presence of a large number of hydrogen bonds between and within glucose molecule chains allows them to aggregate to form multi-layered, aggregated state structures such as microfibrils, fibrils and fibers. Six interconvertible polycrystalline cellulose types have been identified, including cellulose I, II, IIII, IIIII, IVI and IVII. The natural cellulose found in most plant cell walls is cellulose I, which has two isomorphs: the predominant cellulose Iα with a triclinic unit cell and a small amount of Iβ with a monoclinic unit cell.