Plant cell walls are complex and dynamic structures composed of a large degree of cross — linked polysaccharide networks, glycosylated proteins, and lignins. Primary cell walls are formed during early cell growth. The primary cell walls vary in thickness among different cell types and are usually not lignified. However, in older tissue, parenchyma cell walls can show various degrees of lignification. The secondary cell wall is deposited when cell growth has ceased and they are often highly lignified. The tracheary elements (tracheids, vessel elements) and fibers are cells that develop rather thick secondary cell walls. Primary cell walls often become lignified when secondary wall formation begins. The cell wall is generally comprised of long cellulose microfibrils interconnected by hemicellulose polysaccharides, such as xylan and xyloglucan. These amorphous hemicelluloses are generally hydrophilic, thus retaining water in the presence of the hydrophobic and highly crystalline cellulose chains and fibers. Additional polymers, such as pectins and lignins, fill in spaces in structures, such as secondary cell walls and the middle lamella.