|P.77 box 1A: "Cell walls provide structural support, are a physical barrier to pathogens, and house a range of signalling molecules to perceive and respond to developmental and environmental cues. Walls are complex, diverse and dynamic, changing throughout the processes of cell division, growth and differentiation. Walls begin to be formed in the cell plate during cell division which becomes the middle lamella between adjacent cells. As cells expand during growth the cells lay down a primary wall that can then be substantially thickened when cells cease to expand and undergo differentiation, this latter wall is termed the secondary wall which is often lignified. The types of polysaccharides present in the wall vary depending on the plant species, cell type and location, developmental stage and history of environmental stresses. Primary walls are composed predominantly of a complex array of polysaccharides (~90%) and some proteins (~10%)."