plants a structural membrane enclosing the cell membrane and the contents of the protoplast and consisting of cellulose (in bundles of microfibrils or micelles), hemicellulose, and lignin —note cell walls of woody plants are organized in four layers: (a) primary wall, which is the wall of the meristematic cell modified during differentiation and which is a more or less random network of microfibrils, and (b) a secondary wall, which contains lignin, provides the plant with rigidity and strength, and consists of three layers, a narrow S₁, a wide S₂, and a narrow S₃, with the microfibrils in these layers having distinct and differing orientation called microfibril (micellar) angle This definition last updated 10/25/2008