In plants, the cell wall forms as a young plant secretes polysaccharides onto the outer surface of its plasma membrane. being thin and pliable, this primary wall allows the cell to enlarge and change shape. in mature woody plants, cells in some tissues deposit material onto the primary wall's inner surface. why doesn't this secondary wall form on the outer surface of the primary wall?

The secondary cell wall is a structure found in many plant cells, located between the primary cell wall and the plasma membrane. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding.

Secondary cell walls provide additional protection to cells and rigidity and strength to the larger plant. These walls are constructed of layered sheaths of cellulose microfibrils, wherein the fibers are in parallel within each layer. The inclusion of lignin makes the secondary cell wall less flexible and less permeable to water than the primary cell wall. In addition to making the walls more resistant to degradation, the hydrophobic nature of lignin within these tissues is essential for containing water within the vascular tissues that carry it throughout the plant.

The secondary cell wall consists primarily of cellulose, along with other polysaccharides, lignin, and glycoprotein. It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction of the cellulose microfibrils differs between the layers.