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. These variations influence the mechanical properties of the cell wall. The secondary cell wall has different ratios of constituents compared to the
primary wall. An example of this is that secondary wall in wood contains polysaccharides called
xylan, whereas the primary wall contains the polysaccharide
xyloglucan. The cells fraction in secondary walls is also higher.
Pectins may also be absent from the secondary wall, and unlike primary walls, no
structural proteins or
enzymes have been identified. Some secondary cell walls store nutrients, such as those in the
cotyledons and the
endosperm. These contain little cellulose, and mostly other
polysaccharides. == Evolution ==