Secondary cell wall
The secondary cell wall consists mainly of cellulose, but also other polysaccharides, lignin and proteins. In some cells, such as found in xylem, the secondary cell wall contains lignin, which strengthens and waterproofs the wall. Other plant cells have secondary cell wall with additional function, such as food storage. For example, cotyledons and endosperm have secondary cell walls that contain little cellulose but more on polysaccharides.