The right answer is cell wall.
The wall surrounds all the plant cells. It gives an angular aspect to the plant (pentameric, hexameric).
In terrestrial plants, the strongest component of the complex cell wall is a large carbohydrate called cellulose, while those of algae are usually composed of glycoproteins and polysaccharides. However, some species of algae may have a cell wall formed of silicic acid and the seaweed walls are characterized by sulphated polysaccharides which could play a role in the adaptation of the plant to the marine environment.
