Final answer:
Plant cells develop a secondary cell wall inside the primary cell wall after growth ceases, containing cellulose and lignin, which adds rigidity and strength to the wall due to lignin's complex, phenolic structure.
Step-by-step explanation:
Plant Cell Walls and Lignin
Plant cells are encased by a cell wall that provides structural support and protection. Each plant cell initially forms a primary cell wall, which is composed of cellulose microfibrils embedded in a matrix of hemicellulose and pectins. During growth, this wall is flexible to accommodate cell expansion. After cells cease to grow, they often develop a secondary cell wall, found inside the primary wall, that includes cellulose and a polymer known as lignin. Lignin is a complex, phenolic polymer that confers rigidity and strength, making the secondary wall much more rigid than the primary one.
Lignin is crucial for plant structure as it allows plants to stand tall and resist the compressive forces of gravity, contributing significantly to the evolutionary success of vascular plants. It forms a three-dimensional network around the cellulose fibers, enhancing the compressive strength of the cell wall. Another aspect of lignin is its resistance to decomposition, which helps in the longevity of plant tissues after they have ceased their metabolic functions. The presence of lignin is a defining feature of woodiness in plant tissues and is largely responsible for the stiffness and durability of wood.