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Why do many mature plant cells add a secondary cell wall?

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Cutaway illustration of a plant cell showing labeled internal structures including the nucleus, nucleolus, chloroplast, mitochondrion, vacuole, endoplasmic reticulum, ribosomes, Golgi apparatus, cell wall, and other key organelles. — Cell wall Cutaway drawing of a plant cell, showing the cell wall and internal organelles.

Mature plant cells add a secondary cell wall primarily to provide permanent mechanical support and rigidity once the cell has ceased growing.12

While the thin primary cell wall is flexible enough to allow for cell expansion during growth, it relies on turgor pressure—the internal water pressure within the cell—to maintain structural integrity. In contrast, the secondary wall provides a stable, load-bearing structure that does not depend on water pressure to keep the plant upright.12

Furthermore, secondary cell walls are often reinforced with lignin, a complex polymer that significantly increases the wall's strength and resistance to microbial attacks, such as those from fungi and bacteria. These rigid, thickened walls are the essential components that provide the mechanical properties characteristic of wood.312