Because the cell wall surrounding a plant cell prevents it from expanding, the wall’s structure must be loosened when the cell grows. The amount, type, and direction of plant-cell growth are regulated by small-molecule hormones called auxins. The auxin-induced weakening of the cell wall permits the expansion of the intracellular vacuole (see Figure 20-41a) by uptake of water, leading to elongation of the cell. We can grasp the magnitude of this phenomenon by considering that, if all cells in a redwood tree were reduced to the size of a typical liver cell, the tree would have a maximum height of only 1 meter, about a hundredfold less than normal.

The cell wall undergoes its greatest changes at the meristem in a root or shoot tip. Meristems are where cells divide and grow, as described in Chapter 21. Young meristematic cells are connected by thin primary cell walls, which can be loosened and stretched to allow subsequent cell elongation. After cell elongation ceases, the cell wall is generally thickened, either by the secretion of additional macromolecules into the primary wall or, more usually, by the formation of a secondary cell wall composed of several layers. In mature tissues such as the xylem—the tubes that conduct salts and water from the roots through the stems to the leaves—most of the cell eventually degenerates, leaving only the cell wall. The unique properties of wood and of plant fibers such as cotton are due to the molecular properties of the cell walls in the tissues of origin.