A plant stays upright like a balloon inflated inside a wicker basket
Plant cell walls are actually flexible. What makes stems stiff is water pressure: each cell swells against its wall like a balloon blown up inside a wicker basket, as the biologist John Howland put it. When water runs short, the pressure drops and the plant wilts, its leaves and stems drooping.
A cell wall is a structural layer just outside the cell membrane that gives shape, support and protection, filters what gets in and helps cells resist bursting from osmotic pressure. Most bacteria and archaea have one, as do plants, algae and fungi, but animals do not. Plant walls must withstand internal pressures several times that of the atmosphere, and range from a tenth of a micrometre to several micrometres thick.
Recipes differ widely. Land plants build primary walls from cellulose, hemicelluloses and pectin, sometimes reinforced by lignin in wood or suberin in cork. Algae use substances like carrageenan and agar, bacteria use peptidoglycan, fungi use chitin, and diatoms encase themselves in glassy silica. Walls evolved independently many times; the cellulose-making enzyme, for example, originated in cyanobacteria and entered plants through the ancient partnership that produced chloroplasts. One idea holds that fungal walls may have evolved partly to deter viruses.
Robert Hooke described a plant wall in 1665 and called it simply a wall, but for almost three centuries the structure drew interest mainly as industrial raw material or for its effects on health. Karl Rudolphi and J. H. F. Link showed in 1804 that every cell possesses a separate wall, overturning the belief that neighbours shared walls through which fluid flowed. How walls grow sparked a 19th-century dispute: Hugo von Mohl argued for new layers being added on top, while Carl Nageli argued material was inserted within the existing wall.
Terminology kept shifting. In 1930 Ernst Munch coined apoplast for the non-living region of plants that includes the walls, as opposed to the living symplast. By the 1980s some authors proposed replacing cell wall with extracellular matrix, the term used for animal cells, but the older phrase has persisted.
Source: Cell wall
