Editing Cell wall (section)

===Rigidity of cell walls===
In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerable [[tensile strength]]. The apparent rigidity of primary plant tissues is enabled by cell walls, but is not due to the walls' stiffness. Hydraulic [[turgor pressure]] creates this rigidity, along with the wall structure. The flexibility of the cell walls is seen when plants wilt, so that the stems and leaves begin to droop, or in [[seaweed]]s that bend in [[Ocean current|water current]]s. As John Howland explains

{{Blockquote|Think of the cell wall as a wicker basket in which a balloon has been inflated so that it exerts pressure from the inside.  Such a basket is very rigid and resistant to mechanical damage.  Thus does the prokaryote cell (and eukaryotic cell that possesses a cell wall) gain strength from a flexible plasma membrane pressing against a rigid cell wall.<ref name="Howland 2000">{{cite book| last = Howland | first = John L. | name-list-style = vanc | year = 2000 | title = The Surprising Archaea: Discovering Another Domain of Life | pages = 69–71 | publisher = Oxford University Press | location = Oxford | isbn = 978-0-19-511183-5}}</ref>}}

The apparent rigidity of the cell wall thus results from inflation of the cell contained within. This [[turgor pressure|inflation]] is a result of the [[osmosis|passive uptake of water]].

In plants, a '''secondary cell wall''' is a thicker additional layer of cellulose which increases wall rigidity. Additional layers may be formed by [[lignin]] in [[xylem]] cell walls, or [[suberin]] in [[cork cambium|cork]] cell walls. These compounds are [[Structural rigidity|rigid]] and [[waterproof]], making the secondary wall stiff. Both [[wood]] and [[Bark (botany)|bark]] cells of [[tree]]s have secondary walls. Other parts of plants such as the [[petiole (botany)|leaf stalk]] may acquire similar reinforcement to resist the strain of physical forces.