= Cell elongation
In plant development, \b[cell elongation] is an increase in a cell's length, often with growth biased more strongly along one axis than another. It differs from <cell division>, which increases cell number. Tissue extension can therefore reflect changing cell dimensions as well as production of additional cells.
For an illustrative fixed population of ten cells arranged end-to-end, increasing each length from $20$ to $25\,\mathrm{\mu m}$ increases the combined length from $200$ to $250\,\mathrm{\mu m}$ without adding any cells. Each cell has elongated by 25%. This count assumes no intervening spaces or rearrangements and is not a measured growth result.
Plant cells enlarge against a <cell wall>. <Turgor pressure> supplies mechanical loading, while controlled wall relaxation allows irreversible extension rather than only reversible elastic stretching. A landmark reconstitution experiment extracted wall-associated proteins from growing cucumber seedlings and added the extract to isolated walls. Fractionation identified two activities associated with proteins of approximately 29 and 30 kilodaltons; each induced extension without detectable hydrolytic wall breakdown.<Two endogenous proteins that induce cell wall extension in plants by S. McQueen-Mason and others>[1992 abstract: wall-extract reconstitution, fractionation and extension assays.]{ref}
These proteins, associated with the <expansin> family, helped establish that wall loosening need not simply mean wholesale enzymatic digestion of the wall. The assay also separates a physical property from cell multiplication: isolated walls can extend in a preparation that is not dividing. The full growth of a living cell still involves water uptake, wall material and regulation; the reconstitution identifies one experimentally accessible part of that process.
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