= Root elongation zone
The <root elongation zone> is the region behind the <root (botany)> tip where newly produced <cells> undergo substantial <cell elongation>. It lies behind the actively dividing <root apical meristem> and ahead of more mature regions. Movement of a marked surface point and increase in the length of an individual <cell> are related observations, but they are not identical measurements.<Biology 2e by Mary Ann Clark and others>[Section30.3: zones of division, elongation and maturation]{ref}
For a hypothetical measurement, a marked <cell> growing from $20$ to $30\,\mathrm{\mu m}$ increases its length by $10\,\mathrm{\mu m}$, or $50\%$. Merely observing that it moved $10\,\mathrm{\mu m}$ relative to the tip would not prove that it elongated by that amount: neighboring <cells> also change the geometry.
During <gravitropism>, unequal expansion on opposite sides of the <root elongation zone> produces curvature. Tissue-targeted tests in <Arabidopsis thaliana> identified a requirement for <auxin> transport and response in elongating <plant epidermis> <cells>. The resulting bend connects a localized cellular response to the direction of the whole <root (botany)>.<Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal by Ranjan Swarup and others>[Abstract; Figures4–6: kinematics and expanding epidermal cells]{ref}
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