 Root elongation zone

The root elongation zone is the region behind the root 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.^[ref]

For a hypothetical measurement, a marked cell growing from $20$ to $30\mu \mathrm{m}$ increases its length by $10\mu \mathrm{m}$, or $50\%$. Merely observing that it moved $10\mu \mathrm{m}$ 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.^[ref]