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Cotton denotes plants of the taxonomic genus Gossypium, grown especially for the long fibres associated with their seeds. The plant, an individual fibre and a textile made from many fibres are different levels of organization.^[ref]

A cotton fibre is a striking cell elongation example: one cell grows outward from the seed coat plant epidermis. A 2001 study described fibres reaching about $2.5$–$3.0\mathrm{cm}$ in approximately 16 days after flowering. These long cells provide a way to study growth without mistaking an increase in cell number for elongation of one cell.^[ref]

The experiment followed a fluorescent tracer from the seed coat into growing fibres with confocal microscopy. Access through intercellular connections involving a plasmodesma changed during development: tracer entry was seen early, blocked around day 10 and seen again around day 16. Transporter expression and turgor pressure measurements supplied complementary information about solute entry and growth.^[ref]

For an original calculation, a fibre that adds $30\mathrm{mm}$ over 16 days has an average extension rate of $1.875\mathrm{mm}$ per day. That quotient is not an instantaneous rate and does not imply uniform growth throughout development. Measuring fibre length at several ages distinguishes a mean rate from a changing growth trajectory.

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]