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Gravitropism is directional growth of a plant organ in response to gravity. A downward-growing root shows a positive response; an upward-growing shoot shows a negative one. Reorienting a growing plant can therefore produce curvature as growth becomes unequal on opposite sides. The response concerns the direction of growth, rather than passive bending under the organ's weight.

In Arabidopsis thaliana, auxin links perception near the root tip to growth farther back. Ranjan Swarup and collaborators used tissue-targeted expression of the influx carrier AUX1 to map where transport was needed. Their 2005 experiments implicated the lateral root cap and elongating plant epidermis. Separately, blocking auxin responsiveness in expanding plant epidermis cells abolished the gravity response. The comparison distinguishes delivery of a signal from the capacity of receiving cells to respond to it.^[ref]

A horizontal root provides a useful observation: record the tip position over time and inspect curvature through the root elongation zone, rather than expecting the entire existing root to rotate as a rigid rod. Holding the light direction fixed helps distinguish a gravity response from phototropism. Transport, cellular growth and final tip direction are related measurements, but they are not interchangeable.

The lateral root cap is the peripheral part of the root cap, lying around its central columella and adjacent to outer root tissues. It is a tip-associated tissue, not a lateral root, which is an entire branch growing from another root.

Its position makes a transport route possible. In the 2005 Arabidopsis thaliana study, tissue-targeted expression of AUX1 showed that gravitropism required auxin transport through the lateral root cap toward elongating plant epidermis cells. Disrupting auxin response in the receiving expanding epidermal population separately impaired the bend.^[ref]

Signal detection, transport and response need not occur in the same cell. A useful diagram therefore traces a route from the gravity-sensitive cap region through the lateral root cap to the root elongation zone. An experiment removing cells from one region and an experiment blocking a specific transporter there ask different questions; their outcomes should not be collapsed into one claim that the region has only one function.

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]