= Root-cap columella cell
A <root-cap columella cell> is a <cell> in the central <columella (root cap)> of a <plant>'s <root cap>. In <Arabidopsis thaliana>, these <cells> form recognizable tiers and files and contain <starch>-bearing <amyloplasts>. The organelles can sediment after the <root (botany)> changes orientation relative to <gravity>, making these <cells> useful for investigating <gravitropism>.<Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity by Elison B. Blancaflor and others>[Figure1,Figure6 and TableIII: position and amyloplast sedimentation]{ref}
An especially informative experiment removes selected <cells>, rather than the entire growing tip. In 1998, investigators used <laser ablation> and then measured curvature after reorientation. Inner central <columella (root cap)> <cells> contributed more strongly to the response than the other tested regions. Measuring extension alongside bending helped distinguish damage to gravity-associated signaling from a general failure of growth.<Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity by Elison B. Blancaflor and others>[Abstract; Figures2–3 and growth controls]{ref}
The distinction between sensing and response is important: a cap <cell> associated with gravity perception need not be the <cell> whose elongation produces the visible bend. Much of that differential growth occurs farther back in the <root elongation zone>. Likewise, observing an <amyloplast> settle supplies a candidate physical mechanism, while a targeted perturbation tests the functional importance of its cellular setting.
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