= Columella
{disambiguate=root cap}
The <columella (root cap)> is the central, column-like region of the <root cap>. In <Arabidopsis thaliana>, its cells are organized into recognizable files and tiers, distinct from the surrounding <lateral root cap>. Starch-containing <amyloplasts> in these cells can change position after the <root (botany)> is reoriented relative to <gravity>.<Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity by Elison B. Blancaflor and others>[Introduction; Figure1 and amyloplast-sedimentation observations]{ref}
A 1998 study paired two kinds of evidence. Live imaging tracked <amyloplast> displacement, while <laser ablation> removed selected cap cells before measuring <gravitropism>. Inner central <columella (root cap)> cells made a particularly strong contribution to the response and showed rapid amyloplast sedimentation. This agreement linked cellular geometry and a candidate sensing mechanism to whole-root behavior.<Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity by Elison B. Blancaflor and others>[Abstract; Figures3 and6; TableIII]{ref}
A settling particle alone is not a complete proof of sensing: movement might accompany some other process. Selective removal asks whether the relevant cells are needed, while simultaneous growth measurements help exclude a simple failure of root extension. The <columella (root cap)> supplies sensory-associated tissue; much of the resulting bend occurs farther back in the <root elongation zone>.
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