= Root cap
The <root cap> is the covering of <cells> at the growing tip of a <root (botany)>, in front of its <root apical meristem>. Its central <columella (root cap)> and surrounding <lateral root cap> have distinct positions and functions. The <root cap> participates in the response to <gravity>, while the bending growth response is expressed farther back in the <root elongation zone>.
A 1998 experiment mapped function more precisely than removing the entire tip. <Elison B. Blancaflor> and collaborators used <laser ablation> to eliminate selected cap <cells>, allowed recovery, then rotated <Arabidopsis thaliana> seedlings through90degrees. Removing inner central <columella (root cap)> <cells> strongly reduced curvature without reducing axial growth in the same way. Damage to other tested cap regions had smaller effects.<Mapping the Functional Roles of Cap Cells in the Response of Arabidopsis Primary Roots to Gravity by Elison B. Blancaflor and others>[Abstract; Figures1–3 and growth controls]{ref}
This comparison matters because failure to bend could otherwise mean that the <root (botany)> simply stopped growing. Measuring both extension and curvature helps isolate a sensory or signaling defect from a general failure of growth. The observations support functional specialization within the cap, rather than treating every cap <cell> as an interchangeable gravity sensor.
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