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An amyloplast is a nonphotosynthetic plastid specialized for synthesis and accumulation of starch granules. The familiar starch-rich interior of a potato illustrates storage; an amyloplast is an entire membrane-bounded organelle, not merely one starch granule. In certain gravity-sensing plant cells, starch-filled amyloplasts also provide sedimenting material that helps couple orientation to gravitropism.

A revealing test removed starch rather than removing the whole cell. T. Caspar and B. G. Pickard studied an Arabidopsis thaliana mutant lacking plastid phosphoglucomutase activity. Light and electron microscopy confirmed starch deficiency. In normal seedlings, starch-filled plastids moved toward the new lower side after inversion; the mutant's starchless plastids did not show significant sedimentation. Yet the mutant roots still curved in response to gravity. After a five-minute gravity stimulus followed by rotation on a clinostat, the reported root curvature was approximately 10 degrees in the mutant versus 14 degrees in the normal seedlings.^[ref]

The ratio $10/14\approx 0.71$ makes the distinction concrete: the response was reduced under those conditions, but did not disappear. The experiment therefore limits a claim that sedimenting starch-filled amyloplasts are absolutely required for every root gravity response. It does not make starch storage or sedimentation irrelevant; it separates contribution to a response from strict necessity.

B. G. Pickard coauthored a 1989 test of gravity sensing in Arabidopsis thaliana using a starchless mutant. With T. Caspar, B. G. Pickard combined microscopic observations of plastids with measurements of gravitropism in roots and other tissues.^[ref]

Normal starch-containing plastids settled toward the lower side of the tested cells after inversion. The mutant plastids did not behave similarly, yet mutant roots retained a substantial bending response. Comparing short gravity exposures followed by clinostat rotation connected an initial stimulus to later curvature.^[ref]

The work illustrates a useful two-part test of a proposed sensor: establish that the candidate physical process was disrupted, then ask whether the biological response remains. Observing only the mutant's shape would not demonstrate that plastid sedimentation had been altered.

T. Caspar coauthored, with B. G. Pickard, a 1989 experiment testing whether starch-filled plastids are indispensable for gravitropism in Arabidopsis thaliana. The comparison used a mutant unable to synthesize starch in these structures and verified the absence of starch by microscopy.^[ref]

The plastids sedimented in normal seedlings when their orientation changed, whereas the mutant's starchless plastids did not show significant sedimentation. Nevertheless, mutant roots still bent in response to gravity. After one five-minute induction followed by rotation on a clinostat, the reported curvature was about $10^{\circ }$ in mutants and $14^{\circ }$ in controls.^[ref]

The residual response distinguishes a contribution from an absolute requirement. Reduced bending can support a role for starch in sensitivity while continued bending shows that the tested response is not eliminated by its absence.