= T. Caspar
{c}
<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.<Gravitropism in a starchless mutant of Arabidopsis: implications for the starch-statolith theory of gravity sensing by T. Caspar and B. G. Pickard>[Abstract: starchless mutant and microscopic controls]{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.<Gravitropism in a starchless mutant of Arabidopsis: implications for the starch-statolith theory of gravity sensing by T. Caspar and B. G. Pickard>[Abstract: inversion and five-minute induction comparisons]{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.
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