Fig. 2: Examples of the effect of gravity on biological systems.

a The “sine law” of gravitropism⁴⁵: the rate of bending, i.e. the rate of change ∂C/∂t of curvature, is proportional to the component of gravity perpendicular to the plant axis, i.e. to the sine of its inclination angle α. Bending results from the differential growth due to differential auxin concentrations⁴³Δ[auxin]. b Direct⁵⁶ and indirect⁶³ gravitaxis mechanisms illustrated for the biflagellated alga Chlamydomonas (upward arrows: buoyancy; downward arrows: gravitational forces): An inhomogeneous mass distribution (grey area) leads to torques due to “bottom-heaviness” (left); shape asymmetries, e.g. due to the flagellar apparatus, also lead to torques (middle); active regulation of the shapes and density distribution (right) affects gravitactic behaviour. c Biofilm structures of wild-type P. aeruginosa in normal gravity (“flat”) and microgravity (“column-and-canopy”) conditions⁷⁸.