Figure 5.

(a) Schematic of a cell anchored to a surface with a mucous stalk, of length ℓ and orientation $\widehat{\mathbf{\text{r}}}$. The cell has orientation $\widehat{\mathbf{\text{d}}}$. (b) Phase portrait showing the dynamics of a cell (with the aspect ratio of Uronemella and ℓ=10a) moving under the influence of the flow generated by its image cell. The dynamics admit a single stable fixed point (green point) and two unstable fixed points (red points). The basin of attraction (red shading) is roughly described by the strip −π/2<θ<π/2, implying cells initially oriented away from the surface will right themselves. If ϕ≈±π/2, the stalk lies flat on the surface. As the height of the cell above the surface approaches ℓ_c, the cell becomes unstable; this leads to the curved corners of the basin of attraction. As T. majus is more spherical than Uronemella, ℓ_c is correspondingly smaller, leading to a basin of attraction with slightly sharper corners but otherwise identical.