Fig. 5. Sliding of singlet microtubules along the thinner (A) and the thicker (B) bundles. (A) <10^–9 M Ca²⁺. (B) 10^–5 M Ca²⁺. Similar sliding was observed both on the thinner and the thicker bundles regardless of the concentration of Ca²⁺. The top panels in each case were recorded under stronger illumination to observe the paired bundles before singlet microtubules were added. The following three panels in each case, which were recorded under decreased illumination to record clearer image of singlet microtubules, are sequential images of the movements of singlet microtubules along the bundles. Arrowheads indicate the starting positions of the left ends of the singlet microtubules that interacted with the bundles (at 0 seconds). Arrows indicate the direction of sliding of the singlet microtubules.