Images of the horizontal growth of a tethered macrofiber, its contact with the floor, supercoiling and movement of the supercoils toward the tether. The black numbers in each frame represent minutes from the start of the film sequence (white numbers were added during recopying of the sequence and should be ignored). The 12 frames are side view images taken from the sequence shown in additional file 4. The fiber was tethered to the steel wire (76 μm diameter) shown at the left of each frame. A mirror image reflection from the floor of the chamber can be seen and used to determine where the fiber is in contact with the floor. Bar = 1 mm.