FIGURE 4.

Two modes of spreading: anisotropic and isotropic. Comparing the three-dimensional time plots of the spreading cell edge of a typical anisotropic spreading cell (A) versus a typical isotropic spreading cell (B) reveals spreading by STEPs throughout anisotropic spreading, although STEPs appear in the isotropic cell only after 2 min. Above-threshold edge velocities of the anisotropic (C, top) and isotropic cells (D, top) clearly display these two modes. Both cells reach the same total area (C and D, bottom, black curves) and the majority (>75%) of the spreading is accounted for by the above-threshold regions (C and D, bottom, red curves).