Riemann surfaces, arc systems and Weil-Petersson form

Cellular decompositions of the moduli space of pointed Riemann surfaces via ribbon graphs used either the hyperbolic spine or Strebel's quadratic differentials. We show that these two approaches are suitable degenerations of the spine construction for hyperbolic surfaces with geodesic boundary, the bridge being given by infinite grafting. We also discuss a compactification of the moduli space in terms of transverse arc lengths, a formula for the Weil-Petersson Poisson structure in terms of lengths of geodesic arcs associated to the hyperbolic spine and the degeneration of this formula in the long boundary limit.