The cone of effective one-cycles of certain G-varieties
  Let X be a normal projective variety admitting an action of a semisimple group with a unique closed orbit. We construct finitely many rational curves in X, all having a common point, such that every effective one-cycle on X is rationally equivalent to a unique linear combination of these curves with non-negative rational coefficients. When X is nonsingular, these curves are projective lines, and they generate the integral Chow group of one-cycles. 
Michel Brion