  In this paper, we will consider generalised eigenfunctions of the Laplacian
on some surfaces of infinite area. We will be interested in lower bounds on the
number of nodal domains of such eigenfunctions which are included in a given
bounded set.
  We will first of all consider finite sums of plane waves, and give a
criterion on the amplitudes and directions of propagation of these plane waves
which guarantees an optimal lower bound, of the same order as Courant's upper
bound.
  As an application, we will obtain optimal lower bounds for the number of
nodal domains of distorted plane waves on some families of surfaces of
non-positive curvature.
