  Plasmon modes of a two-dimensional lattice of long conducting circular wires
are investigated by using an embedding technique to solve Maxwell's equations
rigorously. The frequency-dependent density of states is calculated for various
values of the wave vector and the filling fraction. At low filling fractions,
collective modes are all found to accumulate at the surface-plasmon frequency
$\omega_p/\sqrt{2}$, $\omega_p$ being the bulk plasmon frequency. As the
filling fraction increases, the interference between the electromagnetic fields
generated by localized surface-plasmon polaritons leads to the presence of {\it
new} resonances, whose frequency strongly depends on the interparticle
separation. For touching wires, a number of multipole resonances fill the
spectral range between dipole resonances, as occurs in the case of a
three-dimensional packing of metal spheres.
