modifications we expect to be necessary are study of a three-dimensional model, allowing fluctuations in directions perpendicular to the array axis, and inclusion of hydrodynamic interactions between the particles. We defer these tasks to future work. We thank O. N. Mesquita, U. Agero and J. G. Moreira for helpful comments. This work was supported by CNPq and Fapemig, Brazil. Acknowledgments [1] B. Schmittmann and R. K. P. Zia, in Phase Transitions and Critical Phenomena , edited by C. Domb and J. L. Lebowitz (Academic Press, London, 1995), Vol. 17; Phys. Rep. 301 , 45 (1998). [2] J. Marro and R. Dickman, Nonequilibrium Phase Transition in Lattice Models (Cam- bridge University Press, Cambridge, 1999). [3] Katz, J. L. Lebowitz, and H. Spohn, Phys. Rev. B 28 , 1655 (1983). [4] K.-t. Leung, B. Schmittmann, and R. K. P. Zia, Phys. Rev. Lett. 62 , 1772 (1989). [5] J. T. MacDonald, J. H. Gibbs, and A. C. Pipkin, Biopolymers 6 , 1 (1968). [6] E. D.Andjel, M. Bramson, and T. M. Ligget, Prob. Theory Rel. Fields 78 , 231 (1988). [7] T. E. Harris, J. Appl. Probab. 2 , 323 (1965). [8] F. Sptizer, Adv. Math. 5 , 2 (1970). [9] J. T. MacDonald and J. H. Gibbs, Biopolymers 7 , 707 (1969). [10] K. Heckmann, in Passive Permeability of Cell Membranes , edited by F. Kreuzer and J. F. G . Slegers (Plenum, New York, 1972), Vol. 3, p. 127. [11] A. Schadschneider, Phys. A 285 , 101 (2001). 25