A simple diffusion model in which the plane is treated through a diffusion coefficient for Coulomb multiple scattering lower than that of a random medium and blocking is depicted as a specular reflection from the plane, has been developed to explain the occurrence either of feeding-in or blocking under different experimental conditions. Calculations were carried out for MeV protons in silicon crystals some microns thick, for angles of incidence on the {110} plane up to 1.5°. The results are in very good agreement with experiments of different authors. Transition between feeding-in and blocking of the kind observed along a star-pattern streak is also obtained. An elementary explanation of the results is given and some limitations of the model discussed. Monte Carlo simulations are used to obtain auxiliary informations on the processes occurring at the microscopic level.