Energy-selective photoluminescence spectroscopy of porous silicon has been used to study the morphology of porous silicon layers prepared from (110) silicon wafers. We demonstrate that the electric field vector of the red emission band is aligned along the principal crystallographic axes of bulk silicon. From these observations, we conclude that large and small silicon nanocrystals assembling the layers have different preferential crystallographic orientations. Oxidized porous silicon layers exhibit a blue emission band which is strongly polarized along the < 100 > crystallographic axes. This indicates that chain-like silicon clusters, preferentially aligned along the same crystallographic directions, are responsible for the blue emission.