We have studied light propagation in p-type waveguiding porous silicon layer. The planar waveguides were made by etching two layers with different porosities, the smaller one located on the top of the structure being the guiding layer. Light (a He-Ne laser beam lambda = 633 nm) is injected in the guide by a prism coupler. Analysis of the different TE and TM guided modes allows to determine with high precision the refractive index of the two porous layers, for both electric field polarisations. For as-formed structures, self-supported or not, we measured a relatively strong index anisotropy which diminishes when the structure is oxidised (anodically and thermally). The origin of the dielectric constant anisotropy is attributed to a morphological anisotropy. We present a model which describes the porous layer as preferentially oriented cylinders immersed in an isotropic medium composed of silicon spheres. The important result of this oversimplified model is that a relatively small proportion of silicon, less than 2% in columns, can determine the optical anisotropy for a 65% porosity sample. The same model also permits to explain the optical anisotropy decrease when oxidised.