  We discuss the discrete spectrum induced by bulges on threadlike mesoscopic
objects, using two models, a continuous hard-wall waveguide and a discrete
tight-binding model with two sorts of atomic orbitals. We show that elongated
bulges induce numerous quasibound states. In the discrete model we also
evaluate the probability of transition between the localized states and
extended ones of the "valence" band. We suggest this as a mechanism governing
the porous-silicon luminescence. In addition, the model reproduces the
dominance of nonradiative transitions, blue shift for finer textures and
luminescence suppression at low temperatures.
