  We analyse the general solutions for the stress field in planar annuli of
isostatic media, a model often used for marginally rigid granular materials in
Couette cells. We demonstrate that these solutions are much richer than in
rectangular symmetries. Even for uniform media, stress chains are found to
curve, broaden away from the stress source, attenuate and leak stress into a
cone of influence. Most spectacularly, stress chains may bend back and transmit
forces oppositely to the original direction. None of these phenomena arises in
solutions for uniform media in Cartesian coordinates. We further analyse
non-uniform media, which exhibit chain branching and stress leakage from the
chains. These results are directly relevant to the many experiments on granular
materials, carried out in Couette cells. They also shed light on, and are
supported by, hitherto unexplained experimental observations of curved and
back-bending chains, which we point out. In particular, we use our results to
provide a new interpretation for the pattern of slip lines observed
experimentally.
