Photon structure and the $\mu$ puzzle

In this talk I first briefly explain the puzzle presented by the high muon content observed in air-showers which point to the ultra high-energy $\gamma$ sources like the HER-X-1. Since it had been suggested that a possible explanation of the puzzle might come from the effects of the hadronic structure of the photon, I briefly explain the concept of photon structure function and comment on the uncertainties in the predictions of the \sigtot. Then I show that while the current experiments at the \eplem\ (TRISTAN and LEP) and $ep$ (HERA) colliders have seen clear evidence for the hadronic structure of the photon, the observed muon excess in the air shower experiments, if confirmed by other experiments to be at the same high level, can not be explained in terms of the photon structure function.