Hadron Structure of High-Energy Photons

We identify a new class of potentially large inclusive cross sections for the production of hadrons when high-energy photons interact with protons. Their origin is the abundant gluon content of very-high-energy photons. They can enhance the hadron (and also muon) content of high-energy electromagnetic showers as indicated by cosmic-ray observations and most dramatically illustrated by our failure to understand the muon content of photon showers from point sources. Experiments at the DESY $\mathrm{ep}$ collider HERA are in an ideal position to identify these contributions to photoproduction.