Staghorn fern leaves are covered in starburst-like clusters of trichomes (leaf hairs). They're quite fascinating up close, if you have a chance to look at one with a magnifying lens. This is the underside of an older leaf turning fertile at the tip and right along the border between the sterile photosynthetic part (triangle at the bottom with green tissue showing through) and the fertile part (raised part on top). The sporangia have not yet pushed through the thick layer of trichomes, but you can see some of them shining through in dark brown. Eventually they will push through to the surface and release their spores, turning the underside of these leaves a velvety brown.