Fertile fern fronds. Collection of various species of fern, upturned to reveal the sori. Left to right: polypody (Polypodium vulgare), male fern (Dryptoteris filix-mas), Hart's tongue (Asplenium scolopendrium) intermediate polypody (Polypodium interjectum) and polypody (Polypody vulgare). Sori are collections of sporangia, which are casings in in which the spores are produced.