View of the underside of a fertile frond of the common polypody fern Polypodium vulgare, showing rows of yellow sori. Each sorus is a circular aggregate of sporangia, each of which contains numerous spores. The walls of the sporangia are composed of thickened cells, filled with water. As the spores mature, the water evaporates and the sporagium bursts open, releasing the spores to be dispersed by the wind. The polypody fern is common in Britain. It is found growing on trees, walls, in hedgerows and sand dunes.