Scanning electron micrograph of a sporangium of the common polypody fern, Polypodium vulgare, which has split open to reveal the spores contained within. Sporangia aggregate in circular clusters called sori, which are located on the undersurface of the fern's frond. The sporangial wall is composed of thickened cells filled with water. As the water evaporates the sporagium bursts open, releasing the spores to be dispersed by the wind. This fern is common in Britain. It is found growing on trees, walls, in hedgerows & sand dunes. Magnification X600 (at 10x8 size).