Scanning electron micrograph of sporangia of the common polypody fern Polypodium vulgare. The central sporanium is splitting open & a glimpse of the spores within is visible. 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 contained within to be dispersed by the wind. This fern is common in Britain. It is found growing on trees, walls, in hedgerows & sand dunes. Magnification X360 (at 10x8 size).