False-colour scanning electron micrograph of sporangia of the common polypody fern Polypodium vulgare. These circular aggregates of sporangia, typical of this species, are called sori. Clusters of sori are found on the undersurface of the fern's frond. This fern is common in Britain. It is found growing on trees, walls, in hedgerows & sand dunes. The sporangial wall is composed of thickened cells filled with water. As the water evaporates the sporagium bursts open, releasing the spores contained within. Magnification X3.5 at 6x4.5cm size.