Fern sori. Coloured scanning electron micrograph (SEM) of two sectioned sori" (spore containing structures) on a leaf of the male fern Polystichum filix-mas. Sori consist of clusters of sporangia (brown, stalked). Each sporangium contains many spores (tiny spheres). When the spores are ready for release, the indusium (pale green) covering each cluster of sporangia uncurls in dry conditions. This drying also occurs in the wall of the sporangium which eventually, due to tension, splits and catapults spores away from the plant. Spores germinate on the ground to form the prothallus of a new fern plant. Magnification: x2 at 5x7cm size. Magnification: x5 at 3x7 inch print size."