Fern spore cases. Polarised light micrograph of a frond of the maidenhair fern (Adiantum capillus- veneris), showing the sporangia (black, centre left) which contain the fern's spores. The spores of some ferns are found in a structure called an indusium, but this fern does not have a proper indusium, and the sporangia occur under a marginal flap.