It is interesting to examine with a good magnifying glass the sporangia borne on the lower surface of a mature fertile frond. In many species each sporangium or spore-case is surrounded with an elastic ring, which at maturity contracts so suddenly as to rupture the spore-case, and cause the expulsion of the numberless spores ([Fig. 7]).
[NOTABLE FERN FAMILIES]
OSMUNDA (Flowering Ferns)
Tall swamp ferns, growing in large crowns, with the fertile fronds or portions conspicuously unlike the sterile; sporangia opening by a longitudinal cleft into two valves.
ONOCLEA
Coarse ferns, with the fertile fronds rolled up into necklace-like or berry-like segments, and entirely unlike the broad, pinnatifid sterile ones. Fertile fronds unrolling at maturity, allowing the spores to escape, and remaining long after the sterile fronds have perished; sporangia stalked, ringed, bursting transversely.
WOODSIA
Small or medium-sized ferns, growing among rocks, with 1-2 pinnate or pinnatifid fronds and round fruit-dots; indusium thin and often evanescent, attached by its base under the sporangia, either small and open, or else early bursting at the top into irregular pieces or lobes; sporangia stalked, ringed, bursting transversely.