How do sporangia in primitive ferns differ from those of most other ferns?

In primitive ferns, such as Ophioglossum and Botrychium, the sporangia are massive, sessile (stalkless) structures with walls composed of multiple layers of cells. These primitive sporangia, known as eusporangia, produce an indefinite and large number of spores that are released without a specialized ejection mechanism.12

In contrast, most other ferns possess smaller, long-stalked sporangia called leptosporangia. These structures have thin, single-layered walls and produce a definite number of spores. Unlike the primitive type, leptosporangia feature a specialized mechanism—a row of differentially thickened cells known as an annulus—that allows them to shed spores explosively by snapping open and shut as the cells dry.12