leptosporangiate

Having sporangia that were each formed from a single epidermal cell; of, pertaining to, or characteristic of ferns of this kind.

[…]which being leptosporangiate cannot be closely related to Isoetes, the sporangia appear late in the history of the leaf.

In fact, it seems that in the Primary rocks the distinctive Leptosporangiate annulus was at least rare, if it existed at all.

Such a fern