Regnellidium diphyllum

When I traveled to California, the last fern that I was expecting to find outdoors was Regnellidium diphyllum, a narrow endemic species from S Brazil, N Argentina, and Uruguay. But there it was, vigorously growing in a garden pond. Almost everything about this fern is unusual. Besides being aquatic, the sori are borne within hardened bean-like structures called “sporocarps”. Each sporocarp has two types of sporangia (megasporangia and microsporangia), a condition that is very rare among ferns (i.e., heterospory). Regnellidium is the only living species of Marsileaceae with two pinnae, thus the common name “two-leaved marsilea”. It is also the only nonflowering plant that produces latex, which is formed in unbranched laticiferous ducts in the cortex of the stem, petiole, and lamina (Gifford & Foster, 1989).