Pillwort

Pilularia globulifera, or pillwort, is an unusual species of fern native to western Europe, where it grows at edges of lakes, ponds, ditches and marshes, on wet clay or clay-sand soil, sometimes in water up to 30 cm (12 in) deep. It has a pea-shaped 4-chambered sporocarp, each chamber formed from a modified leaf and containing several sori bearing both macrosporangia and microsporangia. The species is thus heterosporous.