A perennial fern growing as a cluster of leaves from a rhizome, smooth cliff brake is almost always seen growing out of an exposed limestone or dolomite bluff or rock. The stiff, wiry stems are brown, hairless, or with a few scattered hairs. In general outline, the entire leaf is narrow, lance-shaped or oblong, and is once- or twice-compound. Leaflets can be leathery or not and sometimes have 1 or 2 lobes at the base. Leaflets are narrowly lanceolate to ovate or oblong, lacking hairs. Spores are borne in a continuous band along the outer margin, protected by the recurved edge of the leaflet. Spores are produced April–October.
Similar species: Purple cliff brake (P. atropurpurea) has somewhat dimorphic leaves (two different shapes, with the spore-bearing leaflets thinner and more divided, and the sterile leaflets wider and more rounded). Also, its stems and midveins are densely hairy with short, curly hairs on the upper surface. It can be a larger plant: its leaves can be 20 inches long, and its leaflets can be 1½ inches in length.
Habitat and Conservation
Ferns have a two-part life cycle. The plants we usually see are called sporophytes, because they produce spores that germinate and become the other part of the life cycle, the gametophytes. Gametophytes in this family of ferns are small, green, flat, kidney- or heart-shaped plants that few people notice. The gametophytes produce eggs and sperm, which unite to become a new sporophyte plant.
Cliff brakes, however, are famous for hybridizing with each other, and for creating offspring that are polyploid (possess more than two sets of genetic information in each cell), and for reproducing via apogamy. Apogamy is when a new sporophyte plant forms without fertilization — so it has the same number of genetic copies as the gametophyte.
Missouri has two varieties of smooth cliff brake: var. glabella reproduces apogamously and has larger spores (it is tetraploid, possessing 4 sets of each gene); it is more common. Var. missouriensis reproduces sexually and has smaller spores (it is diploid, possessing 2 sets of each gene); it’s the ancestor of var. glabella and is limited to only a few counties. You can only tell these two varieties apart by using a microscope!