Plant of the Week
Appalachian Bristle Fern (Trichomanes boschianum Strum)
By David Taylor
Appalachian bristle fern, or filmy fern, is a small delicate fern in the Hymenophyllaceae (filmy fern) family. Most of the ferns in this family are found in tropical rainforests in lowland or montane areas. A few, including Appalachian bristle fern, are found in the temperate or subtropical U.S. All of the ferns in the family are delicate with very thin leaves when leaves are produced. Several of the U.S. species produce only filamentous or thalloid gametophyes.
This species produces leaves up to 20 centimeters (approximately 8 inches) long and 4 centimeters (1.6 inches) wide. The leaves are membranous with strong veins. The indistinct form of objects can be seen through the leaf tissue. This thin leaf tissue leads to the name filmy fern. At the tips of the veins a tube-like involucre is produce in which spores are formed. A bristle-like structure called a sporangial receptacle sticks out of the involucre giving the fern the name bristle fern. Spores are formed around the receptacle. The leaves are attached to a wiry rhizome.
The thin leaves of the fern are only a few cells thick and this means it dries out rapidly. For this reason it is found in places where humidity is constantly high and temperatures tend to be moderate throughout the year such as deep recesses and cracks in cliffs and rock shelters, and on boulders along streams or in deep narrow hollows. Often a light is needed to see the plants. Appalachian filmy fern is usually found on sandstone or conglomerate, but can be on other non-calcareous rocks.
The species is found in the Appalachian Mountains (northern Alabama, Georgia and South Carolina, eastern Tennessee and Kentucky, western North Carolina, southeast Ohio, and West Virginia), Arkansas, northeastern Mississippi, and southern Illinois and Indiana in association with cliffs and gorges. It is also known from gorges in Chihuahua, Mexico.