Plant of the Week
Appalachian Threadwort (Drepanolejeunea appalachiana Schust.)
By Duke Rankin
The southern Appalachians are home to an amazing diversity of non-vascular plants. These plants lack the specialized, internal cells that transport water and nutrients characteristic of the vascular plants that dominate most terrestrial ecosystems. As a result, non-vascular plants tend to be small and close to the ground, where water and minerals can diffuse directly throughout the plant. Common non-vascular plants include mosses and the closely related liverworts. Frequently, these plants are just one or two cells thick, and all of them lack specialized structures such as roots and flowers.
Drepanolejeunea appalachiana, the Appalachian threadwort, is a microliverwort. The species exhibits the usual features of liverworts. A microliverwort, however, is so small that it is difficult to see with the naked eye, and must be identified using a microscope to examine structures fashioned from just a handful of cells.
Appalachian threadwort resembles a string of beads strung along a thin thread. The thread portion—technically, the stem—is less than one-half of one tenth of a millimeter wide. Sessile leaves are attached to this thread in an alternate pattern. The leaves are pointed, light green, and less than half the size of a pinhead. At the base of the leaf, where it connects to the thread, is a hidden lobule where the leaf has been folded under itself. The thread also bears characteristic underleaves. These leaves are attached to the bottom of the thread, and can only be seen by flipping the plant over. The underleaf of Appalachian threadwort is wedge-shaped, with two hornlike teeth extending from the upper corners, and resembles a human hand curled into a fist, with the index and pinky fingers extended. Each of the teeth is composed of 1–3 cells, arranged in a straight line.
The natural history of the species is poorly known. It grows primarily on the bark of trees, often mixed with other small liverworts and lichens, and is typically found in moist forests at higher elevations, or in gorges with high humidity and rainfall. Asexual reproduction involves the formation of tiny branches that readily break away and are probably dispersed by wind currents. The environmental factors that produce these small branches are unknown. Sexual reproduction in Appalachian threadwort has not been documented.
Appalachian threadwort is endemic to the southern Appalachians. It is most commonly found in the mountains of southwest Virginia, eastern Tennessee, and western North Carolina, but there are isolated records from the nearby portions of South Carolina, Georgia and Kentucky. Because of its limited range and diminutive size, Appalachian threadwort has been considered a conservation issue, threatened not only by loss of habitat—which, in this case, can be as simple as cutting down the tree the plant is growing on—but by regional issues as well, such as air pollution and acid rain.