Bark of Dirca L.: Tensile properties, anatomy, and utility for handmade Asian-style bark paper
Dirca (Thymelaeaceae) is a genus comprising four species known as leatherwoods. All are characterized by strong fibrous bark that resists tearing perpendicular to its axis. Several Native American peoples are known to have used the bark of Dirca palustris for cordage, but the physical properties and anatomy of all four species has not been reported. I address these voids in the literature. Techniques were developed to determine the ultimate tensile strength and the modulus of elasticity of bark of all species of Dirca and of additional species from the Thymelaeaceae and other plant families. Ultimate tensile strength of Dirca spp. was similar to or greater than that of all other species evaluated. Modulus of elasticity of Dirca spp. was intermediate among species. Bark tissue softened in ethylenediamine, embedded in polyethylene glycol 1500, and supported during sectioning with a particular brand of tape led to sections of quality to allow observation of anatomical traits for analysis. Fiber diameter and length were similar among all species of Dirca. Dirca mexicana and D. palustris share non-lignified fibers. Sieve-tube elements, axial parenchyma, and phellem cells differed between Dirca mexicana and other species of Dirca, but cellular traits cannot be used to differentiate all species. Finally, I used the bark of cultivated plants of D. mexicana to create paper that was evaluated for its durability and potential for use as a medium for printmaking. Paper made from D. mexicana withstood bending, folding, and creasing better than did gampi paper made from the bark of species of Wikstroemia (Thymelaeaceae) native to Japan. The length of fibers of D. mexicana are 2.5 times longer than species of Wikstroemia. Printmakers found my paper to be a suitable medium for relief, intaglio, lithography, screen, and digital printmaking. Long non-lignified fibers contribute to the use of Dirca as cordage and paper. I conclude D. mexicana is a North American source of fibers with properties similar to those of Japanese members of the Thymelaeaceae used to create specialty papers. Bark of Dirca should be collected only from cultivated plants due to the vulnerability of wild populations.