Comparison of Fresh, Paraffin-based, and Resin-based Techniques for Sectioning Bark of Dirca L.

Shrubs in the genus Dirca L. (leatherwood) are notable for their horticultural potential, botanical status in North America, and history of ethnobotanical use.  They are the only members of the Thymelaeaceae native to the United States and develop unusual arborescent growth forms and colorful flowers and leaves.  Products of use to humans have been made from the strong, pliable, and tear-resistant bark of leatherwoods, and observations suggest forest-dwelling animals also harvest bark for functional use.  As part of a more comprehensive analysis of the bark of leatherwoods, we evaluated various techniques for preparing sections of bark tissues for histological characterization.  Analysis of techniques was necessary because previous reports of the stem anatomy of Dirca do not describe methodology, and sectioning techniques recommended for bark tissue were not effective.  We found that the bark of Dirca comprises strata of bast fibers and non-lignified, thin-walled parenchyma; creating high-quality sections of tissue for anatomical analysis is difficult in part because of the heterogeneous composition of cell types.  Three techniques developed specifically for heterogeneous plant tissues were used to section the bark from four-year-old stems of D. mexicana Nesom & Mayfield, which we used to represent the genus.  Embedding with LR White resin led to slides of sectioned bark tissue of better quality than slides of tissue sectioned while fresh or after embedding in paraffin.  Use of LR White resin best prevented damage or folding of tissue during sectioning and mounting.  We conclude that the anatomical study of the heterogeneous cell composition of bark of D. mexicana is facilitated by use of LR White resin to embed fixed samples.