Branching and Anastomosis of Resin Canals in Selected Pinaceae Species

Summary We examine the three-dimensional (3D) anatomy of normal axial (longitudinal) resin canals and fusiform rays in selected Pinus , Pseudotsuga , and Larix species, with a focus on the branching of axial resin canals and their anastomosis with radial resin canals. Helical X-ray microcomputed tomography (micro-CT) was used to acquire 3D information on specimens, which was then reconstructed using supercomputers and segmented using convolutional neural networks (CNN). We found that branching of axial resin canals can be subdivided into two main types: short branches of two or more branches separated by rays (in all species); and Y-shaped extended bifurcation separated by tracheids and subsidiary parenchyma cells (in southern pine and white pine). Anastomosis of resin canals typically occurred along the sides of axial resin canals. However, anastomosis was also observed at the termination of axial resin canals in Pinus , and in southern pine, it occasionally occurred within the axial canals. Extended axial resin canal branches could connect adjacent radial resin canals and ray parenchyma tissues tangentially. We conclude that branching of axial resin canals is common in selected Pinaceae species. The branching and anastomosis of resin canals link resin canals and their adjacent parenchyma tissues into 3D networks. The combination of X-ray micro-CT and CNN segmentation is a promising approach for evaluating the 3D structure of resin canals in softwoods and possibly analogous features in hardwoods.