Extracellular diffusion pathway for heartwood substances in Albizia julibrissin Durazz

A three-dimensional network of intercellular spaces, deemed an extracellular diffusion pathway for heartwood substances, was detected in Albizia julibrissin. Electron microscopy revealed a large number of blind pits in ray parenchyma cells, most facing intercellular spaces. Heartwood substances, which are synthesized in the ray parenchyma cells, are released not only into neighboring cells through pit pairs, but also into the intercellular spaces through the blind pits. There were two types of wood fiber in the heartwood region: one with a lumen surface lined with heartwood substances and one that lacked such lining on its lumen surface. This finding supports the observation that the cellular distribution of heartwood substances is not homogenous throughout the heartwood. There was no positive correlation, however, between this uneven distribution and the distance of a wood fiber from the ray.