The Vessel Network in the Stem

Abstract

The vessels of a dicotyledonous tree stem do not all run neatly parallel; they deviate more or less from their axial path. This deviation differs in successive layers of the xylem; the vessels form a network. This has been known to foresters on a macroscopic scale as “cross grain”. Cross grain was considered an exceptional characteristic of certain species. We know today that virtually all xylem is microscopically “cross grained” in respect to the path of their vessels. This was described first by Braun (1959). He meticulously reconstructed the vessel network of Populus from a series of transverse sections and showed the structure in a block of the dimensions 0.3×1×2.5 mm. He also described the nature of vessel ends. Vessel ends are not easy to recognize, particularly not in single sections. They have occasionally been “seen” where India ink infusions terminated. However, this is by no means reliable, because India ink particles are quite large and ragged; lateral water loss from the vessels concentrates them, and they may clog the vessels somewhere midway. Anyone who has tried to fill a fountain pen with India ink will heartily agree! Vessel ends have occasionally been reported as terminal elements in macerations (e.g., elements that have a perforation only at one end; Handley 1936; Bierhorst and Zamora 1965). This is one of the most reliable methods, but the vessel end is then seen only in isolation.