ABSTRACT
The primary xylem of conducting bundles is usually examined and described on the basis of cross-sections, made most often in the internodes. The question raised by the formation of heterogeneous vessels is one facet of the problem of differentiation of the secondary wall in the xylem in general, more particularly in the wood. In Dicotyledons, the secondary xylem vessels are most often linear; exceptionally they are formed of three branches, two of them being adjacent over their entire length, from a single triperforated element. The vasculature of culms, of entirely primary formation, is organized in bundles, containing, alongside the phloem, a protoxylem and a metaxylem; the former is no longer functional in the adult axes, while the latter is functional. The metaxylem of a bundle is composed of a pair of vessels with pitted elements, the diameters of which are greater in the innermost bundles.
