Many woody plants have a secondary capacity to re-orient axes, or sustain the existing orientation of axes, by producing modified cells,i.e., form reaction tissue.Usually this is eccentric secondary xylem ("reaction wood"). "Compression wood" is formed in conifers on the lower side of branches and leaning trunks, exerting a compressive force ("push");"tension wood" in flowering plants is formed on the upper side and exerts a tensile force ("pull"). Which of these types might be shown by Gnetum, a gymnosperm with vessels in its wood? Experiments on G.gnemon demonstrate the existence of reaction tissues which are extra-xylary, i.e., differing from both conifers and flowering plants. Gnetum gnemon provides a precise example of Roux's model in the Halle-Oldeman system of tree architecture, with clearly differentiated orthotropic and plagiotropic axes. Both kinds of axis produce the reaction tissues and there is a cortical but not xylary eccentricity clearly associated with the secondary growth response. Furthermore, reaction fibers (which resemble "gelatinous fibers") are of both primary and secondary (from the phloem) origin; the primary fibers originate in a very unusual way.

Key words: cortex, gelatinous fibres, Gnetum, reaction anatomy, treearchitecture