TOMLINSON, P.BARRY. Harvard Forest, Harvard University, Petersham MA 01366; National Tropical Botanical Garden, 3530 Papalina Rd., Kalaheo, Kauai, HI 96741. - Does Gnetum show reaction tissue?
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