SCHECKLER, STEPHEN E. Biology, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061-0406, USA.. - Asymmetric cambial growth and reaction wood of the Late Devonian progymnosperm Archaeopteris.
Unequal vascular cambium growth of a small Archaeopteris branch
with Callixylon zalesskyi-type wood from the Late Devonian
(early Frasnian) of New York demonstrates that reaction wood is a
basal lignophyte growth response, rather than a derived feature of
more recent seed plants. Archaeopteris reaction wood shows the
same anatomical signatures of compression wood, opposite wood, and
side wood as occur in modern conifer gymnosperms. Cambial growth is
markedly asymmetric with one side having far more secondary xylem
cells than the other. Secondary phloem production, however, is the
same. Tracheids on the side with more production are smaller, with
some intercellular spaces, and have thicker secondary walls that are
often rounded (some detached from primary walls). Pits of tracheids
with the thickest secondary walls lack obvious pit borders, which
suggests a thick S2 but no S3 wall layer, and have abundant helical
cavities or splits (checks) on their tangential and radial walls.
Tracheids on the opposite side, where many fewer cells are produced,
are larger in XS, lack intercellular spaces, have thinner secondary
walls that are always attached to the primary wall, have fully
bordered pits (complete S2 and S3 layers), and have solid unchecked
walls when seen in LS surface view. Rays on both sides of the axis are
of similar widths and have ray tracheids. Rays on the side with more
growth are curved, although this may result from compression.
Secondary growth mechanisms thus appear to be substantially the same
for all lignophytes, from Devonian progymnosperms to conifers and
angiosperm trees of today.
Key words: Archaeopteris, Devonian, progymnosperm, reaction wood