Geoscience Reference
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(
a
)
(
c
)
Pseudosporochnales
(Cladoxylopsida)
Archaeopteris
leaves
leafless
branches
long-leaved
branches
short-leaved
branches
(
b
)
(
d
)
primary
cortex
secondary
cortex
Fig. 5. Compared arborescent strategies in the Cladoxylopsida as hypothesized in this paper (a, b) and in Archaeopteris
(c, d). (a, c) Main morphological characters; triangles indicating significant growth potential. (b, d) Main anatomical
characters of stems in cross-section; primary xylem in black, secondary type of xylem (i.e. containing radially
aligned tracheids) in grey.
possibly after a growth phase where its diameter
remains constant (menetogenesis; Soria & Meyer-
Berthaud 2005). As for P. levis, the dissected and
peripheral vascular system contributes little to the
rigidity of the stem (8.5 to 10.4% of total flexural
stiffness; Soria 2003).
These results on the genus Pietzschia show that it
is not necessary to have secondary tissues to be big in
the Cladoxylopsida. An unpublished biomechanical
analysis by Soria & Speck (pers. comm., 2005)
suggests that P. levis may have reached 7.5 m in
height and P. schulleri 10 m. These results also
show that,
physical advantage to the evolution of tall plants,
it is not related to biomechanical support.
Stem growth model for self-supporting
Cladoxylopsida
This model is aimed at providing a set of growth
rules, expected to be shared by a wide range of
cladoxylopsid taxa, that explain the record of
morphologies they display. Our main argument for
proposing this model is the similarity in the anatom-
ical stem organization of Pietzschia, Eosper-
matopteris and Duisbergia, the presumed base of
Calamophyton
if the dissected vascular system of
Pietzschia
is an adaptive trait providing some
(Berry & Fairon-Demaret 2002)
