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(Fig. 4e, g, h). From a three-dimensional analysis of
casts, Boyer (1995) and Boyer & Matten (1996)
recognized three zones in Eospermatopteris: a wide
featureless pith-like part in the centre (17 cm wide
in a stem exceeding 20 cm in diameter), a narrow
zone comprised of vascular strands and an outer cor-
tical zone (Fig. 4g). They interpreted the vascular
strands as small, numerous and consisting entirely
of primary tissues. Ongoing investigations by Stein
et al. (2008) show, however, that all strands are sur-
rounded by secondary-like tissue (i.e. comprised of
radially aligned tracheids) and that the most periph-
eral strands are radially elongated. The Duisbergia
stems described by Mustafa (1978) are compressed
but their anatomical structure follows the same
general organization. The remarkable traits shared
by Eospermatopteris, Duisbergia and Pietzschia
are the large amount of ground-like tissue in the cen-
tral part of the stem and the peripheral location of the
radially elongated vascular strands. One difference
is the lack of secondary-like tissues in Pietzschia.
Berry and Fairon-Demaret (2002) hypothesized
that the possession of a secondary type of vascular
tissue consisting of radially aligned tracheids may
have contributed significantly to the realization
and support of large stems in the Pseudosporoch-
nales. Our opinion is different, based on indirect
evidence from non-pseudosporochnalean cladoxy-
lopsids. In the specimens of Xenocladia and Cladox-
ylon (Fig. 2) that we observed, the development of
aligned vascular elements around each primary vas-
cular strand is accompanied by the compression of
the surrounding soft-walled cells of the ground tissue
(Fig. 3f, g). When this development is extensive, pith
cells are no longer visible. The vascular strands that
were separate when composed of primary tissues
only come into contact when surrounded by a sec-
ondary type of xylem. Secondary development of
the vascular tissues in these genera is accommo-
dated by the compression of the voluminous pith
and remains within the limits of the primary body,
determined by the primary cortex (Fig. 3f ).
The lack of evidence for secondary cortex
(periderm) in all known cladoxylopsid (except one
Mississippian specimen from Montagne Noire
referrable to Cladoxylon taeniatum) adds credit to
this alternative hypothesis (Soria et al. 2006). A bio-
mechanical analysis was precisely realized on that
species. Calculation of Young's modulus of Cladox-
ylon taeniatum for different stages of growth shows
that stem stiffness decreases with the development
of the secondary cortex, despite the large amount
of secondary xylem produced (Soria et al. 2006).
This loss of stiffness occurred when the increase
in girth due to the secondary tissues exceeded the
limits of the primary body. This example, taken in
the Cladoxylopsida, indicates that the development
of secondary vascular tissues is not necessarily
linked
to
increased
performances
in
terms
of support.
The hypothesis that we defend here is that the
strategy for building large-sized plants in the
Cladoxylopsida was different from that in the lig-
nophytes such as Archaeopteris (Fig. 5b, d). The
development of secondary vascular tissues was, at
most, moderate compared to that in the arborescent
lignophytes. The major advantages which such
tissues provided to large-sized cladoxylopsids
were probably more significant in terms of water
transport than mechanical support.
The model for stem growth that we propose for
self-supporting Cladoxylopsida (Fig. 4a-c) inte-
grates the anatomical, morphological and develop-
mental information reviewed above. Basic features
of this model are as follows.
1. The stem shows two main growth phases of
unequal importance (Fig. 4a). The epidogenetic
phase where the primary body increases in size
and forms an obconical base is short. It is
mostly or entirely underground. The apoxoge-
netic phase where the primary body decreases
in size is the longest and corresponds to most,
if not all, of the aerial part of the stem that
bears branches. A third menetogenetic phase
where the size of the primary body remains
constant may follow the epidogenetic phase
and precede apoxogenesis.
2. The maximum diameter of the stem above
ground (labelled D
MP
in Fig. 4a, d, f ) results
from the large size of the primary body. This
condition is reached early during growth, at
the end of the short epidogenetic phase.
3. Roots are adventitious and tightly packed
around the obconical base (Fig. 4a, d).
4. The obconical base is the only part that may
undergo some late tangential expansion outside
the limits of the primary body (Fig. 4a, b, f ).
This increase in girth may be due to a proli-
feration of the living cells of the stem. The
stem base, then, may take a swollen shape. Its
maximum diameter (labelled D
MS
in Fig. 4f )
may exceed D
M
and individual roots become
separated from each other.
5. A secondary-type of xylem comprising radially
aligned tracheids may develop around the indi-
vidual vascular strands. This development does
not contribute significantly to the tangential
enlargement of the stem in its aerial part,
above the level of D
MP
(Fig. 4c).
Habitat and geographical distribution
of the pseudosporochnalean trees
By analogy with extant trees, the reduced size and
narrow shape of the crown of the Gilboa trees
