Geoscience Reference
In-Depth Information
height, had a relatively slender trunk tapering to a
distal diameter of 13-15 cm and had attached
branches of about 1 m in length only (Stein et al.
2007). The latter formed a narrow crown. The
regular shedding of branches contributed to the
accumulation of a significant litter. Root casts pre-
served in the palaeosols at Gilboa do not exceed
3 cm in diameter and their vertical extension
30 cm (Driese et al. 1997).
Stem ontogeny in P. levis shows the two succes-
sive growth phases described as epidogenis and
apoxogenesis in other plant taxa (Eggert 1961,
1962; Scheckler 1978) (Fig. 4d). The proximal part
of the stem which is relatively short (10-15 cm
long) is obconical (i.e. shaped as an inverted cone)
and corresponds to an increase in size of the
primary body (epidogenetic growth) until it reaches
itsmaximal diameter (D
MP
in Fig. 4d). The rest of the
stem is conical and corresponds to a decrease in size
of the primary body (apoxogenetic growth). The
obconical part is surrounded by a thick mantle of
adventitious roots that run through the stem cortex
in the proximal part of their course. They emerge
from the stem further down but remain partially
fused and tightly packed outside the stem (Fig. 4d).
The pattern of root production in P. levis is
characterized by two traits: (1) there are few large
roots produced at the very base of the stem and
numerous small roots higher up; and (2) individual
roots increase in size as they grow downwards
(Soria & Meyer-Berthaud 2004). The result is that
all individual roots appear similar in structure and
dimension at any one level. They are wider at the
base of the specimen. In outer morphology, this
base looks swollen despite the fact that the stem
itself is narrow (Fig. 4d). A biomechanical analysis
following the approach of Rowe & Speck (1998)
shows that the plant base is not self-supporting
despite its thick root mantle, a result indicating
that it was probably underground (Soria 2003). In
the aerial conical part, the branch bases and stem
cortex provide most of the support. Although it
was expected that the dissected vascular system of
the stem located in a relatively peripheral position
(Fig. 4e) significantly increased stem stiffness, the
mechanical tests indicate that it contributed little
to its rigidity (only 3.3 to 12.3% of total flexural
stiffness; Soria 2003).
All the available stem specimens of P. schulleri
are incomplete at both ends and the root system in
this species in unknown. Ontogenetical studies
show that the primary body is wider proximally
and decreases
Structure and growth patterns in Pietzschia
The cladoxylopsid genus Pietzschia which compri-
ses three species ranges from the Frasnian to the
early Mississippian and has been recorded in
Germany, USA and Morocco (Fig. 2). The two best-
known species in terms of growth patterns and
anatomy are P. levis, consisting of stems that do
not exceed 9 cm in diameter, and P. schulleri
whose stems, up to 16 cm wide, are the largest
known in the genus (Soria et al. 2001; Soria &
Meyer-Berthaud 2004, 2005).
Pietzschia differs from the Pseudosporochnales
by conspicuous stem internodes and by a different
morphology of the lateral branches. None of the
specimens of Pietzschia examined show any sec-
ondary tissues. Anatomically, stems in cross-section
are characterized by a ring of discrete, radially
elongated, primary xylem strands organized at the
periphery of a wide pith (Figs 3d & 4e). Some
small circular xylem strands are also scattered
within the pith. Depending on species and speci-
mens, the cross-sectional surface area of xylem in
Pietzschia ranges from 2.5 to 10% of the total cross-
sectional surface area of tissues in the stems (Soria
et al. 2001; Soria 2003). This ratio is almost constant
over single specimens. The outer cortex, presumed
to be sclerenchymatous in the stem of P. levis,is
less than 1 mm thick. In P. schulleri the cross-
sectional surface area of the outer cortex ranges
from 17 to 24% of the total cross-sectional surface
area of the stem but it is probably collenchymatous,
therefore not lignous (Soria 2003; Soria & Meyer-
Berthaud 2005).
in size distally (apoxogenesis),
Fig. 4. (Continued)(b) Growth patterns in the basalmost part of the stem. Right: tangential enlargement of stem
base leading to formation of a swollen base, dissociation of roots and increased root spacing. (c) Hypothesized
transverse section of stem above D
MP
showing the potential development of a secondary type of vascular tissue
(radially aligned tracheids) inside a boundary defined by the primary cortex. Primary vascular strands in black.
Central strands not shown here. (d, e) Stem structure and development in Pietzschia. (d) Stem showing two main
primary growth phases (A and E). Basalmost part showing a thick mantle of fused roots around the obconical part of
stem. Distalmost part of roots not preserved. (e) Diagramatic transverse section of Pietzschia stem above D
Mp
,
containing primary tissues only. Central strands, root traces and branch traces not shown here. (f, g, h) Stem structure
and development in pseudosporochnalean trees. (f ) Interpretative diagram of Eospermatopteris/Wattieza stem showing
separated roots radiating from enlarged base. Possible part corresponding to epidogenetic phase limited by dotted line.
(g) Diagrammatic transverse section of Eospermatopteris, adapted from Boyer (1995). (h) Diagrammatic transverse
section of Duisbergia, adapted from Mustafa (1978). D
MP
: maximal diameter of stem resulting from primary growth;
D
MS
: maximal diameter of basal part of stem resulting from late tangential enlargement.
