with less-common features
The Lower Devonian vascular plants preserved in the Rhynie chert,
mostly in various states of decay but also life-like, have
something in common: After the decay of soft tissue, several components
are usually left: xylem strand, spore capsules, the highly
durable spores, and occasionally the epidermis
with cuticle and a narrow strip of adjacent cortex. This
rule based on ample fossil evidence had to be
modified with the discovery of Ventarura
. This plant has
unique structural element which can be conspicuous on
cross-sections as a concentric ring consisting of well-preserved cells,
positioned within the cortex between central strand and
epidermis, in the upper parts of the plant (Fig.1).
usually dark aspect of these tube cross-sections is due to the
thick dark cell walls which suggest an interpretation as a
reinforcing element which increases bending strength and stiffness of
the plant. Such interpretation is supported by the shape of
the forking site of the tube, which seems to be optimized
with respect to load-bearing capacity,
as illustrated in Rhynie
Chert News 3.
on a cut face of Rhynie chert, with shrivelled surface, dark ring of
well-preserved cells, xylem strand, and quartz-lined
cavity replacing part of the decayed soft tissue. Picture taken under oil with a bubble creeping out of the cavity.
Fig.2 (left): Ventarura
fragment with distinctly seen cells of the characteristic tube.
There is evidence, however, that the argumentation concerning the
effect is not quite as simple. The well-preserved cells
forming the cylindrical tube do not always look dark and thick-walled
sclerenchyma. In Fig.2 they are thin-walled and mostly pale
hollow before silicification, shrivelled outside,
shape stabilized by the characteristic cylindrical tube 5mm across,
uncommonly pale, one third broken away.
Apparently the pale tubes are found in a few specimens only
(Figs.3-5), among them those which are not shrivelled (Figs.4,5). It is
suggested by the latter and other observations that all tubes were pale
at first and became dark later by some unknown process.
From Figs.2,3 it is also apparent that the characteristic
cylindrical tube retains its shape while the tissue within and without
vanishes. The space left by the decayed tissue between tube
epidermis is either left open, filled by mineral precipitate, or
vanishes by shrivelling of the decaying epidermis with cuticle.
cross-section of uncommon structure and preservation: outer boundary 5.5mm
across, not shrivelled, two pale concentric tubes
consisting of preserved cells, the inner
one non-circular with two conducting strands inside,
cavity replacing part of decayed outer tissue, yellowish quartz in
by decayed inner tissue.
cross-section, same shoot as Fig.4 but cut at lower
position. Note the
discontinuity of the outer tube and the combination of tube
sections with pale and darker aspect.
The arrangement of tubes seen as concentric rings in Figs.4,5, the
inner one non-circular and thin, is quite
unexpected, and no explanation is offered here. Being incompatible
with the forking as illustrated in a tentative drawing in Rhynie
Chert News 3,
it is one of the features of
which are not yet understood.
The persistent tube cross-sections may not be closed loops but may
consist of loop fragments whose arrangement varies along the stem.
Some of the facts seen in these pictures are summarized here:
- Before silicification, the upper parts of Ventarura
been either shrivelled (Figs.1-3) or smooth and cylindrical (Figs.4,5).
The epidermis, like most of cortex, is usually not preserved but
the cuticle is seen as a dark line,
possibly a thin deposit stained
characteristic tube consisting of decay-resistant cells, usually seen
as a dark ring on cross-sections, can be pale and
- In case of shrivelling surface, the persistent
tube keeps its shape (Fig.3), thus appearing as mechanically
suggesting an interpretation as a strengthening structural
component. (The deviation from circular shape in Fig.1 is due to
- The apparent strength of the tube may be an illusion due to
the weakness of the decaying tissue around.
- There are exceptional cases of two concentric tubes,
the inner one being thin-walled.
- Irregular-shaped thick coatings with abundant
enclosed, surrounded by much cleaner chert (Figs.3-5), are
probably of microbial origin.
Since the persistent tube tissue
is neither connected to the epidermis nor to the central strand, the
question arises how it is brought about, and for which purpose. Hence,
any finds of Ventarura
in the Rhynie chert deserve particular attention as they may lead to an
answer. Also it would be interesting to know
whether a tubular component of
the type found in Ventarura
is unique or present in some other fossil or extant plant, too. A
different type of tube, a hollow straw with or without epidermis
often seen with Aglaophyton.
Edwards, N.H. Trewin: A new vascular plant from the
Lower Devonian Windyfield chert, Rhynie, NE Scotland.
Trans. Roy. Soc. Edinburgh, Earth Sci.
90(2000 for 1999), 331-349.