Aglaophyton
Above: Slightly oblique transverse
section through prostrate stems of Aglaophyton major. The stems show
partial decay, shrinkage (s) and minor compaction (c) (scale
bar = 1mm).
Introduction
Morphology
Relationships
Palaeoecology
Aglaophyton was originally described as Rhynia gwynne-vaughanii
by Kidston and Lang in 1917. In 1920,
however, they split the genus into two species and assigned this plant to the
species Rhynia major. D. S. Edwards
(1986) re-interpreted the plant, based primarily on the lack of thickenings
on the xylem cells (a feature originally attributed to poor preservation by Kidston
and Lang (1920a)) and renamed the plant Aglaophyton major.
Both the male and female gametophytes of Aglaophyton have been identified
though, to date, only the male gametophyte, Lyonophyton rhyniensis,
has been formally described and named (Remy &
Remy 1980b). The overall morphology and palaeoecology of Aglaophyton
is outlined below.
'Aerial' Axes
| The axes of this plant are superficially similar to Rhynia
gwynne-vaughanii though there are a number of distinct differences. They
exhibit a maximum diameter of 6mm and the plant probably attained a height of
around 15cm. Branching in Aglaophyton is predominantly dichotomous
(the angle of dichotomy being between 60 and 900) with minor adventitious
branching.
The epidermis of Aglaophyton
is smooth and the cuticle appears deeply flanged. The stomata
are flanked by two reniform or kidney-shaped guard cells. Most of the axis
comprises the cortex (see inset
right), this is divided into two, the division occasionally marked by a
distinct brown layer.
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Above: Transverse section through an
axis of Aglaophyton showing the cuticle (c), epidermis (e),
outer cortex (oc), inner cortex (ic), phloem (p)
and xylem strand (x) (scale bar = 2mm).
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| The outer cortex comprises closely packed elongate cells of
more or less uniform size. The inner cortex comprises more loosely packed
cells with a well-developed inter-cellular air space network. The dark
layer is formed by the presence of vesicular arbuscular mycorrhizae
within intracellular air spaces. Partially decayed Aglaophyton axes
exhibit a characteristic cellular decay pattern in the cortex (see inset
right).
Right: Transverse section through an Aglaophyton axis
showing the characteristic cellular decay pattern in the cortex (d)
(scale bar = 2mm).
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| The 'vascular strand' of
Aglaophyton comprises a zone of 'phloem'
surrounding a central xylem strand. The
phloem is of uniform thickness, the individual cells showing acute apices.
The xylem is terete and displays an endarch
maturation pattern (see inset right). A significant difference between the
xylem in this plant and that in Rhynia is the fact that the xylem
cells in Aglaophyton exhibit no thickenings.
Right: A slightly oblique cross-section through the
'vascular tissue' of Aglaophyton. The phloem (p) surrounds
the central xylem strand which shows the smaller, thin-walled protoxylem
cells (px) surrounded by larger thicker-walled metaxylem cells (mx)
(scale bar = 250µm). |
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Rhizomal Axes
| Aglaophyton exhibits creeping rhizomes that in life
were subaerial, laying directly on the substrate surface. These branch
repeatedly locally turning upwards and passing into the 'aerial' axes
described above. The rhizomal axes are cylindrical, naked and generally exhibit
a similar morphology and internal anatomy to the upright aerial axes, also
bearing stomata.
Unicellular rhizoids are present as tufts on 'bulges' on the ventral side
of the rhizomes, the bulges formed by elongate cortical cells (D.S.
Edwards 1986).
Right: Rhizoids (r) on an Aglaophyton
rhizomal axis (scale bar = 200µm). |
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Sporangium
| The sporangia of Aglaophyton are
elongate and fusiform in shape and relatively large with a maximum size of 12mm
by 4mm (see inset right). The dehiscence mechanism was determined by Remy
(1978); it would split obliquely along its length.
The disposition of the sporangia is terminal and they usually occur in
pairs above the last point of dichotomy (see reconstruction below).
Right: An empty sporangium of Aglaophyton (scale
bar = 2mm).
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The spores of Aglaophyton are retusoid
and have smooth walls with a trilete mark located in a thinning of the exine.
They are relative large, ranging in diameter from 64µm to 85µm. These spores
are comparable to species of the spore genus Retusotriletes. The
spores of Aglaophyton have often been found preserved at various stages
of germination (Lyon 1957) (see the section
on spores for images of some of these).
Gametophytes
| Both male and female gametophytes of this plant have been
identified though to date only the male form has been formally described.
The male gametophyte has been
assigned the name Lyonophyton rhyniensis (Remy
and Hass 1980b). This free-living gametophyte of Aglaophyton
consists of an aerial axis that widens and terminates in a
conspicuous cup-like structure which bears the antheridia
(see inset right). Although smaller in size, the axis of the gametophyte
is very similar to the sporophyte in its anatomy
Right: Longitudinal section of the male gametophyte Lyonophyton
rhyniensis bearing antheridia (a). Click on the image for a
close up! (scale bar = 1mm) (Copyright owned by University Münster).
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Reconstruction
| Right: Diagrammatic reconstruction of the
sporophyte Aglaophyton major (after D.S.
Edwards 1986) showing the nature of the creeping rhizome with localised
rhizoid tufts; predominant dichotomous branching and fertile axes bearing
a number of terminal fusiform sporangia. |
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Right: Model of Aglaophyton major, sculpted by
Stephen Caine for the Rhynie Research Group, University of Aberdeen.
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The systematic position of Aglaophyton, like a number of other Rhynie
plants, remains unresolved because it shows a mixture of anatomical and
morphological features that are not typical of any one group of plants. It shows
many features characteristic of the rhyniophytes, a group of primitive
plants, known only from fossils, showing simply branched naked stems of which Rhynia
is an example. The water-conducting cells of the xylem strand do not show
thickenings and as such are more reminiscent of the hydroids
of some bryophytes (a group of plants including mosses and liverworts).
Since the xylem of Aglaophyton does not possess true tracheids, this
suggests it is not a true vascular plant.
Aglaophyton was a common plant and significant component of the Rhynie
ecosystem during the Early Devonian. It appears to have preferred growing on litter-covered,
organic-rich surfaces and was never a primary coloniser of sinter
substrates, though the creeping rhizomal axes of Aglaophyton probably allowed it
to occupy large surfaces of substrate. It occasionally grew as monotypic stands
but is often seen associated with other plants, particularly Nothia, Asteroxylon,
Horneophyton and occasionally Rhynia.
Since Aglaophyton exhibits stomata on the rhizomal axes as well as the
upright aerial axes, it seems likely that the plant colonised mainly dry
substrates. The fact that the cuticle and stomata of Aglaophyton
display adaptations to prevent water loss suggests it could also tolerate
periods of drought (Powell 2000b).
However, the matrix of chert beds containing Aglaophyton in growth
position occasionally contain the charophyte Palaeonitella, in
association with the freshwater crustacean Lepidocaris and the 'aerial'
axes of the plant may display infestation by chytrids (chytridomycetes)
which are a type of tiny, simple fungi that thrive in damp and especially
aquatic conditions. Another feature noted by Remy
and Hass (1996) is the association of germinating Aglaophyton spores
with these aquatic elements. Although the plant could probably also tolerate
humid or damp conditions, this does not necessarily mean that Aglaophyton
lived in standing water, perhaps these particular beds represent instances where
areas colonised by the plant were occasionally flooded. It does seem likely,
however, that wet conditions may have been necessary
for spore germination.
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