Rhynia
Above: Transverse sections through axes
of Rhynia gwynne-vaughanii. The star-shaped axes, right of centre,
are a result of shrinkage during decay and desiccation of the stems (scale
bar = 1mm).
Introduction
Morphology Relationships Palaeoecology
One of the first Rhynie chert plants to be described and perhaps the most abundant is the form Rhynia.
The plant was originally described and classified by Kidston and Lang in 1917,
1920a and assigned the
species name Rhynia gwynne-vaughanii. Another plant now known as Aglaophyton,
possessing a slightly similar anatomy, was originally described by Kidston and
Lang in their 1917 paper as also belonging to Rhynia gwynne-vaughanii,
but differs significantly in its vascular anatomy and was subsequently
reassigned (see also Aglaophyton). Unequivocal
gametophytes of Rhynia have recently been discovered (Kerp
et al. in press) and will be illustrated here once published. The overall
morphology and palaeoecology of Rhynia is outlined below.
'Aerial' Axes
The axes of Rhynia exhibit a maximum diameter of 3mm and the plant
probably attained a height of up to 20cm. The aerial or rather the 'upright' axes are cylindrical, naked
and upwardly tapering. The branching of Rhynia is both dichotomous
and adventitious or monopodial, with dichotomy occurring at an angle between
17 and 350 (D. S. Edwards 1980).
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The surface of the axis bear numerous conspicuous emergences or hemispherical
projections (see insert below right) from the epidermis which are occasionally located beneath
stomata and at the base of adventitious branches and in other instances
internally display fungal activity and dark necrotic tissue.
Right: Transverse cross section through
a stoma showing the two guard cells (g) with the stomatal chamber
(c) beneath (scale bar
= 20µm) (Copyright owned by University Münster).
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| The stomata
typically appear circular on the cuticle surface and are flanked by two guard
cells (see inset above right). The cells of the cuticle
often exhibit a median ridge giving the cuticle a flanged appearance.
Right: Transverse cross section through the cuticle and
epidermis of Rhynia showing the median ridges on the
cuticle (r) (click on the image for a close up!) (scale bar = 100µm) (Copyright owned by University
Münster). |
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| The cortex is divided into two distinct
zones separated by a brown line of amorphous material. The outer cortex
comprises closely packed uniform cells, becoming noticeably elongate below the
hemispherical projections. The inner cortex comprises uniform cells with a
well-developed inter-cellular air space network and commonly exhibits vesicular
arbuscular mycorrhizae. The vascular
tissue or stele comprises a zone of 'phloem'
of uniform thickness surrounding a central xylem
strand. The phloem cell walls exhibit what appear to be pores (Satterthwait
and Schopf 1972, Kenrick and Crane 1991). The xylem strand is terete,
exhibiting endarch cell development. The
xylem cells also exhibit annular and rare spiral thickenings.
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Above: Transverse section through a
Rhynia axis showing xylem (x), 'phloem' (p), inner cortex (ic), outer
cortex (oc), epidermis (e), cuticle (c) and a small hemispherical
projection (h) (scale bar = 1mm) (Copyright owned by University Münster). |
Rhizomal Axes
| Like Aglaophyton, Rhynia laid directly on the
ground surface. Rhynia possesses a creeping rhizome displaying repeated dichotomous
and adventitious branching, locally turning upright, passing upwards into the
'aerial' axes. The rhizomal axes are cylindrical and naked and generally exhibit
a similar morphology and internal anatomy to the aerial axes though they lack
stomata. The other main
difference is exhibited by the hemispherical projections, which commonly support
tufts of unicellular rhizoids (see inset right). However, occasionally
hemispherical projections on the upright 'aerial' axes may also bear
rhizoids.
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Above: A hemispherical projection on a Rhynia
axis bearing tufts of rhizoids (scale bar = 250µm).
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Sporangium
| The sporangia of Rhynia are not particularly common. They are
fusiform, displaying a maximum size of 3.6mm by 2.4mm. The disposition of the
sporangia is terminal, being located on the adventitious branches of fertile
aerial axes. No dehiscence mechanism has been observed though a dark cellular
layer or 'sterile pad' at the base of the sporangium has been interpreted as a
site of abscission by D. S. Edwards (1980)
(see inset right). The sporangial wall comprises three layers: an outer
epidermis, a poorly preserved parenchymatous layer
and an inner tapetal layer.
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Above: A split, empty sporangium of Rhynia
still attached to a prostrate, partially decayed axis showing the dark
'sterile pad' (a) (scale bar = 1mm) (Copyright owned by The
Natural History Museum).
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Reconstruction
| Right: Diagrammatic reconstruction of the
sporophyte Rhynia gwynne-vaughanii
showing two fertile adventitious branches with terminal fusiform sporangia (rhizomal
axes not shown) (after D. S. Edwards 1980). |
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Rhynia gwynne-vaughanii being a naked, simply branched sporophyte has
been assigned to a primitive group of vascular plants known only from fossils,
and called the rhyniophytes.
The presence of hemispherical projections on the axes remains a point of
speculation and their like is not seen in any of the other Rhynie plants.
Various interpretations have been proposed:
- Damage from arthropods sucking sap (Kevan
et al. 1975).
- Wounding by nematodes, mites, parasites or fungi (Edwards
and Selden 1993).
- Damage from splashes of hot water from geysers,
or from volcanic ash (Kidston and Lang 1921a);
an unlikely explanation considering the small size and disposition of the
projections on the plant axes, and also since they have not been observed on
any of the other Rhynie plants.
- Pant (1962) and Lemoigne
(1968a, b) interpreted the hemispherical projections as sites of archegonia,
though this interpretation has never been generally accepted.
Rhynia was the most common vascular plant in the Early Devonian
ecosystem at Rhynie, at least in the areas of sinter deposition, both
numerically and in terms of ground cover (Powell
et al. 2000b). The plant appears to have been entirely subaerial with its
naked, branching 'rhizomes' creeping across the ground surface with the upright
portions of the plant growing to give a thicket-like appearance. Although the hemispherical projections on the 'rhizomes' bear the rhizoids
for taking up water from the ground surface, the fact that those on the upright
stems occasionally exhibit rhizoid tufts suggest Rhynia was also capable
of taking up atmospheric water.
Where
found in growth position in the chert beds it is typically, though not
exclusively the only in situ vascular plant present, and is commonly found
above sandy chert layers and allochthonous litter horizons. This suggests Rhynia
commonly grew as monotypic stands, an early colonizer of well-drained sinter and sandy
substrates.
However, the plant is also found associated with all other Rhynie plants,
though only very rarely with Horneophyton. This suggests Rhynia was
tolerant of a wide range of habitats and could also withstand interspecies
competition within the Early Devonian ecosystem at Rhynie (Powell
et al. 2000b).
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