Horneophyton
Above: Aerial axis of Horneophyton showing
sporangia (s) (scale bar = 1mm).
Introduction
Morphology
Relationships
Palaeoecology
Horneophyton was originally described by Kidston
and Lang (1920a) and assigned the name Hornea lignieri, however, the
generic name was already occupied and it was therefore renamed Horneophyton
lignieri by Barghoorn and Darrah in 1938.
Although one of the better known Rhynie plants, it's systematic position is
still very much debated. As well as the sporophyte, the female gametophyte of
this plant, Langiophyton mackiei, has also been described (Remy
and Hass 1991c). The overall morphology and palaeoecology of Horneophyton
is outlined below.
Aerial Axes
| The aerial axes of Horneophyton display a maximum diameter of 2mm and
are cylindrical and naked. The plant probably reached a maximum height of 20cm.
The branching of Horneophyton is dichotomous
and repeated.
The cuticle of the epidermis
displays a regular arrangement of elongate cells. Stomata
are relatively rare and are enclosed by distinctly modified cells (Hass
1991) (see inset right). The cortex is generally poorly
preserved and where present appears to be undivided.
The vascular tissue comprises an endarch
xylem strand with irregular spiral and reticulate thickenings. This
is surrounded by a zone of thin-walled cells interpreted as phloem.
The vascular strand becomes less distinct towards the base of the aerial
axis (see inset below right).
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Above: A stoma in the cuticle of Horneophyton
lignieri (st) surrounded by modified cells (click on the
image for a cross section!) (scale bar = 20µm) (Copyright owned by University
Münster).
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Rhizomal Axes
| The 'rhizomal axes' of Horneophyton distinguish it easily from all
other Rhynie plants. They comprise a corm-like lobe at the base of the aerial
axis with numerous unicellular rhizoids emerging from the epidermis (see insets
right and below right). A vascular
strand is not present in the rhizome, emerging from parenchymatous
tissue upwards from the base of the aerial axis.
Occasionally, fungal activity is evident in the cortex of the rhizome
(see inset below left).
Right: Horneophyton lignieri showing base of an aerial
axis with vascular strand (v) and the corm-like rhizome (c) bearing numerous
rhizoids (r) (scale bar = 2mm).
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Above: Horneophyton rhizome
showing fungal cysts in the cortex (f) and rhizoids (r)
(scale bar = 500µm).
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Above: Close up on the rhizoids of
Horneophyton lignieri (scale bar = 100µm).
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Sporangium
| The sporangia of Horneophyton are very
distinctive. Their disposition on the aerial axes is terminal. They are
cylindrical to globate in shape and may be branched displaying a number of
connected lobes (see heading photograph and reconstruction below); the maximum
dimensions of a single sporangium being 7.5mm by 5mm. The dehiscence mechanism is
apical and particularly well-developed (Eggert
1974; El-Saadawy & Lacey 1979a).
One of the more curious features of the sporangia of Horneophyton
is the presence of a central columella in the sporangial cavity (see inset
right).
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Above: A sporangium of Horneophyton
showing spores (s) and the central columella (c) (scale bar =
500µm).
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The in situ spores of Horneophyton are
relatively well known (Bhutta 1973a) and
may be assigned to the spore species Emphanisporites decoratus.
These vary between 42 and 54µm in size, the proximal face bearing a distinct
trilete mark. The proximal face of the spore is also ornamented with radial
ribs; the distal face characteristically showing an apiculate ornament,
typically of closely spaced micron-sized spines.
Gametophytes
| To date the female gametophyte of this plant has been described and has been
assigned the name Langiophyton mackiei (Remy
and Hass 1991c). This free-living gametophyte of Horneophyton
probably grew to about 6cm in height, the aerial axis terminating in a
conspicuous cup-like structure with numerous tubular outgrowths bearing the archegonia
(see inset right).
Right: Thin section of the female gametophyte Langiophyton
mackiei showing vascular strand (v) and the tubular outgrowths
bearing the archegonia (a) (Copyright owned by University Münster).
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Reconstruction
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Right: Diagrammatic reconstruction of the
sporophyte Horneophyton lignieri showing bulbous corm-like rhizomes with
rhizoids; dichotomously branching aerial axes and branching terminal
sporangia (based on Eggert 1974). |
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Left: Model of Horneophyton lignieri sculpted by Stephen
Caine for the Rhynie Research Group, University of Aberdeen.
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The presence of a sterile central columella in the sporangia of Horneophyton
is a feature in extant plants seen only in some bryophytes (e.g. mosses) suggesting
the plant may have some affinity with the latter, however, the fact that the aerial
axes of the plant exhibit a well-developed vascular strand with tracheids
would suggest it is not part of the bryophyte lineage. This mixture of
features seen in Horneophyton, as with other Rhynie plants, has led
to much debate on the systematic position of the plant which still remains in some
doubt.
Horneophyton appears to have been one of the more common plants in the
Early Devonian ecosystem at Rhynie. Where found in situ and in life
position, Horneophyton often occurs at the base of composite
chert beds, it's subterranean corm-like rhizomes cutting through pre-existing
plant litter and it's rhizoids probably helped to anchor the plant. It therefore appears that Horneophyton preferred sandy and
organic-rich substrates. The plant is commonly present as monotypic
stands, perhaps indicating it was also an early coloniser of sinter surfaces
being able to tolerate environmental conditions unfavourable to many other
Rhynie plants (Powell, et al. 2000b).
For example the plant is seldom associated with Rhynia gwynne-vaughanii
in growth position suggesting the two plant taxa required different optimum
conditions for growth.
It is also likely that Horneophyton flourished in damp to wet
conditions (Powell, et al. 2000b; Remy
& Hass 1991c). This seems likely for two main reasons. Firstly, looking
at modern sinters, their hygroscopic nature generally means they easily retain
water and in some cases the sinter surface where 'early colonising'
plants are growing is covered by an intermittent film of water (see also the
section on The Ancient Environment & Modern
Analogues). Secondly, Remy and Hass (1991c)
noted that Horneophyton was often associated with chytrids (chytridiomycetes)
which are a type of tiny, simple fungi that thrive in damp and especially
aquatic conditions (see also the sections on Fungi and Evidence
for Plant/Animal Interactions).
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