Nothia
Above: Slightly oblique section through a
prostrate aerial axis of Nothia aphylla showing irregular surface to
epidermis and divided vascular strand (scale bar = 1mm).
Introduction
Morphology
Relationships
Palaeoecology
Nothia was originally described as the probable fertile region of
another Rhynie plant, Asteroxylon mackiei by Kidston
and Lang (1920b). This was assumed to be the case until Geoffrey Lyon
discovered the unequivocal fertile leafy shoots belonging to Asteroxylon
(Lyon, 1964). In the same paper Lyon gave a
preliminary description of this enigmatic new vascular plant and assigned it the
name Nothia aphylla though no diagnosis was given. El-Saadawy
and Lacey (1979b) wrote a more detailed account of Nothia and were
the first to provide a diagnosis. The most recent work, concentrating on the
rhizomal anatomy, has been written by Kerp et
al. (2001).
The systematic position position of Nothia remains unresolved. As well
as the sporophyte, the male gametophyte of this plant, Kidstonophyton
discoides, has also been described (Remy
& Hass 1991b). The overall morphology and palaeoecology of Nothia
is outlined below.
Aerial Axes
The aerial axes of Nothia are quite distinctive. They are naked and display a conspicuous, very
irregular surface (see heading photograph). The maximum diameter of the axes
is 2.5mm and the axes exhibit commonly repeated dichotomous
branching giving the plant a thicket-like appearance. The plant probably
attained a height of about 15cm.
| The epidermis of Nothia shows a
characteristic pattern of longitudinally orientated files of short cells
alternating with 'giant cells' (Kerp et al.
2001) (see inset right). It displays elliptical to lenticular stomata-bearing emergences up to
350µm comprising longitudinal files of short cells with up to four intervening
'giant cells'; the files of short cells bearing the stomata.
The pores of the stomata are extremely narrow and their two guard cells are
usually wider than long. The cortex is
generally poorly preserved (see inset below right) though an inner and outer cortex can
occasionally be
differentiated based on cell shape (Edwards et al., in press), being
short in the outer cortex and elongated in the inner cortex. Below the epidermal
emergences the intercellular spaces of the outer cortex are large and within the
emergences the outer cortex appears spongy (Kerp et
al. 2001).
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Above: Close-up of epidermis of Nothia
in slightly oblique transverse cross-section showing alternations of
short (s) and 'giant' cells (g) (scale bar = 300µm).
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| In many cross-sections the aerial axes of Nothia appear to show a
double 'vascular strand', reflecting the
repeated dichotomy of the plant. In transverse section the xylem
appears elliptical or crescentic in shape (see inset right). It is sub-terete
and endarch with small central cells being surrounded by larger cells. Unlike
most of the other Rhynie plants the xylem does not exhibit thickenings (El-Saadawy
& Lacey 1979b) and therefore appears fibrous. Thus Nothia does not possess a vascular strand in the
strictest sense, a feature it shares with Aglaophyton.
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Above: Transverse section through
aerial axes of Nothia aphylla showing irregular epidermis (e),
poorly preserved cortex (c) and split, elliptical xylem strand (x)
(scale bar = 1mm).
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Rhizomal Axes
| The rhizomes of Nothia have only recently been adequately described (Kerp
et al. 2001). Though they have never been discovered in organic
connection with the aerial axes, they are assigned to Nothia on the basis
of their similarity in epidermal and 'vascular' morphology and anatomy. They have primarily been found in situ in a
number of cherty sandstone beds, exhibiting repeated branching and characteristically show a ridge on the
ventral surface that bears the rhizoids (see inset right). These smooth primary axes
locally turn upright and are believed to pass upwards into the aerial axes.
Right: Transverse cross section through a primary rhizomal
axis of Nothia
aphylla showing ventral rhizoidal ridge (r) with rhizoids (scale bar =
500µm) (Copyright owned by University Münster).
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The morphology of epidermal
and 'vascular' cells of the rhizome are very similar to that of the aerial axes.
The epidermis again consists of alternations of files of short cells and
'giant' cells and the xylem strand exhibits small cells surrounded by larger
water-conducting cells. Again the xylem cells do not show partial
thickenings and thus have a fibrous appearance. The main differences between the
rhizomal and aerial axes is the lack of 'emergences' (the axes consequently
appearing 'smooth'), lack of stomata and the thickening of walls in the short
cells of the epidermis.
Sporangium
| The sporangia of Nothia have a
lateral disposition, each sporangium being attached to the aerial axes by a
sporangial stalk (see inset right). The sporangia occur in numerous arrangements, being paired,
whorled, as terminal clusters or random (El-Saadawy
& Lacey 1979b). The shape of the sporangia ranges from typically
reniform (kidney-shaped) to pear-shaped and they display a well-developed
marginal dehiscence mechanism; the maximum
dimensions of a single sporangium being 3.1mm by 1.8mm by 1.4mm. Characteristically the sporangial epidermis is
very similar to that of the aerial and rhizomal axes showing the
alternation of 'giant' cells and files of short cells (Kerp
et al. 2001).
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Above: Two empty sporangia of Nothia
aphylla the left one showing the sporangial stalk (st). The
dehiscence slit is also evident (d) (scale bar = 1mm).
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Gametophytes
| To date the male gametophyte of this plant has been described and has been
assigned the name Kidstonophyton discoides (Remy
& Hass 1991b). The aerial axis of the free-living gametophyte show the
same general morphology and anatomy as that of the sporophyte, however, the
distal end terminates in a distinct cup or disc with tubular projections that
bear the antheridia (see inset right).
Right: Thin section of the male gametophyte Kidstonophyton
discoides (scale bar = 1mm) (Copyright owned by University Münster).
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Reconstruction
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Right: Diagrammatic reconstruction of the
sporophyte Nothia aphylla showing clonal growth from primary
rhizomal axes with upright stems passing upwards into dichotomously
branched aerial axes with distinctive irregular epidermis. Terminal
branches locally bearing lateral sporangia (based on Kerp
et al. 2001). |
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Left: Model of Nothia aphylla sculpted by Stephen Caine for the Rhynie Research Group, University of Aberdeen.
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Nothia aphylla is yet another Rhynie chert plant whose taxonomic relationship remains unclear, exhibiting morphological and anatomical
features characteristic of a number of plant groups. Firstly it shows features
characteristic of the bryophytes, namely the unthickened water-conducting cells
(a feature also seen in the 'hydroids' of Aglaophyton major), a
feature which suggests it is not a true vascular plant. Secondly, Nothia
also shares features with the primitive rhyniophytes since its axes are naked
and show similar simple branching. Also the sporangia of this plant show
similarities with zosterophylls, being lateral in their disposition on
the axes, reniform in
shape with a well-developed marginal dehiscence mechanism.
Nothia is a relatively common plant in a number of chert beds
occurring in allochthonous and autochthonous
plant litter, though, with the exception of rhizomal axes, not commonly preserved in
growth position. In a few horizons rhizomes and aerial axes of Nothia
occur as monotypic assemblages. In most cases the rhizomal axes of Nothia appear
to be preserved in cherty sandstone beds along with plant litter. The presence of rhizoids and lack of stomata on the rhizomal axes suggest the
rhizomes of Nothia were subterranean, apparently preferring sandy
soils.
In some beds Nothia rhizomes
appear to have penetrated earlier Nothia rhizomes or the rhizomes of other plants (e.g.: Asteroxylon)
which also suggests they grew within plant litter. The rhizomes or primary axes
are always far better preserved and less decayed than the aerial axes, even in
the few instances where the latter are found in situ in vuggy and massive
cherts that usually contain very well preserved axes of other Rhynie plants.
These observations tend to suggest a much longer lifespan for the rhizomal
(subterranean) axes than the aerial axes the growth of which was perhaps
determined by seasonal changes (Kerp et al.
2001).
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