Nothia

Nothia aerial axis

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

 

Introduction

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.

 

Morphology

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).

 

Epidermis of Nothia

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).

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.

 

 

 

Aerial axis

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).

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).

 

Rhizomal axis of Nothia bearing rhizoids

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).

 

 

Sporangia

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).

 

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).

 

Kidstonophyton discoides

 

Reconstruction

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).

Reconstruction

 

Model of Nothia aphylla

Model of Nothia aphylla, close-up of sporangia

Left: Model of Nothia aphylla sculpted by Stephen Caine for the Rhynie Research Group, University of Aberdeen.

 

Relationships

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.

 

Palaeoecology

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).