Horneophyton

Horneophyton axis with terminal sporangium

Above: Aerial axis of Horneophyton showing sporangia (s) (scale bar = 1mm).

Introduction

Morphology

Relationships

Palaeoecology

 

Introduction

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.

 

 

Morphology

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).

 

 

Stoma

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).

 

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).

 

 

Base of aerial axis and 'corm'

 

Rhizome

Rhizoids

Above: Horneophyton rhizome showing fungal cysts in the cortex (f) and rhizoids (r) (scale bar = 500µm).

Above: Close up on the rhizoids of Horneophyton lignieri (scale bar = 100µm).

 

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). 

Sporangium

Above: A sporangium of Horneophyton showing spores (s) and the central columella (c) (scale bar = 500µm).

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).

 

Langiophyton mackiei

 

 

Reconstruction

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).

Reconstruction

 

Model of Horneophyton lignieri

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

 

Relationships

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.

 

Palaeoecology

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).