Aglaophyton

Prostrate Aglaophyton axes

Above: Slightly oblique transverse section through prostrate stems of Aglaophyton major. The stems show partial decay, shrinkage (s) and minor compaction (c) (scale bar = 1mm).

Introduction

Morphology

Relationships

Palaeoecology

 

Introduction

Aglaophyton was originally described as Rhynia gwynne-vaughanii by Kidston and Lang in 1917. In 1920, however, they split the genus into two species and assigned this plant to the species Rhynia major. D. S. Edwards (1986) re-interpreted the plant, based primarily on the lack of thickenings on the xylem cells (a feature originally attributed to poor preservation by Kidston and Lang (1920a)) and renamed the plant Aglaophyton major. Both the male and female gametophytes of Aglaophyton have been identified though, to date, only the male gametophyte, Lyonophyton rhyniensis, has been formally described and named (Remy & Remy 1980b). The overall morphology and palaeoecology of Aglaophyton is outlined below.

 

Morphology

'Aerial' Axes

The axes of this plant are superficially similar to Rhynia gwynne-vaughanii though there are a number of distinct differences. They exhibit a maximum diameter of 6mm and the plant probably attained a height of around 15cm. Branching in Aglaophyton is predominantly dichotomous (the angle of dichotomy being between 60 and 900) with minor adventitious branching.

The epidermis of Aglaophyton is smooth and the cuticle appears deeply flanged. The stomata are flanked by two reniform or kidney-shaped guard cells. Most of the axis comprises the cortex (see inset right), this is divided into two, the division occasionally marked by a distinct brown layer.

 

Aglaophyton axis

Above: Transverse section through an axis of Aglaophyton showing the cuticle (c), epidermis (e), outer cortex (oc), inner cortex (ic), phloem (p) and xylem strand (x) (scale bar = 2mm).

The outer cortex comprises closely packed elongate cells of more or less uniform size. The inner cortex comprises more loosely packed cells with a well-developed inter-cellular air space network. The dark layer is formed by the presence of vesicular arbuscular mycorrhizae within intracellular air spaces. Partially decayed Aglaophyton axes exhibit a characteristic cellular decay pattern in the cortex (see inset right).

Right: Transverse section through an Aglaophyton axis showing the characteristic cellular decay pattern in the cortex (d) (scale bar = 2mm).

 

Cortex decay

The 'vascular strand' of Aglaophyton comprises a zone of 'phloem' surrounding a central xylem strand. The phloem is of uniform thickness, the individual cells showing acute apices. The xylem is terete and displays an endarch maturation pattern (see inset right). A significant difference between the xylem in this plant and that in Rhynia is the fact that the xylem cells in Aglaophyton exhibit no thickenings.

Right: A slightly oblique cross-section through the 'vascular tissue' of Aglaophyton. The phloem (p) surrounds the central xylem strand which shows the smaller, thin-walled protoxylem cells (px) surrounded by larger thicker-walled metaxylem cells (mx) (scale bar = 250µm).

'Vascular tissue'

 

Rhizomal Axes

Aglaophyton exhibits creeping rhizomes that in life were subaerial, laying directly on the substrate surface. These branch repeatedly locally turning upwards and passing into the 'aerial' axes described above. The rhizomal axes are cylindrical, naked and generally exhibit a similar morphology and internal anatomy to the upright aerial axes, also bearing stomata. Unicellular rhizoids are present as tufts on 'bulges' on the ventral side of the rhizomes, the bulges formed by elongate cortical cells (D.S. Edwards 1986).

Right: Rhizoids (r) on an Aglaophyton rhizomal axis (scale bar = 200µm).

Aglaophyton rhizoids

 

Sporangium

The sporangia of Aglaophyton are elongate and fusiform in shape and relatively large with a maximum size of 12mm by 4mm (see inset right). The dehiscence mechanism was determined by Remy (1978); it would split obliquely along its length.

The disposition of the sporangia is terminal and they usually occur in pairs above the last point of dichotomy (see reconstruction below).

Right: An empty sporangium of Aglaophyton (scale bar = 2mm).

 

Aglaophyton sporangium

The spores of Aglaophyton are retusoid and have smooth walls with a trilete mark located in a thinning of the exine. They are relative large, ranging in diameter from 64µm to 85µm. These spores are comparable to species of the spore genus Retusotriletes. The spores of Aglaophyton have often been found preserved at various stages of germination (Lyon 1957) (see the section on spores for images of some of these).

 

Gametophytes

Both male and female gametophytes of this plant have been identified though to date only the male form has been formally described. The male gametophyte has been assigned the name Lyonophyton rhyniensis (Remy and Hass 1980b). This free-living gametophyte of Aglaophyton consists of an aerial axis that widens and terminates in a conspicuous cup-like structure which bears the antheridia (see inset right). Although smaller in size, the axis of the gametophyte is very similar to the sporophyte in its anatomy

 

Right: Longitudinal section of the male gametophyte Lyonophyton rhyniensis bearing antheridia (a). Click on the image for a close up! (scale bar = 1mm) (Copyright owned by University Münster).

 

Lyonophyton rhyniensis

 

Reconstruction

Right: Diagrammatic reconstruction of the sporophyte Aglaophyton major (after D.S. Edwards 1986) showing the nature of the creeping rhizome with localised rhizoid tufts; predominant dichotomous branching and fertile axes bearing a number of terminal fusiform sporangia.

Reconstruction

 

Model of Aglaophyton major

Right: Model of Aglaophyton major, sculpted by Stephen Caine for the Rhynie Research Group, University of Aberdeen.

 

Relationships

The systematic position of Aglaophyton, like a number of other Rhynie plants, remains unresolved because it shows a mixture of anatomical and morphological features that are not typical of any one group of plants. It shows many features characteristic of the rhyniophytes, a group of primitive plants, known only from fossils, showing simply branched naked stems of which Rhynia is an example. The water-conducting cells of the xylem strand do not show thickenings and as such are more reminiscent of the hydroids of some bryophytes (a group of plants including mosses and liverworts). Since the xylem of Aglaophyton does not possess true tracheids, this suggests it is not a true vascular plant.

 

Palaeoecology

Aglaophyton was a common plant and significant component of the Rhynie ecosystem during the Early Devonian. It appears to have preferred growing on litter-covered, organic-rich surfaces and was never a primary coloniser of sinter substrates, though the creeping rhizomal axes of Aglaophyton probably allowed it to occupy large surfaces of substrate. It occasionally grew as monotypic stands but is often seen associated with other plants, particularly Nothia, Asteroxylon, Horneophyton and occasionally Rhynia.

Since Aglaophyton exhibits stomata on the rhizomal axes as well as the upright aerial axes, it seems likely that the plant colonised  mainly dry substrates. The fact that the cuticle and stomata of Aglaophyton display adaptations to prevent water loss suggests it could also tolerate periods of drought (Powell 2000b).

However, the matrix of chert beds containing Aglaophyton in growth position occasionally contain the charophyte Palaeonitella, in association with the freshwater crustacean Lepidocaris and the 'aerial' axes of the plant may display infestation by chytrids (chytridomycetes) which are a type of tiny, simple fungi that thrive in damp and especially aquatic conditions. Another feature noted by Remy and Hass (1996) is the association of germinating Aglaophyton spores with these aquatic elements. Although the plant could probably also tolerate humid or damp conditions, this does not necessarily mean that Aglaophyton lived in standing water, perhaps these particular beds represent instances where areas colonised by the plant were occasionally flooded. It does seem likely, however, that wet conditions may have been necessary for spore germination.