Asteroxylon

Above: Transverse cross-sections through aerial axes of Asteroxylon mackiei showing stellate xylem strand (x) and 'leaf traces' (t) (scale bar = 1mm).

Introduction

Morphology

Relationships

Palaeoecology

Introduction

Asteroxylon, one of the better known Rhynie plants, was originally described by Kidston and Lang (1920b) and assigned the name Asteroxylon mackiei. They noted fertile elements probably belonging to Asteroxylon within their material; however, the fertile elements of the sporophyte were not finally resolved until Lyon (1964) discovered sporangia in organic connection with the plant and he concluded that the fertile axes observed by Kidston and Lang were in fact of another new plant, Nothia aphylla. To date the gametophytes of Asteroxylon remain unknown. The overall morphology and palaeoecology of Asteroxylon is outlined below.

Morphology

Aerial Axes

The aerial axes of Asteroxylon exhibit a maximum diameter of 12mm and possess characteristic 5mm long scale-like 'leaves' or enations surrounding each axis (see inset below right). Asteroxylon is perhaps the largest of the known plants from the chert, in life probably attaining a height above ground of about 40cm (it's rhizomes penetrating the substrate to a depth of up to 20cm). Branching is dichotomous and monopodial.

One of the characteristic features of Asteroxylon are the scale like enations that emerge from the epidermis in a spiral arrangement around the axes. These are not true leaves since they do not possess a vascular strand (see inset right). The cuticle on both axes and enations bear abundant stomata with distinctive dark-coloured guard cells. The surface of the epidermis of the enations varies from smooth to papillate whereas that of the axes is smooth.     Right: Longitudinal section of the outer cortex and epidermis of an aerial axis of Asteroxylon mackiei showing three non-vascularised scale-like enations or 'leaves' (e) (scale bar = 2mm).

The cortex may be divided into a narrow outer zone of closely packed cells and a broader inner cortex that can be further subdivided into three zones: an outer and inner layer of compact cells with a trabecular middle layer of elongate cells with a well-developed inter-cellular air space network. Occasionally the inner zones of the cortex display fungal infestation (see insert below right).

The vascular strand is quite distinctive. Asteroxylon possess an actinostele, in other words the vascular strand appears star shaped or stellate in transverse cross-section (see insert below left and heading photograph). The xylem is exarch to locally mesarch where the protoxylem occurs at the ends of the 'lobes' of the actinostele, and displays spiral thickenings (see insert below right). Phloem tissue is best developed between the 'lobes' of the xylem strand. 'Leaf traces' are often seen where vascular tissue splits from the central stele. These traces end at the bases of the enations.

Above: Transverse cross-section through a prostrate aerial axis of Asteroxylon showing the distinctive lobed stellate xylem strand (x) (scale bar = 2mm).	Above: Slightly oblique longitudinal cross-section of the xylem strand of Asteroxylon (x) showing spiral thickenings. Click on the image for a close up! (scale bar = 500µm).

Rhizomal Axes

Sporangium

Reconstruction

Right: Reconstruction of Asteroxylon mackiei by Kidston and Lang (1920b, 1921a) showing aerial axes, dichotomous and lateral branches with scale-like 'leaves' or enations and naked cylindrical rhizomes.

Relationships

Asteroxylon is rather more anatomically complex than the other known Rhynie chert plants. The plant is considered a true lycophyte (a group of plants which includes the 'club mosses') based on the structure of its apical meristem (Hueber 1992) together with the characteristic exarch actinostele and the lateral disposition of the sporangia. The spirally thickened and reticulate cell walls of the xylem cells are also typical of lycophytes (Kenrick & Crane 1991). Modern lycophytes are also characterised by their enations or microphylls which possess a single vascular strand. In Asteroxylon the vascular trace stops at the base of the enations which may suggest the plant represents an intermediate stage in the evolution of lycopsid leaves.

Palaeoecology

Asteroxylon is quite commonly encountered in a number of chert beds and apparently formed a significant component of the Rhynie flora during the Early Devonian. Its rhizomes are usually found traversing plant litter and the plant is primarily found to occur in situ with two or more other genera, commonly Nothia, Rhynia and Aglaophyton and occasionally Ventarura. It seems therefore that Asteroxylon primarily lived as part of a diverse plant community rather than as monotypic stands (Powell et al. 2000b).

The fact that Asteroxylon possesses an extensive, substrate-penetrating 'root' system suggests it was capable of exploiting larger volumes of water and nutrients than the other Rhynie plants. Also the presence of microphylls increase the surface area of the aerial axes. This would have created a larger photosynthetic surface and with the greater density of stomata the plant possesses would also have meant greater efficiency in gaseous exchange and transpiration. It is likely that Asteroxylon could tolerate quite dry habitats compared with most of the other Rhynie flora..