A fossil Pteridophyta; RHYNIA

RHYNIA; The fossil Pteridophyta Above: Transverse sections through axes of Rhynia gwynne-vaughanii. The star-shaped axes, right of centre, are a result of shrinkage during decay and desiccation of the stems (scale bar = 1mm).   Introduction One of the first Rhynie chert plants to be described and perhaps the most abundant is the form Rhynia. The plant was originally described and classified by Kidston and Lang in 1917, 1920a and assigned the species name Rhynia gwynne-vaughanii. Another plant now known as Aglaophyton, possessing a slightly similar anatomy, was originally described by Kidston and Lang in their 1917 paper as also belonging to Rhynia gwynne-vaughanii, but differs significantly in its vascular anatomy and was subsequently reassigned (see also Aglaophyton). Unequivocal gametophytes of Rhynia have recently been discovered (Kerp et al. in press) and will be illustrated here once published. The overall morphology and palaeoecology of Rhynia is outlined below.   Morphology 'Aerial' Axes The axes of Rhynia exhibit a maximum diameter of 3mm and the plant probably attained a height of up to 20cm. The aerial or rather the 'upright' axes are cylindrical, naked and upwardly tapering. The branching of Rhynia is both dichotomous and adventitious or monopodial, with dichotomy occurring at an angle between 17 and 350 (D. S. Edwards 1980). The surface of the axis bear numerous conspicuous emergences or hemispherical projections (see insert below right) from the epidermis which are occasionally located beneath stomata and at the base of adventitious branches and in other instances internally display fungal activity and dark necrotic tissue. Right: Transverse cross section through a stoma showing the two guard cells (g) with the stomatal chamber (c) beneath (scale bar = 20µm) (Copyright owned by University Münster).  The stomata typically appear circular on the cuticle surface and are flanked by two guard cells (see inset above right). The cells of the cuticle often exhibit a median ridge giving the cuticle a flanged appearance. Right: Transverse cross section through the cuticle and epidermis of Rhynia showing the median ridges on the cuticle (r) (click on the image for a close up!) (scale bar = 100µm) (Copyright owned by University Münster). The cortex is divided into two distinct zones separated by a brown line of amorphous material. The outer cortex comprises closely packed uniform cells, becoming noticeably elongate below the hemispherical projections. The inner cortex comprises uniform cells with a well-developed inter-cellular air space network and commonly exhibits vesicular arbuscular mycorrhizae. The vascular tissue or stele comprises a zone of 'phloem' of uniform thickness surrounding a central xylem strand. The phloem cell walls exhibit what appear to be pores (Satterthwait and Schopf 1972, Kenrick and Crane 1991). The xylem strand is terete, exhibiting endarch cell development. The xylem cells also exhibit annular and rare spiral thickenings. Above: Transverse section through a Rhynia axis showing xylem (x), 'phloem' (p), inner cortex (ic), outer cortex (oc), epidermis (e), cuticle (c) and a small hemispherical projection (h) (scale bar = 1mm) (Copyright owned by University Münster). Rhizomal Axes Like Aglaophyton, Rhynia laid directly on the ground surface. Rhynia possesses a creeping rhizome displaying repeated dichotomous and adventitious branching, locally turning upright, passing upwards into the 'aerial' axes. The rhizomal axes are cylindrical and naked and generally exhibit a similar morphology and internal anatomy to the aerial axes though they lack stomata. The other main difference is exhibited by the hemispherical projections, which commonly support tufts of unicellular rhizoids (see inset right). However, occasionally hemispherical projections on the upright 'aerial' axes may also bear rhizoids. Above: A hemispherical projection on a Rhynia axis bearing tufts of rhizoids (scale bar = 250µm). Sporangium The sporangia of Rhynia are not particularly common. They are fusiform, displaying a maximum size of 3.6mm by 2.4mm. The disposition of the sporangia is terminal, being located on the adventitious branches of fertile aerial axes. No dehiscence mechanism has been observed though a dark cellular layer or 'sterile pad' at the base of the sporangium has been interpreted as a site of abscission by D. S. Edwards (1980) (see inset right). The sporangial wall comprises three layers: an outer epidermis, a poorly preserved parenchymatous layer and an inner tapetal layer. Above: A split, empty sporangium of Rhynia still attached to a prostrate, partially decayed axis showing the dark 'sterile pad' (a) (scale bar = 1mm) (Copyright owned by The Natural History Museum). Reconstruction Right: Diagrammatic reconstruction of the sporophyte Rhynia gwynne-vaughanii showing two fertile adventitious branches with terminal fusiform sporangia (rhizomal axes not shown) (after D. S. Edwards 1980). Relationships Rhynia gwynne-vaughanii being a naked, simply branched sporophyte has been assigned to a primitive group of vascular plants known only from fossils, and called the rhyniophytes. The presence of hemispherical projections on the axes remains a point of speculation and their like is not seen in any of the other Rhynie plants. Various interpretations have been proposed: Damage from arthropods sucking sap (Kevan et al. 1975). Wounding by nematodes, mites, parasites or fungi (Edwards and Selden 1993). Damage from splashes of hot water from geysers, or from volcanic ash (Kidston and Lang 1921a); an unlikely explanation considering the small size and disposition of the projections on the plant axes, and also since they have not been observed on any of the other Rhynie plants. Pant (1962) and Lemoigne (1968a, b) interpreted the hemispherical projections as sites of archegonia, though this interpretation has never been generally accepted. Palaeoecology Rhynia was the most common vascular plant in the Early Devonian ecosystem at Rhynie, at least in the areas of sinter deposition, both numerically and in terms of ground cover (Powell et al. 2000b). The plant appears to have been entirely subaerial with its naked, branching 'rhizomes' creeping across the ground surface with the upright portions of the plant growing to give a thicket-like appearance. Although the hemispherical projections on the 'rhizomes' bear the rhizoids for taking up water from the ground surface, the fact that those on the upright stems occasionally exhibit rhizoid tufts suggest Rhynia was also capable of taking up atmospheric water. Where found in growth position in the chert beds it is typically, though not exclusively the only in situ vascular plant present, and is commonly found above sandy chert layers and allochthonous litter horizons. This suggests Rhynia commonly grew as monotypic stands, an early colonizer of well-drained sinter and sandy substrates. However, the plant is also found associated with all other Rhynie plants, though only very rarely with Horneophyton. This suggests Rhynia was tolerant of a wide range of habitats and could also withstand interspecies competition within the Early Devonian ecosystem at Rhynie (Powell et al. 2000b).