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Rhynia a Fossil Pteridophyte from Scotland
- 1. Rhynia a fossile Pteridophyte Dr Manoj Joshi Department of Botany KCMT
- 2. Distribution and habitat • It was discovered by Kidston and Lang 1917 from old sand stone bed (middle devonian) of Rhynie in district aberdeenshire of scotland. • Two species of Rhynia ar known till this date namely R. major and R. gwynae-vaughani • These plant are grown in acidic soil neat peaty habitatenear volcanoes. • Sulpher vapour are found inenvironment • Plant are petrified* form • *(of organic matter) changed into a stony substance
- 3. Rhynia major
- 5. External feature • 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. • 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.
- 6. • 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.
- 8. 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. •
- 9. Transverse section (T.S.) of Aerial shoot and Rhizome: • Anatomically, the aerial shoots and rhizome are almost similar. T. S. of aerial shoot can be differentiated into three parts: epidermis, cortex and stele (Fig. 2 A). • (a) Epidermis: It was the outer-most surrounding layer. It was one cell thick and covered by thin cuticle. In aerial shoots it was interrupted at certain places by stomata but stomata (Fig. 2 B) were absent in rhizome. • (b) Cortex: Epidermis was followed by cortex. It is differentiated into outer cortex and inner cortex. The outer cortex was only 1-4 cells thick, thin walled and without intercellular spaces. The inner cortex had large intercellular spaces and its cells had chloroplast. It is thought that this was the chief photosynthetic region of the plant. The endodermis and pericycle layers were absent.
- 10. Stele: • The centre of the aerial shoot/rhizome was occupied by stele. The stele was a protostele (haplostele). The xylem was made up of annular tracheids and there were no sieve plates in phloem.
- 11. 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. • The sporangia were borne singly on the apices of some aerial branches, each sporangium being oval or slightly cylindrical structure with a little greater diameter than that of aerial branch on which it is developed. They were 12 mm long and 4 mm in breadth in R. major and 4 mm long and 1 mm broad in R. gwynne-vaughani.
- 12. A longitudinal section (L.S.) of sporangium shows that it had a five cells thick wall. The outermost layer was 1 cell thick cuticularized epidermis. It was followed by 3 cells thick middle layers of thin walled cells. The inner-most layer was 1 cell thick tapetum. The wall was surrounding a spacious sporangial cavity which was without columella and contained large number of spores. The spores were of same size and measured upto 60 µ in diameter. It means that Rhynia was homosporous. In many specimens the sporangium contained tetrahedral tetrads of spores (Fig. 3 B, C) which suggest that they were formed by reduction division and the plant bearing them represented the sporophytic generation. There was no special mechanism of sporangium dehiscence. The liberation of spores seems to have taken place by disintegration of the sporangial wall. Nothing definite about the gametophyte of Rhynia is known.
- 13. 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.
- 14. 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).