Rhynie Chert Flora:

Basic Information on the Higher Land Plants

 

Introduction

There are seven genera of terrestrial macroplants described from the Rhynie chert. Of these, five are considered to be true vascular plants, displaying tracheids in the water-conducting tissue, but the status of Aglaophyton and Nothia, which appear to lack tracheids, remains in doubt. Rhynia and Aglaophyton are the most abundant, Asteroxylon, Nothia and Horneophyton fairly common, and Trichopherophyton and Ventarura scarce. These plants seldom exceed 20cm in height.

Aglaophyton major

A simple plant with a creeping rhizome and smooth, naked, upright axes. This plant grew to around 15cm in height and its axes were up to 6mm in diameter. The rhizomes occasionally had bulges bearing tufts of rhizoids for taking up water and nutrients. Branching was mainly dichotomous and fertile axes terminated with pairs of fusiform sporangia (see inset right). Aglaophyton was a sporophyte, the male gametophyte stage of the plant, called Lyonophyton rhyniensis, was much smaller, the upright axis ending in a cup-like structure that bore the antheridia.

The water-conducting cells in this plant did not have thickenings like true vascular plants, being more similar to those seen in some modern bryophytes (a group including mosses and liverworts). The simply branched naked stems are more comparable with the extinct rhyniophytes and as such its systematic position remains unknown.

Aglaophyton grew mainly on dry, litter covered, organic-rich substrates, on its own as monotypic stands or with other Rhynie plants, though it seems to have required wet conditions for germination.

Right: Reconstruction of Aglaophyton major with fertile upright axes bearing fusiform sporangia (after D. S. Edwards 1986).

 

Reconstruction of Aglaophyton major

Asteroxylon mackiei

One of the more advanced and complex Rhynie plants, Asteroxylon had an extensive subterranean system of branching rhizomes. The aerial upright axes of the plant grew to about 40cm in height with a maximum diameter of 12mm. Branching was dichotomous and monopodial. The aerial axes possessed scale like 'leaves' or enations (see inset right). Fertile axes bore stalked kidney-shaped sporangia attached between the 'leaves' and the stem.

In cross-section the water-conducting strand of this plant forms a characteristic star-shape pattern from which smaller strands radiate to meet the base of the 'leaves'.

Asteroxylon is a member of a group of plants called the lycophytes which includes modern club mosses.

Asteroxylon mainly grew in organic-rich soils as part of a diverse community together with other Rhynie plants and could probably tolerate quite dry habitats.

Right: Reconstruction of Asteroxylon mackiei (after Kidston & Lang 1921a).

 

Reconstruction of Asteroxylon mackiei

Horneophyton lignieri

Horneophyton comprised naked upright aerial axes with a subterranean bulbous, corm-like rhizome bearing rhizoid tufts. The aerial axes were smooth and naked, growing up to 20cm in height with a maximum diameter of 2mm. Branching was mainly dichotomous and repeated. Fertile axes terminated in branched, tubular sporangia (see inset right) that internally possessed a central 'strand' or collumella. Horneophyton was a sporophyte, the female gametophyte stage of the plant, called Langiophyton mackiei, was much smaller, the upright axis ending in a cup-like structure that bore the archegonia.

The water-conducting cells in this plant, like true vascular plants, possessed thickenings. However, the presence of a collumella in the sporangia shows similarities with bryophytes (a group including mosses and liverworts). As such the systematic position of this plant remains uncertain.

Horneophyton preferred to grow on sandy and organic-rich substrates, often on its own as monotypic stands and probably flourished in damp to wet conditions.

Right: Reconstruction of Horneophyton lignieri showing bulbous corm-like rhizomes with rhizoids; dichotomously branching aerial axes with branching terminal sporangia (based on Eggert 1974).

 

Reconstruction of Horneophyton lignieri
Nothia aphylla

Nothia possessed a laterally extensive, branched subterranean rhizome network, bearing a ventral ridge supporting tufts of rhizoids (see inset right). Locally branches turned upright to form the aerial axes. These axes were naked with a very irregular surface and showed repeated dichotomous branching, giving the plant a thicket-like appearance. Fertile terminal axes bore lateral kidney-shaped sporangia attached by stalks. Nothia was a sporophyte, the male gametophyte stage of the plant, called Kidstonophyton discoides, was much smaller, the upright axis ending in a cup-like structure with tubular outgrowths bearing the antheridia.

The water-conducting cells in this plant did not have thickenings like true vascular plants, being similar to those seen in some modern bryophytes (a group including mosses and liverworts). The stalked, lateral kidney-shaped sporangia are comparable with zosterophylls and the simply branched naked axes with rhyniophytes and as such its systematic position remains unknown.

Nothia preferred to grow in sandy soils and plant litter, on its own or with other plants.

Right: Reconstruction of Nothia aphylla showing primary rhizomal axes with upright stems passing upwards into aerial axes with distinctive irregular epidermis. Terminal branches locally bearing lateral sporangia (based on Kerp et al. 2001).

 

Reconstruction of Nothia aphylla
Rhynia gwynne-vaughanii

Rhynia was one of the most common plants in the Rhynie ecosystem. Like Aglaophyton it was completely subaerial with creeping, branched rhizomal axes and smooth, naked upright axes (see inset right). The plant grew to a height of about 20cm, individual axes being up to 3mm in diameter. Branching was dichotomous and adventitious or monopodial. The plant possessed curious hemispherical projections on its axes, those on the rhizomes bearing tufts of rhizoids. Fertile axes bore terminal fusiform sporangia.

Rhynia is a member of an extinct group of primitive plants called the rhyniophytes, characterised by their simple branching and naked stems.

Rhynia commonly grew in thickets, typically on its own as monotypic stands and was often an early coloniser of well-drained sinter and sandy substrates. It also grew with other plants and was tolerant of a wide range of habitats.

Right: Reconstruction of Rhynia gwynne-vaughanii showing two fertile branches with terminal sporangia (rhizomal axes not shown) (after D. S. Edwards 1980).

 

Reconstruction of Rhynia gwynne-vaughanii
Trichopherophyton teuchansii

Trichopherophyton was quite a rare plant in the Rhynie ecosystem. The height of this plant is uncertain but the aerial axes had a maximum diameter of 2.5mm. Branching was mainly dichotomous. Subterranean rhizomal axes were smooth with small blunt-tipped emergences which probably acted as rhizoids. Aerial axes bore curious spiny projections (see inset right) and the tips of the axes displayed a feature called circinate vernation. Fertile terminal axes bore lateral, stalked, kidney-shaped sporangia, also bearing spiny projections.

The water-conducting cells in this plant, like true vascular plants, possessed thickenings. This together with the shape and position of the sporangia suggest Trichopherophyton belongs to a group of plants called the zosterophylls.

Trichopherophyton was a late coloniser of organic-rich substrates, always growing with other Rhynie plants as part of a diverse flora.

 

Trichopherophyton teuchansii

Above: Longitudinal section of an aerial axis showing conspicuous unicellular spinose projections (s) (scale bar = 500Ám).

Ventarura lyonii

Ventarura is the most recently discovered higher land plant from the Windyfield chert. The height of this plant is uncertain but was at least 12cm and the aerial axes had a maximum diameter of 7.2mm. Branching was mainly dichotomous and repeated (see inset right). Subterranean rhizomal axes were smooth with small blunt-tipped emergences which probably acted as rhizoids. Aerial axes bore curious peg-like projections. Characteristically, internally the axes had a lignified middle layer to the cortex called the sclerenchyma. Fertile terminal axes bore lateral, stalked, kidney-shaped sporangia.

The water-conducting cells in this plant, like true vascular plants, possessed thickenings. This together with the shape and position of the sporangia suggest Ventarura belongs to a group of plants called the zosterophylls.

The palaeoecology of Ventarura is not fully known, but it probably grew in localised patches, at least in the vicinity of freshwater ponds and probably in sandy and organic-rich substrates.

 

Ventarura lyonii

Above: Polished slab of Windyfield chert showing aerial and rhizomal axes of Ventarura lyonii. Many of these axes are inverted and therefore not in growth position.