Trichopherophyton

Trichopherophyton axes

Above: Transverse sections through rhizomal axes of Trichopherophyton teuchansii (scale bar = 1mm).

Introduction

Morphology

Palaeoecology

 

Introduction

Trichopherophyton is one of the more recent additions to the Rhynie chert list of vascular plants. It was formally described by Lyon & Edwards (1991) and assigned the species name Trichopherophyton teuchansii, and was the first true zosterophyll recorded from the cherts. This is the scarcest of the Rhynie plants having only been found in a few beds of chert.

A reconstruction of the whole plant has not yet been attempted and the gametophyte stage of the plant remains unknown. The overall morphology and palaeoecology of Trichopherophyton is outlined below.

 

 

Morphology

Aerial Axes

The aerial axes of the plant exhibit a maximum diameter of 2.5mm. The branching of Trichopherophyton is both dichotomous and pseudomonopodial and it is the only plant known in the Rhynie flora to display circinate vernation.

Generally the cuticle of Trichopherophyton is poorly preserved and as  a result stomata have not been observed, the rest of the epidermis, however, is usually well preserved and very often exhibits unicellular spinose projections emerging at 900 to the axis of the plant (see inset right). These spiny outgrowths are one of the most characteristic features of Trichopherophyton.

Right: Longitudinal section of an aerial axis showing epidermis with conspicuous unicellular spinose projections (s) (scale bar = 500µm).

Epidermal projections
The cortex can be divided into a distinct outer cortex comprising closely packed cells and an inner cortex or more loosely packed cells with an inter-cellular air-space network. Many cells of the inner cortex contain dark coloured residues (see inset right).

 

Right: Transverse section through an axis of Trichopherophyton teuchansii showing xylem (x), phloem (p), inner cortex (ic) and outer cortex (oc) (scale bar = 1mm).

Trichopherophyton axis

The vascular tissue comprises a very distinct xylem strand (see inset above right). It is sub-terete, exarch and displays both annular and spiral thickening. A narrow zone, uniform in thickness, of thin-walled cells surrounds the xylem strand and probably represents phloem (see inset above right).

 

 

Rhizomal Axes

Observed within many of the chert specimens containing the spinose aerial axes of Trichopherophyton are other generally more common axes of a slightly differing but distinctive morphology that are believed to represent the rhizomes of the plant although there remains no unequivocal evidence for organic continuity between the two.

These axes are generally slightly smaller than the aerial axes, having a maximum diameter of 2.3mm.

They exhibit a generally smooth epidermis, lacking the spinose outgrowths though epidermal cells may be papillate and occasionally may bear blunt-tipped emergences that may represent rhizoids. The cortex is quite narrow and less distinctly divided compared with the aerial axes, though again cells of the inner cortex commonly show dark coloured residues (see inset right). A zone of thin-walled cells surrounding the xylem strand may represent phloem. As with the aerial axes the sub-terete xylem strand is very distinctive being exarch and showing narrow annular and spiral thickenings (see inset right).

Trichopherophyton rhizome

Above: Slightly oblique section of a rhizomal axis clearly showing the sub-terete xylem strand (x) (scale bar = 500µm).

 

Sporangium

The sporangia or fertile elements of Trichopherophyton have been observed in a few specimens but have not as yet been fully described. However, sporangia do appear to be reniform (kidney-shaped) with dimensions up to a maximum 3.7mm by 2.5mm. They are attached laterally to the aerial axes with a vascularised sporangial stalk though their spatial distribution is uncertain. The sporangia appear to be bi-valved with a marginal dehiscence mechanism. Characteristically the sporangia display the same unicellular spinose projections emerging from the epidermis as seen in the aerial axes.

The in situ spores of Trichopherophyton appear to vary between 55 and 80µm in size, are retusoid and smooth walled and display a triangular thickening associated with the trilete mark. These spores are comparable to species of the spore genus Retusotriletes.

 

Relationships

Trichopherophyton teuchansii is undoubtedly a zosterophyll, fitting into the classification of Banks (1975), showing lateral, reniform sporangia with a well-developed marginal dehiscence mechanism as well as an exarch xylem strand with thickenings. However, there remain other features, although characteristic of this plant, that are rarely encountered in true zosterophylls; namely a sub-terete xylem strand and the presence of unicellular spiny epidermal projections on the aerial axes and sporangia.

The circinate vernation seen exclusively in Trichopherophyton is a feature that is also observed in most extant ferns. This suggests that the plant has a more advanced anatomy than that of the other Rhynie plants.

 

Palaeoecology

Generally Trichopherophyton appears to be relatively rare in comparison with the other vascular plants and has been found  to occur in only a few beds of chert. This may partially reflect the scarcity of the plant within the original ecosystem or may simply reflect sampling bias. However, within the few beds within which it occurs, it is preserved both in autochthonous growth position and as litter, but always as part of a diverse flora including Nothia, Horneophyton and locally Rhynia. It seems likely therefore that Trichopherophyton was a late coloniser of humus-rich substrates (Powell et al. 2000b).