Diversity in conducting cells in early land plants and comparisons with extant bryophytes

Abstract

Anatomical screening using scanning electron microscopy (SEM) of short lengths of smooth coalified axes (mesofossils) from a Lochkovian (Lower Devonian) locality in the Welsh Borderland, Shropshire has revealed extensive diversity in the architecture of centrally aggregated, elongate cells. At least 14 types have been discovered, each distinguished by variation in wall architecture and combination of the cells in the central strand. End walls have not been seen. These elongate cells may have smooth, uniformly thick or thin walls, walls with smooth projections either traversing or lining the lumen, or bilayered walls, the innermost perforated by pores of plasmodesmata dimensions. The latter type may be further divided on presence or absence of projections which may line the lumen, but usually cross it and are highly disorganized. Indeed, none of the cells shows the regularity associated with the secondary thickenings of tracheids, but the imperforate/pitted forms with projections superficially resemble the S-type tracheids of the Rhyniopsida in basic construction. Simply pitted types show greater similarity with the water-conducting cells (WCCs) of liverworts and Takakia. To facilitate direct comparison with bryophyte conducting elements, SEM studies were undertaken on the WCCs of a number of mosses and liverworts and on the leptoids of mosses, in conjunction with a range of degradation experiments designed to assess the fossilization potential of these cells. With the exception of polytrichaceous hydroids, the latter demonstrated the resilience of hydroids and leptoids to the chemical treatments. In addition, dehydration of the leptoids produced globular residues similar to those seen in some of the fossils. This combination of techniques raises the possibility that food-conducting cells might well be preserved in coalified fossils, and hence extends the interpretation of the functions of the elongate cells. Broadly speaking, imperforate bilayered examples may have been involved in water conduction, cells with globular residues with or without pitting involved in metabolite movement, and smooth walled examples with or without projections involved in support. The wider affinities of the plants which produced the axes remain equivocal and in the absence of sporangia it is impossible to assign them to a genus. However, this anatomical diversity in vegetative remains of extreme simplicity demonstrates far greater diversity in early land vegetation than is apparent from perusal of species lists.