Evidence for a conducting strand in early Silurian (Llandoverian) plants: implications for the evolution of the land plants

Abstract

Macerations of fragmented plant compressions of Silurian (Llandoverian) age yield sheets of organic material bearing ridges and depressions (interpreted as corresponding to surficial dimensions of cells), fragments of smooth walled tubular elements, and fragments of tubular elements with differentially thickened walls (“banded tubes”). A fragment of tissue (1.2 mm long), consisting of smooth walled tubes (17 ± 6.9 μm in diameter), surrounding 2–3 larger (18.8 ± 2.1 μm in diameter) banded tubes, was isolated from an irregularly shaped, relatively large (1 × 3 mm) compression. Comparisons between (1) fragments of tubular cell types, not organized into strands, isolated from 122 compressions, and (2) dispersed tubular cell types previously reported from the Massanutten Formation and other Silurian formations (Tuscarora and Clinton), reveal no significant morphologic differences. Comparisons between the organization of smooth-walled and banded tubular cell types found in the tissue strand and the organization of cell types in nematophytic plants (Nematothallus, Nematoplexus, Prototaxites) indicate a similarity in construction (tubular) but a lack of correspondence in organization. The strand of tissue is interpreted as representing part of the internal anatomy of a nonvascular land plant of unknown taxonomic affinity. On the basis of analogy with present-day embryophytes, the strand of tubular cell types is inferred to have functioned as a conductive tissue. The significance of “banded tubes” in Silurian strata is discussed, and it is concluded that, until more is known about the anatomy, morphology, and biochemistry of the parent plant(s), the habitat and systematic affinity of these organisms are conjectural.

The development of early terrestrial ecosystems

“…Edwards & Davies, 1990;Fanning, Edwards & Richardson, 1990) and it persisted into the early Devonian. The recent demonstration of tracheids in LowerDcvonian C. pertoni confirms its vascular status (Edwards, Davies & Axe, 1992), but tracheids have not been seen in other Devonian rhyniophytoids nor in Silurian fertile axes (Edwards & Rogerson, 1976;Niklas & Smokovitis, 1983), As the Cooksonia tracheids were very small and few, failure to demonstrate them in these taxa is understandable. A growing body of evidence from late Silurian sporangia has revealed that some of the spores in the dispersed record, particularly the oldest ones (e.g.…”

“…Maceration of circular, elliptical and coalified compressions with irregular outlines, just a few millimetres in diameter from Llandovery sediments (Pratt et al, 1978) have yielded sheets of organic matter with possible cellular imprints and fragments of wefts of both smooth and ornamented tubes (Niklas & Smokovitis, 1983). In one example longitudinally aligned smooth tubes surround similarly orientated banded forms.…”

The Structure and Function of Xylem in Seed-Free Vascular Plants: An Evolutionary Perspective

“…Fossil spore and possibly micro-fossil records suggest that vascular plants made their appearance in the Silurian (Niklas and Smocovitis 1983 ;Kenrick and Crane 1997 ;Taylor et al 2009 ), with the Lower Devonian giving rise to a diversity of plants with transport tissues (Niklas 1985 ). Of these proto-vascular plants, the Rhyniophytes (so-named for the Early Devonian Rhynie Chert locality) are probably the best characterized.…”

Cells and tissues in the vegetative sporophytes of early land plants

“…Such ornamented tubes are usually found within wefts dominated by associations of smooth-walled smaller tubes often showing no preferred orientation and interpreted as part of a thalloid organism (Lang, 1937;Edwards, 1982;Strother, 1988). However Lang thought that the wider tubes may have been concentrated towards the interior of the thallus, and Niklas & Smocovitis (1983) isolated a strand with longitudinally aligned banded tubes from a small thalloid compression. In contrast, Nematasketurn Burgess & Edwards, 1988 differs from the nematophyte, Prototaxites, mainly in its possession of tubes with internal annular to anastomosing thickenings.…”