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.