A vascular conducting strand in the early land plant Cooksonia

THE late Silurian-early Devonian genus Cooksonia, characterized by smooth isotomously branching axes and solitary, terminal sporangia¹, has long been regarded as the archetypal vascular plant because of its age and simplicity of organization. The discovery of stomata, sterome² and thick-walled spores^1,3 in Cooksonia pertoni and C. hemisphaericaconfirmed its land-plant status, but tracheids have never been demonstrated in attached axes⁴. Here we report on tubes with differentially thickened walls typical of tracheary elements found in the central region of axes of Lower Devonian unequivocal C. pertoni, vindicating Lang's belief that Cooksonia was a vascular plant¹.