Early fossil land plants of simple construction

Although trilete spores are known from the mid-Ordovician and early Silurian, undisputed vascular plants are not found until halfway through the Silurian. A number of these lack true tracheids (“protracheophytes”) or are insufficiently well preserved to allow us to be sure of the presence of tracheids (“rhyniophytoids”).

The earliest accepted tracheophyte-like plant is Cooksonia. Several species are now known ranging from the upper Silurian to the Lower Devonian. Cooksonia was evidently widespread, occurring in a number of localities in North and South America and Europe. The plants were dichotomously branched (Fig. 6.1a) and probably formed swards, perhaps in swampy areas, a few centimetres in height. The axes, which were bare of any appendages, terminated in reniform (Fig. 6.1b and c) or globose sporangia with little evidence of predetermined sites of dehiscence. So far as known, Cooksonia was homosporous. Vegetative axes of some species have been found with a simple strand of tracheids (true tracheophytes), but others appear to lack them (“rhyniophytoids”). The presence of stomata-like pores has been confirmed only in forms from the earliest Devonian. There is as yet no evidence of extensive aerating systems in Cooksonia which might support the suggestion that carbon dioxide was taken up through the underground organs (as in Isoetes and a few other living plants).