Cooksonia were small, a few centimetres tall, and had a simple structure: They didn't have leaves or flowers. They had a simple stalk, that branched a few times. Each branch ended in a Sporangium, a rounded structure that contained the spores. No specimen has been found attached to roots. Either it connected to the ground with very fine root hairs, or the fossils are of fragments. Some specimens have a dark stripe in the centre of their stalks which is interpreted as being the remains of water carrying tissue. Not all specimens have this stripe, either some Cooksonia lacked vasular tissue, or it was destroyed in the fossilization process.
The relationships between the known species of Cooksonia and modern plants remain unclear. They appear to represent plants that are near to the branching between Rhyniophyta and to the club mosses. It is considered likely that Cooksonia is not a Clade but rather represents an evolutionary grade.
Five species of Cooksonia have been clearly identified. C. pertoni, C. hemisphaerica, C. cambrensis, C. caledonica and C. paranensis. They are distiguished primarily by the shape of the sporangia.
The Cooksonia were discovered by W.H. Lang in 1937 and named in honour of Isabel Cookson, with whom he had collaborated.