Among the Sphenophyllums (although there was at first some confusion in his nomenclature) he gave the first correct account of the anatomy, and of the organization of the cone.
Plate XXIV
Cone of Calamostachys Binneyana; sporangia and sporangiophores
Drawn by Williamson
Concerning the Lycopods, the greater part of our knowledge is due to him. He described the structure in ten species referred to Lepidodendron, besides other allied forms, and placed our knowledge of the comparative anatomy, once for all, on a broad and secure basis. His great monograph of Stigmaria, by some considered his best work, is still our chief authority for the subterranean organs.
In the Ferns he made important contributions to our knowledge of the group now familiar to botanists as the Primofilices of Arber. In particular his account of the plant now known as Ankyropteris corrugata is still among the best we possess of any member of the family.
In Pteridosperms, to use the modern name, Williamson may fairly be called the discoverer of the important family Lyginodendreae. He appreciated their intermediate position, speaking of them, in 1887, as "possibly the generalised ancestors of both Ferns and Cycads."