Sachs maintain, in a lecture to his class, that the descent of the Cycads could be traced, not merely from Ferns, but from a definite family of Ferns, the Marattiaceae, a view which, though in a somewhat crude form, anticipated more modern ideas.
Williamson appears to have been the first to recognise the presence, in the Carboniferous flora, of plants combining the characters of Ferns and Cycads. (See especially his "Organisation of the Fossil Plants of the Coal-Measures", Part XIII. "Phil. Trans. Royal Soc." 1887 B. page 299.)This conclusion was first reached in the case of the genera Heterangium and Lyginodendron, plants, which with a wholly fern-like habit, were found to unite an anatomical structure holding the balance between that of Ferns and Cycads, Heterangium inclining more to the former and Lyginodendron to the latter. Later researches placed Williamson's original suggestion on a firmer basis, and clearly proved the intermediate nature of these genera, and of a number of others, so far as their vegetative organs were concerned. This stage in our knowledge was marked by the institution of the class Cycadofilices by Potonie in 1897.
Nothing, however, was known of the organs of reproduction of the Cycadofilices, until F.W. Oliver, in 1903, identified a fossil seed, Lagenostoma Lomaxi, as belonging to Lyginodendron, the identification depending, in the first instance, on the recognition of an identical form of gland, of very characteristic structure, on the vegetative organs of Lyginodendron and on the cupule enveloping the seed. This evidence was supported by the discovery of a close anatomical agreement in other respects, as well as by constant association between the seed and the plant. (F.W. Oliver and D.H. Scott, "On the Structure of the Palaeozoic Seed, Lagenostoma Lomaxi, etc." "Phil. Trans. Royal Soc." Vol. 197 B.
1904.) The structure of the seed of Lyginodendron, proved to be of the same general type as that of the Cycads, as shown especially by the presence of a pollen-chamber or special cavity for the reception of the pollen-grains, an organ only known in the Cycads and Ginkgo among recent plants.
Within a few months after the discovery of the seed of Lyginodendron, Kidston found the large, nut-like seed of a Neuropteris, another fern-like Carboniferous plant, in actual connection with the pinnules of the frond, and since then seeds have been observed on the frond in species of Aneimites and Pecopteris, and a vast body of evidence, direct or indirect, has accumulated, showing that a large proportion of the Palaeozoic plants formerly classed as Ferns were in reality reproduced by seeds of the same type as those of recent Cycadaceae. (A summary of the evidence will be found in the writer's article "On the present position of Palaeozoic Botany", "Progressus Rei Botanicae", 1907, page 139, and "Studies in Fossil Botany", Vol. II. (2nd edition) London, 1909.) At the same time, the anatomical structure, where it is open to investigation, confirms the suggestion given by the habit, and shows that these early seed-bearing plants had a real affinity with Ferns. This conclusion received strong corroboration when Kidston, in 1905, discovered the male organs of Lyginodendron, and showed that they were identical with a fructification of the genus Crossotheca, hitherto regarded as belonging to Marattiaceous Ferns. (Kidston, "On the Microsporangia of the Pteridospermeae, etc.""Phil. Trans. Royal Soc." Vol. 198, B. 1906.)The general conclusion which follows from the various observations alluded to, is that in Palaeozoic times there was a great body of plants (including, as it appears, a large majority of the fossils previously regarded as Ferns) which had attained the rank of Spermophyta, bearing seeds of a Cycadean type on fronds scarcely differing from the vegetative foliage, and in other respects, namely anatomy, habit and the structure of the pollen-bearing organs, retaining many of the characters of Ferns. From this extensive class of plants, to which the name Pteridospermeae has been given, it can scarcely be doubted that the abundant Cycadophyta, of the succeeding Mesozoic period, were derived. This conclusion is of far-reaching significance, for we have already found reason to think that the Angiosperms themselves sprang, in later times, from the Cycadophytic stock;it thus appears that the Fern-phylum, taken in a broad sense, ultimately represents the source from which the main line of descent of the Phanerogams took its rise.
It must further be borne in mind that in the Palaeozoic period there existed another group of seed-bearing plants, the Cordaiteae, far more advanced than the Pteridospermeae, and in many respects approaching the Coniferae, which themselves begin to appear in the latest Palaeozoic rocks.
The Cordaiteae, while wholly different in habit from the contemporary fern-like Seed-plants, show unmistakable signs of a common origin with them.