VI. FILICALES, or ferns ; these comprise not only the living ferns, with about i 5o gen era and over 6,000 species, but also a rich sequence of fossils, from the Palaeozoic Coenopteridaceae to the most recent strata.
I. Psilophytales.—This new class of plants was constituted by Kidston and Lang to receive certain fossils of early Devonian time, discovered by Dr. Mackie at Rhynie in Aberdeenshire. Its name is taken from the old genus Psilophyton of Dawson, and the class now comprises a number of other genera of like age, more recently discov ered, forming together a very distinctive flora. But of these only the sporophyte is known (fig. 5). The vege tative system consists of upward-growing, forked, aerial shoots that spring from a rhizome sometimes creeping, sometimes tuberous and mycorhizic; sometimes the underground branchlets are root like, forking in the substratum (Asteroxylon). The erect shoots are cylindrical, and were evidently green, covered by an epidermis, with stomata, and they are traversed by conducting strands of simple structure. Various superficial growths, often with the ap pearance of thorns or prickles, are borne upon the larger branches, as in Psilophyton and Asteroxylon. The plants were of low stature, and growing gregariously they must have looked rather like grass. The genera Hornea, Rhynia and Asteroxylon, described by Kid ston and Lang were so well preserved that their structure is as well known as though they were modern plants. The class stands conspicuously apart as leafless and rootless vascular plants.
The most distinctive feature of the class for diagnosis is that the large sporangia, protected by a wall of many layers and con taining numerous homosporous spores, are terminal on the vege tative twigs.
The very early existence thus demonstrated of leafless, and rootless and homosporous vascular plants, with distal sporangia of primitive construction is a fact of the first comparative im portance. It is true it does not demonstrate any nearer connection with the Algae, but as regards other relations the new facts are highly suggestive. Long ago it was remarked that the widest
gap in the sequence of plants was that between the Bryophytes and Pteridophytes. It is within this gap that the newly discovered fossils take their natural place, acting as synthetic links for the whole sequence of land-living, sporangium-bearing plants.
II. Psilotales.—Whereas the Psilophytales are known only by their fossil sporophytes, the Psilotales are represented by two living genera, Psilotum and Tmesipteris, of which both genera tions are now known. They form a natural family of the Psilo taceae. By their features they appear remarkably isolated among living plants, and their nearest affinity is to be sought among the Psilophytales and Sphenophyllales, both being classes of plants long extinct. These two genera appear in fact as living fossils.
They live epiphytically, or in soil rich in humus, and are root less. The green, more or less shrubby shoot is fixed in the sub stratum by much branched leafless rhizomes, which are infected by a mycorhizic fungus. The saprophytic nutrition by these is supplemented by photosynthesis in the green leafy shoots. In Psiloturn the aerial stems bifurcate, bearing small and simple scaly leaves, which however pass upwards into bifid "sporophylls," and between the two teeth is seated a large trilocular synangium, containing numerous homosporous spores (fig. 6). Tmesipteris resembles it in general character, but branching is infrequent, the leaves and "sporophylls" are larger, and the synagium has only two large loculi. The anatomy of the green stem of Psilotum, with its epidermis and stomata, its photosynthetic cortex and conducting protostele, is on the same general plan as that of Rhynia or Asteroxylon; the bifurcation and presence of minute leaves, and the rootless mycorhizic base all support the corn parison with the Devonian types of Rhynie.