A couple of specimens from the older Mesozoic have been recently described, with well-preserved structure, and they belong to the family of the Osmundas (the so-called “flowering ferns”, because of the appearance of special leaves on which all the sporangia are crowded), and show in the anatomical characters of their stems indications that they may be related to an old group, the Botryopterideæ, in which are the most important of the Palæozoic ferns.
In the Palæozoic rocks there are numerous impressions as well as fern petrifactions, but in the majority of cases the connection between the two is not yet established. There were two main series of ferns, which may be classed as belonging to
I. Marattiaceæ.
II. Botryopterideæ.
Of these the former has still living representatives, though the group is small and unimportant compared with what it once was; the latter is entirely extinct, and is chiefly developed in the Carboniferous and succeeding Permian periods.
The latter group is also the more interesting, for its members show great variety, and series may be made of them which seem to indicate the course taken in the advance towards the Pteridosperm type. For this reason the group will be considered first, while the structure of the Pteridosperms is still fresh in our minds.
The Botryopterideæ formed an extensive and elaborate family, with its numerous members of different degrees of complexity. There is, unfortunately, but little known as to their external appearance, and almost no definite information about their foliage. They are principally known by the anatomy of their stems and petioles. Some of them had upright trunks like small tree ferns (living tree ferns belong to quite a different family, however), others appear to have had underground stems, and many were slender climbers.
Fig. 86.—Stele of Asterochlaena, showing its deeply lobed nature
In their anatomy all the members of the family have monostelic structure (see [p. 62]). This is noteworthy, for at the present time though a number of genera are monostelic, no family whose members reach any considerable size or steady growth is exclusively monostelic. In the shape of the single stele, there is much variety in the different genera, some having it so deeply lobed that only a careful examination enables one to recognize its essentially monostelic nature. In [fig. 86] a radiating star-shaped type is illustrated. Between this elaborate type of protostele in Asterochlaena, and the simple solid circular mass seen in Botryopteris itself ([fig. 88]) are all possible gradations of structure.