5. On the Fossil Osmundaceae.
This most important memoir gives detailed descriptions, illustrated by admirable microphotographs, of a number of stems of fossil ferns referred by the authors to the Osmundaceae.
Two of the specimens described were discovered in certain Jurassic rocks near Gore, Southland, the one by Mr. R. Dunlop, formerly of Orepuki, and the other by Mr. R. Gibb. Both plants are described as new species, under the names of Osmundites Dunlopi and O. Gibbiana respectively.
The two specimens agree in all essential characters with the stems of the modern Osmundaceae.
An examination of the distribution of the various sclerotic strands that occur at the base of the fully developed petiole of a number of living species of Osmundaceae showed that it was characteristic of, and practically constant in, each species examined, but it varies sufficiently to be suitable for a mark of comparison. Judged from this standpoint, the leaf-base of Osmundites Dunlopi comes very near that of Todea barbara of the present New Zealand flora; while that of O. Gibbiana, although in some respects unique, approaches nearest to O. regalis and O. javanica,
The most important character that shows any considerable variation in the living species (apart from Osmunda cinnamomea) is the extent of the interruption in the continuity of the xylem ring caused by the departure of the leaf-traces. Osmunda regalis represents one extreme, where the xylem ring is broken up into many distinct strands free from one another, while the other extreme is shown by Todea barbara and T. superba, in which the strands are fused with each other and with the xylem of the leaf trace, so forming continuous bands. As regards the two fossil species, Osmundites Dunlopi, with its continuous xylem ring, points in the direction of Todea barbara, while O. Gibbiana points in the direction of O. regalis, and the authors consider that amongst the living Osmundaceae, “so far as our data permit us to judge,” T. barbara shows most resemblance to O. Dunlopi, and O. regalis to O. Gibbiana. But until the sporophylls are known the species are to be kept in the fossil genus Osmundites.
An unnamed Osmundites in the British Museum collection, said to have come from New Zealand, was examined, and was found identical with O. Dunlopi.
Besides the two species mentioned above, all the recorded fossils of an Osmundaceous character are dealt with excepting one. A table is given showing the chronological order of the species, and a synopsis of their more important anatomieal features. This latter shows that the medullation of the stele and the subsequent breaking-up of the xylem ring takes place part passu with the advance from the older to the younger strata. The age of the fossils is as follows: Upper Permian, 5; Jurassic, 2; Upper Jurassic, 1; Lower Cretaceous, 1; Lower Eocene, 1; Lower Pliocene or Upper Miocene, 1.
Dealing with the ancestry of the Osmundaceae, the authors consider them, as a whole, as “an ascending series of forms whose vascular system is to be derived from a primitive protostele with a solid homogeneous xylem,” by the medullation of which and the subsequent breaking-up of the peripheral xylem ring thus formed into separate strands the typical osmundaceous stele has been derived.
By a consideration of the methods through which the medullation of the stele has come about, the authors find the existence of an intermediate stage in which the pith consists of tracheae mixed with parenchyma, and it becomes inevitable that the mixed pith of the Zygopterideae is of the same nature and origin as that of the Osmundaceae, especially as the authors believe the two families have descended from a common ancestor. Further, the authors postulate the discovery of a primitive zygopterid stele with a solid xylem mass, the central tracheae of which are short, and transitional toward a pith, as in two of the osmundaceous genera dealt with,
This prophecy was confirmed by the discovery of a stele exactly as anticipated by W. T. Gordon, a figure of which is given.
The monograph concludes with an attempt to explain the derivation of the specialized leaf-trace of the Zygopterideae from the simple primitive form of the Osmundaceae.