Polypodiaceae.
Sections of petrified sporangia from the English Coal-Measures (Pteridotheca sp.) occasionally exhibit a striking resemblance to those of recent Polypodiaceae[927], but in the absence of material in which it is possible to recognise the true orientation of the sporangia, the exact position of the annulus is almost impossible to determine. We have as yet no satisfactory evidence of the existence of true Polypodiaceae in the Palaeozoic era. It is noteworthy that apart from the absence of ferns which can reasonably be included in this family, the anatomical features of the Botryopterideae (Coenopterideae) and of the Cycadofilices or Pteridosperms do not foreshadow those of Polypodiaceous ferns. On the other hand, as we have already noticed, anatomical characters of such families as the Gleicheniaceae, Hymenophyllaceae, and Schizaeaceae are met with in certain generalised Palaeozoic types. These facts are perhaps of some importance as supplying collateral evidence in favour of the relatively more recent origin of the dominant family of ferns in modern floras.
Fig. 279.
- Adiantides antiques (Ett.). (½ nat. size.)
- A. Lindsayoides (Sew.). (B′ nat. size.)
(A, after Kidston.)
The use of the generic name Adiantites for fern-like fronds of Lower Carboniferous age characterised by cuneate pinnules like those of species of Adiantum, suggests an affinity which is in all probability non-existent. It has been pointed out that this generic name was applied in the first instance to the leaves of the Jurassic plant Ginkgo digitata[928] and should, therefore, be discarded. Schimper[929] used the designation Adiantides, and Ettingshausen[930], more rashly than wisely, preferred Adiantum. The specimens described by Kidston[931] as Adiantides antiquus (Ett.) ([fig. 279], A) from the Carboniferous limestone of Flintshire are portions of tripinnate fronds bearing cuneate segments with numerous forked veins radiating from the contracted base of the lamina. It is not improbable, in view of Dr White’s[932] discovery of seeds on a very similar plant from the Pottsville beds of North America, that this characteristic Lower Carboniferous genus is a Pteridosperm.
From Jurassic rocks in various parts of the world numerous fossils have been described under the generic names Aspidium, Asplenium, Davallia, Polypodium, and Pteris. In the great majority of cases such records leave much to be desired from the point of view of students who appreciate the dangers of relying on external similarity between vegetative organs, and on resemblances founded on obscure impressions of sori. The generic term Woodwardites[933], which suggests affinity with the recent genus Woodwardia, has been used for Rhaetic plants belonging to the Dipteridinae.
A plant described as Adiantides Lindsayoides from Jurassic rocks of Victoria[934], characterised by marginal sori which appear to be protected by the folded-over edge of the leaflets, and by the resemblance of the pinnules to those of recent species of Lindsaya, may be a true Polypodiaceous fern; but in this case, as in many similar instances, nothing is known of the structure of the sporangia. Some sterile pinnae described by Yabe from Jurassic rocks of Korea as Adiantites Sewardi[935] may perhaps be identical with the Australian species.
In such a species as Polypodium oregonense Font., from Jurassic rocks of Oregon, the generic name is chosen because the “fructification seems near enough to that of Polypodium to justify the placing of the plant in that genus[936].” But the fact that no sporangia have been found is a fatal objection to this identification.