PTERIDOSPERMS REPRESENTED BY SEED-BEARING LEAVES ONLY OR LEAVES IN
CONSTANT ASSOCIATION WITH SEEDS.
Before describing other genera represented by petrified vegetative
organs exhibiting in their anatomical features points of contact with
the Medulloseae, a short account is intercalated of some imperfectly
known seed-bearing fronds and seeds belonging to the Pteridosperms but
which do not afford sufficient data to admit of their reference to a
more precise position in a natural classification.
_Pecopteris Pluckeneti_ (Schlotheim).
In the section in Volume +ii.+ devoted to the genus _Pecopteris_
reference was made to the species _P. Pluckeneti_[439], further
treatment being deferred until other Pteridosperms had been described.
The fern-like fronds originally described by Schlotheim as _Filicites
Pluckeneti_[440] and afterwards transferred by Brongniart and other
authors to _Pecopteris_[441] are now recognised as the leaves of a
Pteridosperm. Some doubt has been expressed as to the specific identity
of the specimens figured by Schlotheim and Brongniart respectively,
but Potonié’s examination of the type-specimen of the earlier author
convinced him that Brongniart’s leaves were correctly named. The
large fronds of _Pecopteris Pluckeneti_ are characterised by the
bifurcation of the principal axis which bears opposite pairs of bi-
or tri-pinnate branches and in the angles of the bifurcations of the
rachis undeveloped buds occur on prolongations of the axis, a habit
recalling recent species of _Gleichenia_[442] (figs. 225 and 226, vol.
+ii.+). The variation in the form of the pinnules is shown in fig.
442, A, which represents both the apical portion and parts of pinnae
10 cm. lower on the rachis of a large leaf from the Coal Measures of
Radstock. The species is characteristic of the Upper Coal Measures and
is recorded also from Permian strata. In 1883 Sterzel adopted for this
species the generic name _Dicksonites_ because of the occurrence of
shallow circular cups at the end of the lowest lateral vein on some of
the pinnules which he believed to be sori of the Cyatheaceous type.
The cups have an involute margin and occasionally a small scar in the
centre (fig. 442, B). Stur[443] declined to accept Sterzel’s evidence
as satisfactory and suggested a fungal origin for the sorus-like
impressions, a view, as Sterzel objected, that is rendered improbable
by the constant position of the single cups on several pinnules. The
nature of Sterzel’s ‘sori’ has not been demonstrated: it is suggested
by Grand’Eury[444] that they mark the position of microsporangia.
Some seed-bearing specimens in Dr Kidston’s collection show cups,
like those figured by Sterzel, on pinnules from which the seeds have
fallen, and it is not improbable that they are the scars of seeds. In
1905 Grand’Eury published a description and photographs of specimens
of _P. Pluckeneti_ from the St Étienne coal-field showing hundreds of
well-preserved seeds, many of them attached to pinnules characterised
by a very slightly reduced lamina. Some fronds were found to be
entirely fertile, while others bore both sterile and fertile pinnae.
The smallest seeds, 5 mm. long and 3 mm. broad, were found at the tips
of unexpanded leaves: the mature seeds, only slightly larger, agree in
their broadly oval form and narrow marginal ‘wing’ with small examples
of _Samaropsis_[445]. The seeds were figured by Grand’Eury[446] in
an earlier work as _Carpolithes granulatus_. They are believed to
have hung free from the lamina, a conclusion based on the position
of the seeds relative to the plane of the pinnule in well-preserved
examples. Prof. Zeiller informs me that he is by no means certain
that Grand’Eury’s seed-bearing fronds should not be referred to
_Pecopteris Sterzeli_; but as that species and _P. Pluckeneti_ are
very closely allied forms and may well have borne the same general
type of fructification, the question of specific difference does not
affect the significance of Grand’Eury’s discovery. A statement was made
in vol. +ii.+[447], quoted from Grand’Eury, that the fronds of _P.
Sterzeli_ were borne on a _Psaronius_ stem, but Prof. Zeiller told me
that in his opinion the fronds and stem are merely in association and
not in organic contact. It is probable that the _Psaronius_ stem bore
fronds of some species of _Pecopteris_ with sori of the _Asterotheca_
or _Scolecopteris_ type and not seeds. _Pecopteris Pluckeneti_ and _P.
Sterzeli_ are no doubt the fronds of a Pteridosperm[448], but apart
from the seed-impressions there is no evidence as to the nature of
the reproductive organs or stem. The form of the seeds with a fairly
thick sarcotesta, which gives them a winged appearance, suggests a
member of the Medulloseae rather than a plant with seeds like those of
_Lyginopteris_ and _Heterangium_.
[Illustration: Fig. 442. _Pecopteris Pluckeneti._ A, apex and lower
pinnae (Kidston Coll., 234); B, pinnule with supposed sorus (after
Sterzel); C, pinnule with seeds (after Zeiller).]
_Eremopteris artemisaefolia_ Steinberg with _Samaropsis acuta_ Lindley
and Hutton.
The generic name _Eremopteris_ was instituted by Schimper for a type
of frond from the Coal Measures of Newcastle described by Sternberg as
_Sphenopteris artemisaefolia_. He included also a second species, _E.
Neesii_, from the Permian of Bohemia: this was removed by Zeiller to
_Callipteris_. The type-species of _Eremopteris_ is included in this
chapter on the ground that the almost constant association with the
fronds of seeds comparable with those described by White as _Aneimites_
(_Wardia_) _fertilis_ affords a strong argument in favour of assigning
_Eremopteris artemisaefolia_ to the Pteridosperms.
1826. _Sphenopteris artemisaefolia_ Steinberg, Flor. Vorwelt, Fasc.
+iv.+ p. 15, Pl. +lvi.+ fig. 1.
1833. _Sphenopteris crithmifolia_ Lindley and Hutton, Foss. Flor. Vol.
+i.+ Pl. +xlvi.+
1833. _Cardiocarpon acutum_, _Ibid._ Pl. +lxxvi.+
1869. _Eremopteris artemisaefolia_ Schimper, Trait. Pal. Vég. Vol. +i.+
p. 416.
1914. _Samaropsis acuta_ Kidston, Trans. R. Soc. Edinb. Vol. +l.+ Pt
+i.+ p. 156.
The large compound fronds of this species[449] are characterised by
the regular dichotomy of the main branches, a feature frequently met
with in Palaeozoic fern-like leaves: the cuneate or oval-cuneiform
pinnules (fig. 443, A, C) vary considerably in breadth from the typical
cuneate type of segment as figured by Brongniart[450] to narrow,
almost linear, leaflets like those of _Sphenopteris crithmifolia_.
Several spreading veins traverse the lamina. Lindley and Hutton, while
admitting a very close resemblance between their species and _S.
artemisaefolia_, adopted a distinctive name. The only evidence so far
obtained as to the stem of the plant is furnished by some specimens
in the Hutton collection (Newcastle-upon-Tyne) one of which shows a
piece of rhizome bearing several petioles (fig. 443, B): there are no
pinnules attached to the rachises but some occur in close association.
Brongniart noticed the frequent association of _Eremopteris_ fronds
with small seeds, but he regarded it as accidental. Dr Kidston[451]
has recently drawn attention to a note by Prof. Duns published in
1872 on the juxtaposition of seeds and fronds, and Mr Howse[452] in
his Catalogue of the Hutton plants considers that the seeds were borne
on the _Eremopteris_ leaves; in his synonymy of _E. artemisaefolia_
he includes _Cardiocarpon acutum_ Lind. and Hutt. as the ‘spore-cases
or sporangia.’ The _Eremopteris_ seeds are of the platyspermic
(_Samaropsis_) type, broadly oval and about 7 mm. long with an obtuse
base and two slightly divergent acute processes at the apex (fig. 444).
Some specimens in Dr Kidston’s collection from the Lower Coal Measures
of Midlothian, which were associated with _Eremopteris_ fronds, are
preserved as mummified cuticular membranes and on microscopical
examination they show clearly the presence of a pollen-chamber.
The seeds are of the _Samaropsis_ type. The drawings reproduced in
fig. 444 were made for me by Dr Kidston from two specimens, in his
collection, of exceptionally well-preserved seeds from Midlothian:
the seeds of this species vary considerably in size and form; some are
almost orbicular and show no distinction between nucule and border
(fig. 444, A) while in others (B) the impression of the flattened
and longitudinally striated sarcotesta is clearly distinguished.
Kidston is of opinion that in younger seeds there is a single apical
point replaced in a later stage of development by two cusps, as seen
in figs. A and B, formed by the opening of the micropylar tube. A
ridge in the middle of the flattened surface indicates the position
of the vascular bundles in the principal plane as in _Cardiocarpus_.
The fully developed seeds are 8–9 mm. long. The correlation of the
seeds represented in figs. 443 and 444 with _Eremopteris_ fronds
furnishes an additional illustration of the impossibility of trusting
to external form as a criterion of affinity, for it is known that
seeds of the _Samaropsis_ type were produced by Pteridosperms with
foliage represented by _Eremopteris artemisaefolia_ and _Pecopteris
Pluckeneti_, also by some members of the Cordaitales (_e.g._ fig. 480).
Dr Arber[453] has recently proposed a new generic name _Cornucarpus_
for _Cardiocarpon acutum_, but the drawings that he gives of seeds from
the Kent coal-field referred to this species suggest a type distinct
from that of Lindley and Hutton. In the absence of specimens showing
actual attachment it is impossible to say how the seeds were borne,
but the analogy of _Wardia fertilis_ and _Pecopteris Pluckeneti_ lends
support to the view that the seeds were attached to pinnules with a
reduced lamina. _Eremopteris artemisaefolia_ occurs in the Lower and
Middle Coal Measures of England: a species recorded by Kidston from
the Calciferous sandstone of Scotland as _E. Macconochii_[454] is now
believed by that author to be generically distinct[455]. With the
exception of the unsatisfactory specimen reproduced in fig. 443, B, we
have no information with regard to the habit of the stem to which the
_Eremopteris_ leaves were attached.
[Illustration: Fig. 443. _Eremopteris artemisaefolia._ A, part
of a frond with associated seeds of _Samaropsis acuta_; B, rhizome
with fragments of fronds; C, pinnule. (Drawn by Mr L. D. Sayers from
specimens in the Hutton Collection, Newcastle. A, B, ⅔ nat. size.)]
[Illustration: Fig. 444. _Samaropsis acuta._ (From drawings by Dr
Kidston of specimens in his collection, 3475 (A), 3316 (B); × 2.)]
[Illustration: Fig. 445. _Wardia fertilis_ (White). A, sterile
portion of frond; B, D, enlarged pinnules; C, seed. (After White. A,
_ca_, nat. size; B–D, × 2.)]
=WARDIA.= White.
_Wardia fertilis_ (White). Another example of a fern-like frond bearing
seeds is afforded by specimens from the Lower Pottsville series of
Virginia (correlated with the Millstone grit of British geologists)
described by Mr David White[456] as _Aneimites_ (_Wardia_) _fertilis_.
The compound fronds usually referred to the genus _Adiantides_ or
_Adiantites_[457] are characterised by cuneate pinnules with a thin
lamina and forked, slightly divergent veins (fig. 445, A, D). White
discards the name _Adiantides_ in favour of Dawson’s genus _Aneimites_
on the ground that Goeppert[458], who instituted the former term,
applied it in the first instance to leaves of _Ginkgo_ which he
identified as simple Fern fronds. In spite of this misapplication of
the name it has been constantly used and is well established. The
discovery of seeds is, however, a reason for the adoption of a new
generic name, and as White proposed _Wardia_ for the seeds it may
appropriately be extended to the fronds in place of the provisional
term _Aneimites_. The seeds which occur at the apices of slender
pedicels on pinnae bearing relatively small pinnules with a reduced
lamina (fig. 445, B, C) are rhomboidal in shape, 4·5 mm. long and 2·5
mm. broad. The bilaterally symmetrical seeds were probably enclosed,
as White suggests, in a fleshy integument which on pressure became
laterally extended as a wing-like border. In some of the seeds there
is an indication of a ‘slight collapse within the apex of the nutlet,’
which may mean the presence of a pollen-chamber; but while the
preservation is too imperfect to afford any decisive evidence as to
anatomical features, there is no reason to doubt the conclusion as to
the seed-nature of the organs described by White. Nothing is known of
the stem, though the opinion may be hazarded that _Wardia_ is a member
of the Medulloseae.
_Adiantites bellidulus_ Heer and _Lagenospermum Arberi_ Nathorst.
Reference is made to the genus _Lagenospermum_ in the account of
_Lagenostoma_[459]. The species _Lagenospermum Arberi_ has recently
been founded by Dr Nathorst[460] on some seeds obtained from Lower
Carboniferous rocks in Spitzbergen: a brief description is intercalated
here because it is probable that they were borne on fronds of the
_Adiantites_ type similar to those on which White found the seeds
described by him as _Wardia_. The seeds of _L. Arberi_, 14–18 ×
5 mm., are spindle-shaped with an obtuse apex and longitudinally
ribbed with a stalk at least 7 mm. long. Nathorst considers that
a cupule was probably present: the specimens do not convey the
impression of naked seeds and in some examples there are indications
of an investing envelope, though this may be the result of tearing
of the testa. Nathorst regards the Spitzbergen seeds as probably
specifically identical with a specimen described by Schmalhausen[461]
from Carboniferous rocks in the Urals as _Rhabdocarpus orientalis_
Eich., a species which agrees closely with _Lagenospermum nitidulum_
as described by Heer[462] (under the name _Carpolithes nitidulus_) and
Nathorst[463] from Spitzbergen. It is also possible that Kidston’s
_Rhabdocarpus elongatus_[464], from the Lanarkshire coal-field and
elsewhere, recently transferred by Arber[465] to _Platyspermum_, is
an example of the same species. The chief interest of _Lagenospermum_
lies in the fact, assuming Nathorst’s correlation of the seeds with
_Adiantites bellidulus_ Heer to be correct, that it is a typical
radiospermic seed, while _Wardia_, borne on foliage of the same general
type, is an equally typical platyspermic seed.