Coenopterideae. ╭ I. Botryoptereae.
╰ II. Zygoptereae.
The term Botryopterideae, first used by Renault, has been applied to a
group of Palaeozoic ferns ranging from the Lower Carboniferous to the
Permian and containing several genera, the distinguishing features of
which are supplied by the anatomical structure of the stems or, in many
cases, by that of the petiolar vascular strand. Scott[1109] subdivides
the Botryopterideae into the Botryopteris and the Zygopteris sections.
In an admirable monograph recently published by Paul Bertrand[1110]
considerable changes are proposed in current nomenclature; he
substitutes the name Inversicatenales for Botryopterideae, a
designation, which as Scott remarks, is “probably too technical to
command general acceptance.” A more serious criticism is that the name
Inversicatenales has reference to a character (the inverse curvature
of the leaf-trace in relation to the axis of the stem) which is by no
means universal in the group[1111].
In the following account, necessarily incomplete, the generic
terminology of Bertrand is adopted, but this decision does not carry
with it any obligation to accept the name Inversicatenales. We may
speak of the types of Palaeozoic ferns dealt with in the following
pages as members of a group differing in many respects from any
existing genera of the Filicales, and exhibiting the characteristics
associated with generalised plants. Williamson, as early as 1883,
spoke of Renault’s Botryopterideae as comprising “altogether extinct
and generalised” types[1112]. For these generalised Palaeozoic ferns
I propose to use the name Coenopterideae[1113]. This term may be
adopted in a wider sense than Renault’s name Botryopterideae. The
name Primofilices proposed by Arber[1114] might be employed, but the
implication which it carries is an argument against its adoption. We
have not yet reached a stage in the investigation of extinct types at
which we are able to recognise what are actually primary or primitive
ferns. The search for origins will continue; as new discoveries are
made our point of view shifts and the primitive type of to-day may
to-morrow have to take a higher place. The epithet primitive or primary
is in reality provisional: to adopt such a name as Primofilices
suggests a finality which has not been, or is likely to be, achieved.
The true ancestral type—the _Urform_—which we strive to discover eludes
the pursuer like a will-o’-the-wisp.
Seeing that the number of true ferns of Palaeozoic age has been
recently considerably reduced and is likely to suffer further
reduction, the consideration of such undoubted Carboniferous and
Permian examples of the Filicales as are left acquires a special
importance. In the first place it is natural to ask whether the
Palaeozoic ferns include any types which, if not themselves ancestral
forms, may serve to indicate the probable lines of evolution of
existing families. It is probable that in the near future our knowledge
of the Coenopterideae will be considerably extended; as yet we possess
meagre information in regard to those characters on which most stress
has generally been laid in the classification of recent ferns,
namely the structure of the spore-bearing organs. The sporangia of
_Diplolabis_ and _Stauropteris_ (figs. 309, A; 322) are exannulate;
in the former genus they occur in sori or synangia consisting of a
small number of sporangia, while in the latter they are borne singly
at the tips of ultimate ramifications of a highly compound leaf. The
resemblance of the synangium of _Diplolabis_ to that of _Kaulfussia_
(fig. 245, C) is not shared in an equal degree by the sporangia of
_Stauropteris_, which are in some respects comparable with those of
the Ophioglossaceae. In the Zygoptereae, or at least in the case of
such fertile fronds as are known, and in _Botryopteris_ (fig. 319),
the sporangia occur in groups, and the pedicel of each sporangium
is supplied with vascular tissue as in _Helminthostachys_. Another
characteristic of the sporangia of the extinct types is the possession
of an annulus several cells in breadth, a peculiarity which supplies a
point of contact with the Osmundaceae. In the sporangia of _Kidstonia_
we have a similar though not an identical type (fig. 256, E, p. 340).
So far, then, as the evidence afforded by sporangial characters is
concerned, it points to comparison with the Ophioglossaceae, the
Osmundaceae, and the Marattiaceae. When we compare the steles of the
stems we find a wide range of structure. All the genera agree in being
monostelic; in _Tubicaulis_ and _Grammatopteris_ the protoxylem is
exarch, in _Botryopteris_ it is internal, while the foliar strand of
_Stauropteris_ and the stele of _Ankyropteris corrugata_ are mesarch.
The axillary branching of species of _Ankyropteris_ suggests comparison
with the Hymenophyllaceae.
The investigation of the vascular system of the petioles has afforded
results which in the hands of P. Bertrand have led to conclusions in
regard to inter-relationships. We must, however, not overlook the
danger of attributing can excessive importance to this single criterion
and of neglecting the facts of stem anatomy.
I. Botryoptereae.
_Grammatopteris._
Renault instituted this genus for petrified stems from the
Permo-Carboniferous beds of Autun. _Grammatopteris Rigolloti_[1115],
the type-species, is represented by a fragment, 12–15 mm. in diameter,
surrounded by crowded petioles characterised by a vascular strand in
the form of a short and comparatively broad plate with the smallest
tracheae at each end. The solid xylem of the stem stele (protostele)
has peripheral groups of protoxylem. Nothing is known as to the
form of the leaves, but sporangia similar to those of _Etapteris_
(_Zygopteris_) were found in association with the stem. It is possible,
as P. Bertrand suggests, that Renault’s species may be the stem of a
_Tubicaulis_.
_Tubicaulis._
_Tubicaulis solenites_ (Sprengel)[1116]. Fig. 304.
This species from the Lower Permian of Saxony has been fully described
by Stenzel[1117]. It is characterised by a very slender erect stem
bearing numerous spirally disposed leaves associated with adventitious
roots; the single stele (protostele) consists exclusively of tracheae,
described as intermediate between the scalariform and reticulate type,
surrounded by phloem. Leaf-traces are given off from the periphery
of the stele where groups of smaller elements occur; these have the
form of a wide-open =U=-shaped strand with the base of the =U= facing
the axis of the stem. As the trace passes out towards the leaves, the
ends of the =U= become more or less incurved. The stem is said to
reach a metre in length and to bear compound fronds a metre long. The
orientation of the leaf-trace with its concavity turned outwards is in
striking contrast to the relation between leaf-trace and stem in recent
ferns.
[Illustration: FIG. 304. _Tubicaulis solenites._ (From Tansley, after
Stenzel.) Stem and petioles: the latter numbered in the order of
their age.]
_Tubicaulis Sutcliffii_, Stopes[1118].
In this species the vascular axis, 2 mm. in diameter, is almost
cylindrical and of the protostelic type with the protoxylem “near
to or at the edge”: the tracheae are scalariform or reticulate. The
leaf-traces, when first separated from the edge of the stele, are oval
and gradually assume the curved form seen in _T. solenites_ (fig. 304)
with the convex side towards the axis of the stem. The transition from
the scalariform to the reticulate type of pitting on the tracheal walls
referred to by Miss Stopes has also been noticed in some recent ferns
(e.g. _Helminthostachys_) and in _Sigillaria_ (fig. 200, C, p. 212).
The fact that the scalariform type of pitting is practically universal
in the xylem of recent ferns, would seem to show that this character
has been acquired in the course of evolution and retained in preference
to the reticulate form characteristic of several Palaeozoic species.
The distinction between the two methods of pitting is one of little
phylogenetic importance.
_Botryopteris._
This genus, founded by Renault on a specimen from Autun, is represented
in the Lower Coal-Measures of England by _Botryopteris hirsuta_ (=
_Rachiopteris hirsuta_ Will.), _B. ramosa_ (= _R. ramosa_ Will.[1119])
(fig. 306) and _B. cylindrica_ (fig. 305), also by _B. antiqua_ (fig.
307) from the Culm of Pettycur, Scotland.
An important characteristic of the genus is the solid stele of the stem
which agrees with that of _Tubicaulis_ and _Grammatopteris_, except in
the central or peripheral position of the smallest tracheae.
_Botryopteris forensis_ Renault[1120]. Figs. 309, B; 319, D–G.
The stem of this species from the Upper Carboniferous of St Étienne
is 1·7 cm. x 7·5 mm. in diameter. The solid stele consists of
reticulate tracheae with the smallest elements on the outer edge.
The comparatively broad cortex of the type-specimen is traversed by
a leaf-trace in an almost vertical course and by vascular strands
passing horizontally to roots. The petioles are circular in section
and their vascular strand has the form of an ω in transverse section
(fig. 319, G), the three projecting arms pointing to the axis of the
stem. Both stem and leaves bore large multicellular hairs, spoken of
by Renault as equisetiform because of the finely toothed sheaths of
which they are composed. The compound fronds had fleshy lobed pinnules
with dichotomously branched veins (fig. 309, B); stomata are said to be
confined to the upper surface, an observation which leads Renault to
describe the plant as aquatic on evidence which is hardly convincing.
The pyriform and pedicellate sporangia are borne in groups of two to
six on the ultimate divisions of the frond; the wall is composed of
two layers of cells and on one side of the sporangium is an annulus
several cells in breadth (fig. 319, D, F). An interesting type of
sporangium described by Oliver[1121] from Grand’Croix in France may,
as he suggests, belong to _Botryopteris forensis_; the differences
between Renault’s and Oliver’s specimens being the result of the more
perfect preservation of the tissues in the latter. The sporangium
described by the English author is circular in section and measures
0·65 × 0·53 mm.; the wall is in part composed of a single layer of
cells and in part of two to three layers, a character recalling the
“annulate” sporangia of _Botryopteris_. Between the spore-mass and the
wall is an interrupted ring of short tracheal elements similar to the
xylem-mantle which occurs at the periphery of the nucellus of certain
Palaeozoic gymnospermous seeds. In the absence of proof of a connexion
between this sporangium and _Botryopteris_ it is convenient to use the
generic name _Tracheotheca_ subsequently proposed by Oliver[1122].
In the recent ferns _Helminthostachys_ and _Botrychium_, and, as
Oliver notices, in the microsporangia of the Australian Cycad _Bowenia
spectabilis_, vascular strands extend almost to the sporogenous tissue,
but the fossil sporangium is unique in having a tracheal layer in
immediate contact with the spores. These xylem elements may, as Oliver
suggests, have served the purpose of conveying water to the ripening
spores.
_Botryopteris hirsuta_ (Will.)[1123].
This English species has a slender axis bearing numerous leaves with
petioles equal in diameter to the stem. The surface of the vegetative
organs bears large multicellular hairs. The leaf-traces resemble
those of _B. forensis_, but the projecting teeth which terminate in
protoxylem elements are less prominent than in the French species;
the petioles were named by Felix _Rachiopteris tridentata_[1124]. As
a leaf-trace passes into the stele of the stem the three protoxylem
strands unite and take up an internal position in the solid stele. The
stele may, therefore, be described as endarch. The small tracheae at
the edge of the stele supply the xylem strands of adventitious roots.
Sporangia similar to those of _B. forensis_ have been found in
association with the English species.
_Botryopteris cylindrica_ (Will.). Fig. 305.
A plant originally described by Williamson[1125] from the Lower
Coal-Measures of England as _Rachiopteris cylindrica_ (fig. 305)
and afterwards more fully dealt with by Hick[1126], has a slender
stem with a cylindrical stele characterised by well-defined central
protoxylem elements in one or two groups. The leaf-traces are
semi-lunar in section with the protoxylem on the flatter side. The
stele of _Botryopteris cylindrica_ (fig. 305, A) is more cylindrical
in section than that of _B. ramosa_ (fig. 306) and shows more clearly
the differentiation into smaller central and larger peripheral
tracheae. In the section reproduced in fig. 305, B the stele is giving
off a branch almost identical in structure with the main vascular
axis. Scott[1127], in referring to the inclusion of this type in the
genus _Botryopteris_, expresses the opinion that its habit must have
been very different from that of other species, and he suggests the
institution of a new genus.
[Illustration: FIG. 305. _Botryopteris cylindrica_ (× 30). From
sections in the Cambridge Botany School.]
_Botryopteris ramosa_ (Williamson). Fig. 306.
This species, which bears a close resemblance to _Botryopteris
hirsuta_, was originally described by Williamson from the Lower
Coal-Measures of England as _Rachiopteris ramosa_[1128], the specific
name being chosen on account of the numerous and crowded branches
given off from the main axis. The section shown in fig. 306, A,
illustrates Williamson’s description of the stem as being “always
surrounded [when seen in transverse sections] by a swarm of similar
sections of the large and small branches, though of varying shapes and
sizes.” The stele is composed of a solid and more or less cylindrical
rod of xylem tracheae of the reticulate type surrounded by phloem
(figs. A and D): one or more internal groups of smaller protoxylem
elements occur in an approximately central position (fig. A, _px_).
The stele is in fact endarch like those of _Selaginella spinosa_ and
_Trichomanes reniforme_, a feature which, as Tansley[1129] believes,
probably entitles the vascular axis to be considered a primitive form
of protostele. In the specimens represented in fig. 306 the phloem
and inner cortical tissues were almost completely destroyed before
petrifaction. The thick-walled outer cortex bears at its periphery
numerous multicellular hairs. Some of the xylem strands given off from
the stele no doubt supplied adventitious roots, but in most cases the
outgoing branches are leaf-traces and the numerous sections of axes
of different sizes seen in fig. A point to a repeated subdivision of
the crowded fronds. The structure of a petiole is shown in figs. C and
D. As seen in fig. C, the oval vascular strand has three protoxylem
groups, _px_, on its flatter side; a well-defined epidermal layer is
shown at _e_ in fig. C.
Fig. B shows at _a_ a section of a leaf-axis in the act of branching
and the row of branchlets at _b_ represents a further stage in
subdivision. At _sp_ in fig. A the section has cut through a single
sporangium characterised by a group of larger (“annulus”) cells on one
side of the wall.
[Illustration: FIG. 306. A–D. _Botryopteris ramosa_; stem and frond
axes. (A × 7; B × 15; C × 26; D × 13. From sections in the Cambridge
Botany School Collection.) _px_, protoxylem; _sp_, sporangium; _e_,
epidermis.]
This slender fern with its numerous repeatedly branched leaves may
perhaps have lived epiphytically on more robust plants.
_Botryopteris antiqua_, Kidst. Fig. 307.
This species, recently described by Kidston[1130] from the Culm of
Pettycur near Burntisland, is represented by sections of a small
stem with a cylindrical stele 0·40 mm. in diameter composed entirely
of scalariform tracheae without any recognisable protoxylem. The
petioles are larger than the stem; the meristele (fig. 307) is oval
with protoxylem elements on the slightly more rounded adaxial face.
As Kidston suggests, this stem may belong to a scrambling plant which
required support to bear its relatively large leaves. An interesting
feature is the absence of projecting teeth in the leaf-trace, a
character in marked contrast to the ω form assumed by the petioles of
_Botryopteris forensis_ (fig. 319, G) and _B. hirsuta_. This leads
Kidston to suggest that the vascular strand of the petiole tends “to
become more simple ... as traced back in geological time.” The greater
similarity in this species between the stele of the stem and that of
the petiole is probably another mark of a more primitive type.
[Illustration: FIG. 307. _Botryopteris antiqua_: Petiolar vascular
strand. (After Kidston: × 65.)]
• • • • •
In these three types, _Grammatopteris_, _Tubicaulis_, and
_Botryopteris_, we have monostelic plants, for the most part of very
small size, with leaf-traces varying in shape from the oblong band-form
in _Grammatopteris_, and the oval form of _Botryopteris antiqua_, to
the ω type represented in its most pronounced form by _B. forensis_. In
several species the stem stele is endarch. Our knowledge of the leaves
is very meagre: in _B. forensis_ they were repeatedly branched and
apparently bore small fleshy pinnules; the sporangia, though differing
from those of recent ferns, may be compared with the spore-capsules
of Osmundaceae as regards the structure of the annulus. The abundance
of hairs on the stems and leaves of some species, the tracheal sheath
in the sporangium described by Oliver[1131] as _Tracheotheca_ (=
_Botryopteris?_), and the apparent absence of a large well-developed
lamina, may perhaps be regarded as evidence of xerophilous conditions.
II. Zygoptereae.
Corda[1132] proposed the generic name _Zygopteris_ for petrified
petioles from the Permian of Saxony, included by Cotta in his genus
_Tubicaulis_, which he named _T. primarius_. Corda’s genus has been
generally used for petioles of Palaeozoic ferns characterised by
a vascular strand having the form of an =H= in transverse section
(fig. 308, D). Since the generic name was instituted, information
has been obtained in regard to the nature of the stems which bore
some of the petioles of the _Zygopteris_ type; and for other species
of _Zygopteris_, the stems of which are still unknown, new generic
names have been proposed. P. Bertrand[1133] retains _Zygopteris_ for
one species only, _Z. primaria_. Fig. 308, D, shows the character of
the petiolar vascular strand; the chief points are the comparatively
long cross-pieces (antennae of P. Bertrand) inclined at an angle of
45° to the plane of symmetry of the petiole axis, and the groups of
protoxylem elements shown by the white patches in fig. D. In this as in
other members of the Zygoptereae the main rachis of the leaf gives off
four sets of branches in pairs alternately from the right and left side
of the primary vascular axis. This method of branching of the stele in
the primary rachis of several members of the Coenopterideae shows that
the fronds bore pinnae laterally disposed, in some cases in one row
and in others in two rows on each side of the rachis. In a typical fern
frond, as represented by recent and most fossil species, branching
of the rachis occurs in the plane of the frond, that is in the plane
represented by the horizontal arm of xylem in _Zygopteris primaria_
connecting the two antennae or cross-pieces. In the Zygoptereae the
branches from the petiole vascular axis lie in a plane at right angles
to that of the frond; they lie in the transverse and not in the
horizontal plane. The two strands shown in fig. 308, B, 4, have been
formed by the division of a single strand, 3, in the transverse plane
(i.e. in the plane of the paper). As Tansley[1134] points out, a type
of branching superficially similar to, though not identical with this,
is seen in some recent species of _Gleichenia_ and _Lygodium_. In
this connexion it is worthy of note that a fern figured by Unger from
Thuringia as _Sphenopteris petiolata_ Goepp[1135] bears pinnae in two
rows on the rachis which are characterised by repeated branching and by
a very narrow lamina or by slender naked axes; the occurrence of this
form of frond in rocks containing _Clepsydropsis antiqua_ (fig. 308,
A) suggests a possible connexion between the petrified rachis and the
impressions of the leaves.
[Illustration: FIG. 308.
A. _Clepsydropsis antiqua._
B. _Etapteris Scotti._
C. _Diplolabis forensis._
D. _Zygopteris primaria._
E–G. _Stauropteris oldhamia._
The white patches in the xylem in figs. B–G mark the position of
protoxylem elements.
(A, after Unger; B–G, after P. Bertrand.)]
[Illustration: FIG. 309.
A. _Diplolabis forensis._
B. _Botryopteris forensis._
C, D. _Corynepteris coralloides._
E. _Schizopteris_ (_Etapteris_) _pinnata_.
(A, B, after Renault; C, D, after Zeiller; E, after Renault and
Zeiller.)]
The vascular strand of the rachis of _Zygopteris primaria_ (fig. 308,
D) is simpler than that of most of the Zygoptereae and exhibits a close
resemblance to the type of strand described by Renault as _Diplolabis_
(fig. 308, C).
_Diplolabis._
Renault[1136] instituted this genus for two species from the Culm beds
and Coal-Measures of France based on the structure of the petioles.
The stems are unknown. The main rachis has a stele similar to that of
_Zygopteris primaria_, but distinguished by its greater similarity,
in transverse section, to an =X= rather than to the letter =H=: the
long transverse bar in _Zygopteris_ is here much reduced in size.
The petiole of _Diplolabis forensis_[1137] Ren. (fig. 308, C) has a
diameter of 1·5–2 cm. From the antennae a pair of small bundles is
given off alternately from the right and left side, as in _Zygopteris_;
the members of each pair coalesce after leaving the antennae and then
separate to pass into the lateral branches of the frond. The position
of the protoxylem and the formation of the lateral xylem strands
previous to their separation are shown in fig. 308, C. On the side of
the vascular strand shown in fig. C, 2, the two lateral extensions
of the antennae are converging towards one another previous to their
separation and subsequent union. The ovoid sporangia occur in groups
of three to six and are coalescent below with a central receptacle;
they have no annulus, but the cells on the side next the receptacle are
smaller than those on the external wall (fig. 309, A). The synangial
form of the sorus suggests comparison with Marattiaceae.
The species described by Renault from the Culm of Esnost is regarded by
P. Bertrand as identical with that described by Solms, from the Culm
of Falkenberg, as _Zygopteris Roemeri_[1138]. _Diplolabis_ is compared
by P. Bertrand with _Metaclepsydropsis_, the generic name given to the
Lower Carboniferous petiole described by Williamson as _Rachiopteris
duplex_[1139].
Mr Gordon has recently described in a preliminary note a new type
of stem stele under the name _Zygopteris pettycurensis_ from the
Lower Carboniferous plant bed of Pettycur[1140]: he regards the
petioles attached to the stem as identical with _Zygopteris Roemeri_
Solms-Laubach[1141]. This species, founded by Solms-Laubach on
petioles only, is placed by Bertrand[1142] in the genus _Diplolabis_
and regarded as identical with _D. esnostensis_ Ren. The stele found
by Mr Gordon may therefore be assigned to the genus _Diplolabis_:
it includes two regions composed exclusively of tracheae and is
cylindrical in transverse section. The inner xylem zone consists of
short, square-ended, reticulately pitted elements and the outer zone
is composed of long and pointed conducting tracheae. The scalariform
protoxylem elements are situated between the two metaxylem zones.
As Mr Gordon says: this type of stem occupies a position “in the
Zygopteroid alliance” corresponding to that which _Thamnopteris
Schlechtendalii_ (p. 329) occupies in the Osmundaceous series. The
discovery of this stem supplies another link between the two fern
groups, Osmundaceae and Coenopterideae. Pelourde[1143] has described
an imperfectly preserved vascular strand from a locality near Autun as
the type of a new genus _Flicheia esnostensis_. Mr Gordon has pointed
out to me that this is a partially rotted petiole of _Diplolabis
esnostensis_ (= _Zygopteris Roemeri_).
In their recent account of fossil Osmundaceous genera, Kidston and
Gwynne-Vaughan[1144] speak of the central parenchyma of the existing
medullated stele as being derived from tracheal tissue. They add that
if the Zygopteroid line of descent is at all close to the Osmundaceous,
we must be prepared for the existence of a _Zygopteris_ with a solid
xylem like that of _Thamnopteris_: “such a discovery, in fact, we
hopefully anticipate[1145].” The new Pettycur stem amply justifies this
prophecy. It is noteworthy that Mr Gordon’s stem affords an instance
of the occurrence of a type of stele, similar in its cylindrical form
and in the absence of parenchyma to that of _Botryopteris_, in a plant
bearing leaves characterised by the _Zygopteris_ type of vascular
strand.
_Metaclepsydropsis duplex_ (Will.) fig. 310, A[1146]. [= _Rachiopteris
duplex_, Williamson 1874. _Asterochlaena_ (_Clepsydropsis_) _duplex_,
Stenzel 1889. _Clepsydropsis_, Renault 1896.]
The vascular axis of the main axis of the frond is characterised by the
hour-glass shape of the xylem which consists entirely of tracheae, most
of which are reticulately pitted. In a transverse section (fig. 310,
A) the two ends of the stele are dissimilar; at one end of the long
axis is a small bay of thin-walled tissue (phloem) enclosed by a narrow
band of xylem, and at the other the bay is open and has two protoxylem
groups. The latter represents the earliest stage in the production of
secondary bundles: at a later stage the bay is closed by the elongation
of the edges, the enclosed group of phloem is vertically extended, and
the protoxylem strands are more widely separated. The curved band of
xylem becomes detached as a curved arc and divides into two (fig. 310,
A). In a single section of this species one often sees several strands
of xylem enclosed in a common cortex with the main vascular axis; these
are the xylem bundles of lateral pinnae. _Metaclepsydropsis duplex_
shows the method of branching of the petiole vascular axis which has
already been noticed in _Diplolabis_ and _Zygopteris_. In reference
to this feature, Williamson wrote in 1872—“I know of no recent fern
in which the secondary branches of the petiole are thus given off in
pairs, which pairs are distichously arranged on the primary axis,
and each of which secondary petioles sustains ternary ones arranged
distichously.” By slightly altering the primary stele of this type of
frond, by narrowing of the constricted portion of the hour-glass and
extending the lateral groups of xylem obliquely upwards, the form of
stele shown in fig. 310, A, would be converted into the _Diplolabis_
type (fig. 308, C).
_Clepsydropsis._
Unger[1147] instituted this genus as a subdivision of Corda’s family
Rhaciopterideae[1148], the name having reference to the hour-glass
form of the vascular axis[1149]. The type-species _C. antiqua_ (fig.
308, A) is spoken of as the commonest fossil plant in the Devonian
rocks of Thuringia. In some sections the xylem has the form seen in
fig. 308, A, in which an invagination of thin-walled tissue occurs at
each end; in other sections (fig. 308, A′) the bays become islands in
the xylem. Solms-Laubach speaks of Unger’s species as _Rachiopteris_
(_Clepsydropsis_) _antiqua_. P. Bertrand[1150], who has recently
described Unger’s plant, while recognising that _C. antiqua_ and
_Metaclepsydropsis duplex_ closely resemble one another, draws
attention to certain differences in the structure of the xylem which he
regards as sufficient to justify a generic separation. The leaf-traces
of _Clepsydropsis_ are described by Bertrand as almost circular closed
rings of xylem instead of an arc as in _Metaclepsydropsis_.
[Illustration: FIG. 310.
A. _Metaclepsydropsis duplex._
B, C. _Stauropteris oldhamia._
D. _Ankyropteris scandens._
[A, from a section in Dr Kidston’s Collection (Lower Carboniferous);
B, C, from sections in the Cambridge Botany School; D, after
Stenzel.]]
_Ankyropteris._
Stenzel adopted this name for a subdivision of Corda’s genus
_Zygopteris_, applying it to a species described by Renault as _Z.
Brongniarti_, to a Permian species described by himself as _Z._
(_Ankyropteris_) _scandens_, and to _Z. Lacattii_ Ren.; _Rachiopteris
Grayi_ Will. and _Rachiopteris corrugata_ Will. are also included in
this genus. The characters emphasised by Stenzel[1151] are (i) the
double anchor-like form of the =H=-shaped petiole strand in which the
lateral arms (antennae) are curved like the flukes of an anchor, and
(ii) the emission of four rows of branches instead of two. The latter
distinguishing feature no longer holds good, as _Z. primaria_ also
gives off four rows of bundles and not two as Stenzel supposed. P.
Bertrand has adopted Stenzel’s genus in a narrower sense[1152].
[Illustration: FIG. 311. _Ankyropteris Grayi._ Stele. (From a section
in Dr Kidston’s Collection, × 18.)]
_Ankyropteris scandens_ Stenzel[1153]. Fig. 310, D.
This Lower Permian species is very similar to or perhaps identical with
_Ankyropteris Grayi_ (Williamson). The stem of
_A. scandens_ was found in association with the roots of a _Psaronius_
stem evidently petrified _in situ_ as it burrowed, like _Tmesipteris_,
tropical aroids, and other recent plants, among the living roots of
the tree-fern. The stem, 10–11 mm. in diameter, bore fronds with an
=H=-shaped vascular strand, small scale-leaves, and adventitious roots.
The stele consists of a five-angled cylinder of scalariform tracheae
surrounding an axial strand of parenchyma containing scattered tracheae
of smaller diameter. This axial tissue extends as a narrow strip
into each of the short and obtusely truncated arms (cf. fig. 311).
A striking feature is the production of a shoot in the axil of the
foliage-leaves (fig. 310, D), a manner of branching characteristic of
_Trichomanes_ (see page 365).
_Ankyropteris Grayi_ (Will.). Fig. 311.
In describing this species, Williamson wrote—“That no classification
of these fossil ferns based solely upon transverse sections of the
petiolar bundles is or can be of much value, is clearly shown when
tested amongst those living ferns the classification of which is
chiefly based upon the sporangial reproductive organs[1154].” This is
a view entirely opposed to that which inspires P. Bertrand’s recent
monograph. Whether the value attached to the vascular structure
of petioles as a basis of classification is upheld or not, it is
noteworthy that since Williamson expressed his opinion, our knowledge
of the anatomy of ferns and of the value of anatomical evidence
has enormously increased. The slender stem[1155] of this Lower
Coal-Measures species agrees closely with that of _A. scandens_; it
bore spirally disposed fronds, scale-leaves, and roots. The stele
has the form of an irregular five-rayed star (fig. 311) in which
the relative length of the arms varies in different sections owing
to the separation of the distal ends to form leaf-traces. The axial
region is composed of parenchyma and associated narrow tracheae, as
in _A. scandens_. The xylem, with protoxylem elements at the ends and
especially at the angles of the arms, is completely surrounded by
phloem. The cortex consists internally of parenchyma which becomes
thicker-walled towards the periphery and bears multicellular epidermal
hairs. A leaf-trace is detached in the form of a triangular strand
and is formed by the tangential extension of the distal end of an arm
of the stele. The trace, on its way through the cortex, divides into
two; the outer branch gradually changes from a slightly curved band to
an =H=-shaped meristele; the inner branch, which supplied an axillary
shoot, is similar to the stele of the stem, but smaller. Scott[1156]
has recently recorded the occurrence of scale-leaves (aphlebiae) in
this species like those described by Stenzel in _A. scandens_.
[Illustration: FIG. 312.
A. _Thamnopteris Schlechtendalii._ Leaf-trace: _px_, protoxylem;
_s_, island of parenchyma. (After Kidston and Gwynne-Vaughan.)
B. _Ankyropteris corrugata._ Single trachea with tyloses.
C. _A. bibractensis._ Part of foliar strand. (After P. Bertrand.)]
Bertrand includes in _Ankyropteris_ Renault’s species _Zygopteris
bibractensis_[1157] and Williamson’s species _Rachiopteris
corrugata_[1158]: the former he names _A. bibractensis_ var.
_westphaliensis_. The fossil described by Williamson as _R.
irregularis_ or _inaequalis_[1159] are the secondary branches of _A.
bibractensis_.
_Ankyropteris bibractensis_, var. _westphaliensis_. Figs. 312, C; 313.
The rachis stele of this species, which is represented by portions of
fronds only, has the form of a double anchor (fig. 313); the antennae
are continued at the outer edge of their distal ends into a narrow
band (“filament” of P. Bertrand) (fig. 312, C, and 313, _a_) composed
of smaller tracheae and separated from the xylem of the antennae by a
strip of thin-walled tissue (phloem?). A group of protoxylem occurs at
the junction of the filament and antennae. The whole of the xylem is
surrounded by phloem.
[Illustration: FIG. 313. _Ankyropteris bibractensis_: _s_, stigmarian
rootlet; _a_, narrow loop of xylem. (Cambridge Botany School; × 6).]
The section reproduced in fig. 313 shows the characteristic form of the
petiolar vascular axis, consisting of a horizontal band of metaxylem
with groups of much smaller tracheae on both the upper and lower
margins. At the junction between the antennae, curved like the flukes
of an anchor, and the horizontal band of xylem, the latter is only one
trachea in breadth. The narrow loops of smaller xylem elements are
shown on the outer edge, _a_ (fig. 313), of the antennae separated
from the arcs of larger tracheae by a dark line which represents a
crushed band of delicate tissue. The spaces enclosed by the incurved
antennae are largely occupied by parenchymatous ground-tissue.
The cylinder of outer cortex consists internally of comparatively
thin-walled parenchyma succeeded externally by a zone of dark and
thicker-walled cells characterised by a fairly regular arrangement in
radial series, as if formed by a secondary meristem; there is, however,
no indication of a meristematic layer. Below the small-celled epidermis
are a few layers of thinner-walled cells which are not arranged in
radial series. The structure of the outer part of the cortex is similar
to that in the petiole of recent species of _Angiopteris_ (fig. 243, p.
319) and _Marattia_, in which a more delicate hypoderm is succeeded by
a band of mechanical tissue.
The rachis of this type of frond gives off two rows of lateral branches
from the vascular axis, the plane of symmetry being at right angles to
the primary rachis. Each pinna bore at its base two aphlebiae supplied
with vascular strands from the leaf-traces.
We have no certain information in regard to the sporangia of this
species, but Scott points out that “pear-shaped sporangia, with a
very broad and extensive annulus, are commonly found associated with
_Zygopteris bibractensis_ and _Z. corrugata_ in the petrifactions of
the English Lower Coal-Measures[1160].”
_Ankyropteris corrugata_ (Will.). Figs. 312, B; 314–317.
The stem of this type of Zygoptereae was described by Williamson from
the Lower Coal-Measures of Lancashire as _Rachiopteris corrugata_ and
included by him in the sub-group Anachoropteroides. The stele (fig.
314, B) is oval in transverse section; it consists of a cylinder of
xylem tracheae enclosing a central region occupied by parenchymatous
tissue and scattered narrow scalariform tracheae. The central tissue
extends radially in the form of narrow arms which reach almost to the
outer edge of the tracheal tissue and divide it up into 5–7 groups. A
cylinder of thin-walled tissue encloses the xylem and in this occur
groups of large sieve-tubes (fig. 314, D, _Sv_).
In a section of this species in the Williamson Collection[1161] the
long axis of the stele has a length of 5 mm. and the diameter of the
stem as a whole is 2·5 cm. The greater part of the extra-stelar tissue
consists of large parenchymatous cells passing near the periphery into
a band of darker and thicker-walled tissue.
Reniform vascular strands traverse the cortex in an obliquely
ascending course on their way to the leaves, also smaller bundles,
some of which are given off directly from the stele, while others are
branches of the petiole vascular strands. The petioles described by
Williamson as _Rachiopteris insignis_[1162] were afterwards recognised
by him as those of _Ankyropteris corrugata_, though this conclusion
was not published[1163]. Williamson’s species _R. insignis_ must
not be confused with Unger’s Culm species _Arctopodium insigne_,
which Solms-Laubach[1164] refers to as _Rachiopteris insignis_. The
leaf-bundle of _Ankyropteris corrugata_ is at first reniform in contour
(fig. 314, C, _P_), but as it becomes free from the stem it gradually
assumes the =H=-shaped form (figs. 315–317). This petiolar strand
differs from that of _Ankyropteris bibractensis_ (fig. 313) in the
shorter and less strongly curved antennae; and, as Williamson first
noticed, the tracheae are frequently filled with thin-walled parenchyma
(fig. 312, B). The existence of scale-leaves or aphlebiae like those of
_Ankyropteris scandens_ and _A. Grayi_ has been recorded by Scott in
_A. corrugata_[1165].
The section represented in fig. 314, C, shows the relatively small size
of the stele _S_ in the stem of _Ankyropteris corrugata_. The main mass
of the cortex consists of uniform parenchyma passing near the surface
into darker and stronger tissue: two vascular bundles are shown in the
cortex, one of which forms the conducting strand of a petiole, _P_,
which has nearly freed itself from the stem: the other bundle, as shown
by the examination of a series of sections, eventually passes into
another leaf-stalk. A root of another plant has invaded the cortex at
_R_, fig. 314, C.
[Illustration: FIG. 314. _Ankyropteris corrugata._ _R_, intruded root;
_P_, petiole; _S_, stele, _Sv_ sieve-tubes.
A, B. From a section in the University College Collection.
C. After Williamson.
D. From a section in the Williamson Collection (British Museum).]
The form and structure of the stele is diagrammatically represented
in fig. 314, B. The outer portion (black) consists of a cylinder
of scalariform tracheae in which the position of groups of smaller
elements (protoxylem) is shown by the white patches. The xylem is
thus seen to be mesarch. The prominent group of xylem on the lower
right-hand side of the section consists of tracheae, cut across
in an oblique direction, which are about to pass out as a separate
strand. The centre of the stele is occupied by parenchymatous tissue
in which are included scattered tracheae, either singly or in small
groups. These medullary tracheae are rather narrower than those of the
main xylem cylinder. A characteristic feature is the radial outward
extension into the xylem of the medullary parenchyma, which tends
partially to divide the tracheal cylinder into broad groups.
[Illustration: FIG. 315. _Ankyropteris corrugata_ (Will.). Petiolar
vascular strand. [From a section in the University College (London)
Collection; after Tansley × 35.]]
Fig. 314, A, enlarged from fig. B, _a_, shows the mesarch position of a
protoxylem group, and a few of the parenchymatous cells of one of the
narrow arms of the axial tissue. At _Sv_ in fig. D a group of large
sieve-tubes is seen separated from the xylem by a few parenchymatous
cells, and beyond the sieve-tubes are some tangentially elongated
cells. Both the sieve-tubes, _Sv_, and the flattened cells resemble
tissues in a corresponding position in the steles of modern Osmundaceae.
In a section of _Ankyropteris corrugata_ in the Williamson Collection
the radial arrangement of the more external metaxylem elements suggests
the addition of secondary tracheae[1166]. This suggestion of secondary
thickening, a point which requires much more thorough investigation,
is interesting in relation to a new type of stem named by Scott
_Botrychioxylon_[1167], but not yet fully described. This generic
name has been given to a stem stele which closely resembles that of
_Ankyropteris corrugata_ except in the regular radial arrangement of
the peripheral xylem elements. The name _Botrychioxylon_ was chosen by
Scott because of the secondary xylem characteristic of the recent genus
_Botrychium_ (fig. 247, p. 322).
[Illustration: FIG. 316. _Ankyropteris corrugata._ Petiole. _a_,
narrow xylem loop; _b_, spaces in cortex. From a section in the
Cambridge Botany School Collection. (× 10.)]
In the petiolar vascular strand represented in fig. 315 the narrow band
of tracheae which forms a loop external to the antennae is clearly
seen, also the small-celled parenchyma between the loops and the larger
metaxylem elements of the antennae. The crushed tissue lying on the
outer face of each of the loops probably represents the phloem and
pericycle; the thin-walled elements above and below the horizontal band
of metaxylem are probably sieve-tubes.
Fig. 316 shows a transverse section of a petiole of this species: the
loops, _a_, of small tracheae are seen bending round the outer edge of
the antennae. The inner and more delicate cortical tissue is partially
preserved and spaces, _b_, have been formed in it as the result of
contraction previous to petrifaction. In the petiole represented in
fig. 317 the tracheae of the horizontal band are considerably crushed;
the section is, however, of interest because of the presence of
_Lyginodendron_ roots, _l_, in the space originally occupied by the
inner cortex.
[Illustration: FIG. 317. _Ankyropteris corrugata._ From a section in
the Cambridge Botany School Collection. (× 9.)]
In a paper on the tyloses of _Rachiopteris corrugata_, Weiss[1168]
draws attention to the fact that similar inclusions have not been
found in the tracheae of recent ferns. The occurrence of thin-walled
parenchymatous cells in the large tracheae of _Ankyropteris corrugata_
petioles and of other species is a striking feature. Williamson[1169]
compared these cells with the tyloses in the vessels of recent
flowering plants, and in a later paper[1170] he suggested that the
included cells may belong to saprophytic or parasitic fungi. It is, as
Weiss points out, difficult to explain the occurrence of tyloses in
tracheae not immediately in contact with living parenchyma. It may be
that the pits in the tracheae of _Ankyropteris_ were open spaces as in
the xylem of recent ferns described by Gwynne-Vaughan, and if so this
would facilitate the invasion of the conducting elements by growing
cells. A comparison is made by Weiss between certain cell-groups found
by him in the tracheae of _Ankyropteris_ and by Miss Jordan[1171] in
the vessels of the recent dicotyledon _Cucumis sativus_. In a more
recent paper on tyloses Miss McNicol[1172] expresses the opinion that
pseudoparenchyma in the tracheae of the fossil petioles owes its origin
to fungal hyphae.
Williamson compared the petiole bundles of _Ankyropteris corrugata_
with those of recent Osmundaceae, a comparison based on the structure
of the leaf-trace before its separation from the stem and its
assumption of the =H=-form. It is noteworthy, however, that this
comparison has acquired a greater significance as the result of recent
work. The stele of _Ankyropteris_ bears a fairly close resemblance to
that of _Zalesskya_ described by Kidston and Gwynne-Vaughan; in both
types the xylem is represented by two kinds of tracheal tissue. In the
Permian Osmundaceous genus the centre of the stele consists of short
storage tracheids, while in _Ankyropteris_ we may regard the central
parenchyma and scattered tracheae as derivatives of the solid xylem
core of some ancestral type. Moreover, the appearance and arrangement
of the phloem and the tangentially elongated elements external to it
(fig. 314) remind one of the extra-xylem zone in recent Osmundaceae.
That the Osmundaceae and Zygoptereae are closely related groups
there can be little doubt; of this affinity and common origin[1173]
_Ankyropteris corrugata_ affords striking evidence.
The difference between the steles of _Ankyropteris Grayi_ and _A.
scandens_ (figs. 310, D; 311) and that of _Ankyropteris corrugata_ is
comparatively small. In the two former species the cylindrical form has
become stellate owing to the radial extension of the xylem arms. It may
be that this more elaborate style of vascular construction is connected
with the climbing habit of _A. scandens_ and possibly _A. Grayi_. The
radial extension of the xylem and the consequent alternation of the
yielding parenchymatous cortex and the more rigid tracheal arms would
probably render the water-conducting elements less liable to injury in
a twisting axis[1174]. In _Anachoropteris Decaisnii_[1175], described
by Renault, and more especially in _Asterochlaena laxa_[1176] Stenzel,
a Lower Permian type from Saxony (fig. 324), the xylem of the stele is
much more deeply lobed than in _Ankyropteris Grayi_ or _A. scandens_.
[Illustration: FIG. 318. _Etapteris Scotti_, P. Bert. (From Tansley,
after Renault.)]
_Etapteris Scotti_. Figs. 308, B; 309, E; 318.
P. Bertrand has proposed this name for a species of petiole from the
Lower Coal-Measures of England referred by Binney[1177] to _Zygopteris
Lacattii_ Ren., and included by Williamson[1178] in his comprehensive
genus _Rachiopteris_. Bertrand[1179] regards the English species,
which is recorded also from Germany[1180], as distinct from Renault’s
type[1181] and therefore proposes a new name. The petiole stele has the
=H=-form, but its structure is simpler than that of the _Ankyropteris_
petiole.
The horizontal band of xylem has at each end two oval groups of
tracheae connected with it by a single row of xylem elements (fig.
318). From the lower part of each oval group a small strand is
detached; the two strands from one side of the stele coalesce and then
separate to pass into two pinnae. Fig. 308, B, shows four stages in the
giving-off of the secondary branches. This species, therefore, produces
four rows of branches in alternate pairs from the right and left sides
of the petiole.
The first stage is shown at 0, 0, fig. 308, B; the two projecting
groups of protoxylem mark the points of departure of a pair of small
strands. At 1, the projections are more prominent, and at 2 a pair of
strands has become detached: at a later stage, 3, these two strands
unite to divide later (4) into two slightly curved bundles.
[Illustration: FIG. 319.
A–C. Sporangia of _Etapteris_ (?).
D–G. _Botryopteris forensis_. (After Renault.)]
Our knowledge of the fructification of _Etapteris_ is based on
Renault’s account of sporangia, which he regarded as belonging to
_Zygopteris_ (_Etapteris_) _Lacattii_. They have the form of elongated
slightly curved sacs (2·5 × 1·3 mm.) borne in clusters (fig. 319, A–C)
on slender ramifications of the fertile frond, which is characterised
by the absence of a lamina. Each sporangium has a pedicel, and three
to eight sporangia are attached to a common peduncle; the walls of the
sporangia are at least two cell-layers in thickness and the annulus
consists of a band of thick-walled cells passing from the crest down
each side (figs. B and C), thus differing from the sporangia of
_Botryopteris_ (fig. 319, D, F) in which the broad annulus is confined
to one side.
[Illustration: FIG. 320. _Stauropteris oldhamia_. (After Tansley. From
a section in Dr Scott’s Collection. × 60.)]
It is practically certain that the fronds described by Grand’Eury[1182]
as _Schizopteris pinnata_ (fig. 309, E) and _Schizostachys frondosus_
represent respectively the sterile and fertile leaves of _Etapteris_.
Zeiller[1183] gives expression to this by substituting the generic name
_Zygopteris_ for _Schizopteris_, and we may now speak of the leaves as
_Etapteris_. Dr White[1184] has referred to a new genus, _Brittsia_,
some impressions of pinnate fronds from the Coal-Measures of Missouri
which, as he points out, bear a close resemblance to _Schizopteris
pinnata_ Grand’Eury (fig. 309, E). No sporangia have been found; it is,
however, probable that _Brittsia problematica_ represents fragments of
a leaf borne by a plant closely allied to _Etapteris_ (_Zygopteris_).
The broad rachis bears crowded pinnae given off at a wide angle; the
small pinnules are rather deeply lobed or pinnatifid (3–10 mm. long
by 1·5–3 mm. broad). The lamina is traversed by irregularly lobed and
occasionally anastomosing veins. In the fertile pinnae the segments
have no lamina but bear bundles of pedicellate sporangia.
It should be noticed that the sporangia described by Renault and by
other authors as those of _Zygopteris_ (fig. 319, A–C) have not been
found in organic continuity with a frond showing a well-preserved
vascular strand. It is, however, certain that this characteristic
annulate sporangium, borne on branched and slender pedicels, was
produced on fronds with a much reduced lamina belonging to some species
of the Zygoptereae, _Etapteris_ and probably also _Ankyropteris_.
_Stauropteris._
This genus was instituted by Binney for petioles from the Lower
Coal-Measures of Oldham (Lancashire).
_Stauropteris oldhamia_ Binney[1185] is characterised by a stele (figs.
308, E–G; 310, C; 320; 321) composed of four groups of xylem which
Bertrand regards as homologous with the antennae of _Diplolabis_,
_Ankyropteris_, and _Etapteris_, the horizontal cross-piece of
these genera being absent in _Stauropteris_. Williamson spoke of
this species as “one of the most beautiful and also one of the most
perplexing of the plants of the Coal-Measures”; he discussed its
possible affinity with both Lycopods and ferns, deciding in favour of
the latter group[1186]. In transverse section the petiolar vascular
axis is approximately square, the xylem groups forming the ends of the
diagonals; the tracheal groups are separated by phloem and the centre
of the stele in the primary rachis is also occupied by that tissue,
which is connected by four narrow strips with the external phloem. The
structure of the petiolar vascular axis is very clearly shown in the
drawing by Mrs Tansley reproduced in fig. 320. Protoxylem elements
occur close to the surface of each of the four arms of the xylem; the
bays between the two lateral and the two lower xylem groups contain
large sieve-tubes. Portions of the inner cortex are seen in places
abutting on the small-celled pericyclic tissue.
The right and left halves of the stele are not absolutely identical
(fig. 320; fig. 308, E); this is due to the fact that secondary
branches are given off in four rows, two alternately from the right and
left sides. The preparation for the departure of the lateral strands
alters the configuration of the stelar xylem groups. The protoxylem
groups are not external but separated from the surface by one or two
layers of metaxylem. In fig. 308, E, the occurrence of two protoxylem
strands in the right-hand groups of metaxylem marks an early stage in
the detachment of branches. These two protoxylems are the result of
division of single protoxylem strands like those in the left-hand half
of the stele. At a later stage the petiolar stele assumes the form
shown in fig. 308, F, and two small bundles are detached to supply
aphlebiae: this is followed by the stage shown in fig. G, where two
four-armed strands are passing out to a pair of branches of the leaf
axis. The separation of these two meristeles leaves the right-hand half
of the stele in the condition seen on the left-hand side of fig. E.
The diagrammatic sketch represented in fig. 310, C, shows one pair of
branches in organic connexion with the rachis, and each of these arms
contains an obliquely cut vascular strand like those in fig. 308, G.
The cortex consists for the most part of fairly thick-walled parenchyma
(fig. 321) which in the hypodermal region is replaced by a zone of
thin-walled lacunar tissue. A few stomata have been recognised in the
epidermis[1187]. The lower left-hand branch seen in fig. 310, C, has
been shaved by the cutting wheel so that the aerenchymatous tissue,
_l_, is shown in surface-view: a portion of this tissue is enlarged in
fig. C′. The same delicate chlorophyllous tissue forms a folded and
shrivelled layer with an uneven margin on the surface of the rachis
and lateral branches. This hypha-like tissue, which was discovered by
Scott[1188] and figured by Bertrand[1189], doubtless represents the
much reduced lamina of the highly compound leaves; it may be compared
with the green outer cortex of _Psilotum_ shoots and with the lacunar
tissue in the capsule of the common moss, _Funaria hygrometrica_.
[Illustration: FIG. 321. _Stauropteris oldhamia_: _a_, sections of
pinnae. (× 10. From a section in the Cambridge Botany School Coll.)]
The rachis reproduced in fig. 321 is surrounded by an enormous
number of sections, some transverse, others more or less vertical,
of branchlets of various sizes. Fig. 310, B, shows the three-rayed
vascular axis of a branch of a lower order than those seen in fig. C,
and the single vascular strands of still finer ramifications of the
leaf. The extraordinary abundance of axes of different sizes, many of
which are cut in the plane of branching, in close association with the
rachises of _Stauropteris_ affords a striking demonstration of the
extent to which the subdivision of the frond was carried in a small
space. The leaves must have presented the appearance of a feathery
plexus of delicate green branchlets devoid of a lamina, some of which
bore terminal sporangia. It may be that the delicate fronds were borne
on a slender rhizome which lived epiphytically in a moist atmosphere on
the stouter stems of a supporting plant.
The sporangia[1190] of _Stauropteris oldhamia_ are exannulate and
nearly spherical, with a wall of more than a single row of cells;
they occur at the tips of slender and doubtless pendulous branchlets.
The discovery by Scott[1191] of germinating spores (fig. 323) in a
sporangium of this type supplies an interesting piece of evidence
in favour of the fern nature of these reproductive organs. Similar
germinating spores have been described by Boodle[1192] in sporangia of
_Todea_.
[Illustration: FIG. 322. Sporangia of _Stauropteris oldhamia_. _St_,
stomium: _p_, palisade tissue. (From Tansley, after D. H. Scott,
from a drawing by Mrs D. H. Scott.)]
_Stauropteris burntislandica._
This Lower Carboniferous plant identified by Williamson with the
Oldham plant from the Lower Coal-Measures is referred by Bertrand to
a distinct species. In the structure of the rachis stele it agrees
closely with _Stauropteris oldhamia_; the main vascular strand gives
off four rows of branches, two from each side, and aphlebiae were
present at the common base of each pair of pinnae. Mrs Scott[1193],
who has recently described the sporangia of this species, speaks of
one specimen in which germinating spores were found. The same author
gives an account of some curious spindle-shaped bodies which she found
in association with _S. burntislandica_. The nature of these organs
is uncertain; Mrs Scott inclines to regard them as glands borne in
pairs on lateral pedicels of the frond: she adopts for these the name
_Bensonites fusiformis_ proposed by Dr Scott. If there is a reasonable
probability, as there certainly seems to be, in favour of connecting
these organs with _Stauropteris_, it is legitimate to question the
desirability of adding to the long list of names included in the group
Coenopterideae.
[Illustration: FIG. 323. Germinating spores from a sporangium of
_Stauropteris_. (From Tansley, after D. H. Scott.)]
_Corynepteris._ Fig. 309, C, D.
This genus was founded by Baily[1194] on fragments of a fern from
Carboniferous rocks in County Limerick, Ireland, characterised by a
peculiar type of fructification which he named _Corynepteris stellata_.
More complete examples of the same genus have been described by
Zeiller[1195] from the Coal-field of Valenciennes. The sporangia are
large, ovoid, and sessile; the annulus (fig. 309, D) has the form of a
complete vertical band several cells in breadth: five to ten sporangia
are grouped round a receptacle. Zeiller describes two species as
_Sphenopteris (Corynepteris) coralloides_ Gutb. and _S. (Corynepteris)
Essinghii_ And.; in both the fronds are quadripinnate and bear
aphlebiae at the base of the pinnae. The former species is recorded
by Kidston[1196] from the South Wales Coal-field. A single pinnule of
_C. coralloides_ is shown in fig. 309, C. Potonié[1197] refers this
frond to his genus _Alloiopteris_: the portion of a pinna represented
in fig. 354, G shows the characteristic modified pinnule next the
rachis. Zeiller draws attention to the occurrence of two parallel
lines on the rachis of a specimen of _Corynepteris coralloides_ which
he figures[1198], and suggests that these may indicate the existence
of an =H=-shaped form of vascular strand like that of _Etapteris_ and
_Ankyropteris_. The sorus of _Corynepteris_ is comparable with that of
the Marattiaceae, but the broad annulus is a difference which suggests
affinity to _Etapteris_. The sorus is similar to that in _Diplolabis_
(fig. 309, A), but in that genus the sporangia are exannulate.
• • • • •
The vascular axis in the stems of different members of the
Coenopterideae assumes a variety of types. In _Botryopteris antiqua_
the xylem forms a solid protostele in which no protoxylem strands have
been recognised; in other species, e.g. _B. ramosa_, the cylindrical
stele is similar to that of _Trichomanes radicans_ (Hymenophyllaceae)
in the more or less central position of the protoxylem. In
_Botryopteris forensis_ the protostele is said to be exarch. The
probability is that the central Botryopteris type is the endarch
protostele, a form of vascular axis which may be regarded as primitive.
The leaf-traces of the Lower Carboniferous _Botryopteris antiqua_ are
simple oval strands differing but slightly from the cylindrical stele
of the stem. In the Upper Carboniferous British species the petiolar
vascular strand has become more specialised and farther removed from
that of the stem; in _B. forensis_ the distinction between leaf
and stem steles is still more pronounced. It is perhaps legitimate
to regard these types as representing an ascending series, the more
primitive of which are distinguished by the greater similarity between
leaf and stem, organs differentiated from a primitive thallus[1199],
that is from a vegetative body. Portions of this ultimately became
specialised as lateral members or leaves, while a portion acquired the
character of a radially constructed supporting axis or stem.
[Sidenote: ANKYROPTERIS, ETC.]
The vascular strand characteristic of the Zygoptereae is represented
by the =H=-shaped form as seen in _Ankyropteris corrugata_ or in a
more complex form in _A. bibractensis_. This style of strand may be
regarded as a development from the simple strands of _Grammatopteris_
and _Tubicaulis_ or _Botryopteris antiqua_ along other lines than
those followed by _B. forensis_. The extension of the xylem in two
symmetrically placed arms at the ends of the cross-piece of the =H=
is correlated with the habit of branching of the leaf-system which
forms one of the striking peculiarities of many of the Zygoptereae.
The solid type of stele characteristic of the Botryoptereae is closely
matched by that in the Lower Carboniferous stem discovered by Mr
Gordon[1200]. By the partial transformation of the central xylem region
into parenchymatous tissue and the concentration of water-conducting
elements in the peripheral region the style of _Ankyropteris corrugata_
was developed. The vascular strand of the older plant, which is of
the _Diplolabis_ type, may be regarded as a more primitive style than
that of the =H=-form of petiole strand represented by _Ankyropteris
corrugata_. A further stage in evolution is seen in the stem stele
of _Ankyropteris Grayi_ and _A. scandens_, both of which have the
=H=-form of meristele. This step in increasing complexity of stem
stele, though probably connected with the increasing specialisation of
the leaf-traces, as held by Mr Tansley, may also be associated with the
development of a climbing habit. In _Asterochlaena laxa_ Stenzel (fig.
324) and _A. ramosa_ (Cotta)[1201] the tendency towards a stellate
expansion of the originally cylindrical form of stele reaches a higher
degree, with the result that a style is evolved which agrees closely
with that of the conducting tissue of some existing Dicotyledonous
Lianes.
Attention has already been drawn to the generalised features exhibited
by the Coenopterideae both in the anatomy of the steles and in the
structure of the sporangia. The conclusion arrived at is that while
the Coenopterideae foreshadow in some of their characters more than
one group of more recent ferns, some at least of their members afford
convincing evidence of the correctness of the view—which is also that
of Dr Kidston and Mr Gwynne-Vaughan—that the Osmundaceae and the
Coenopterideae are offshoots of a common stock.
[Illustration: FIG. 324. _Asterochlaena laxa_: part of stem with
petiole and a few roots. From Tansley, after Stenzel.]