spore-bearing bodies from the Coal Measures of Illinois[348]:
nothing is known as to the plant which bore them, but Sellards is
inclined to associate them with _Neuropteris decipiens_ Lesq.[349], a
species abundant in the same coal-field. Whatever may have been the
parent-plant it is probable, as the author of the genus believes, that
_Codonotheca_ is the microspore-bearing organ of a Pteridosperm. As
shown in fig. 427, 5, the form is that of a stalked cup consisting in
the basal portion of a stout axis, the peripheral tissue of which is
believed to have been fleshy, containing an axial rod of conducting
tissue running up to the floor of the cup, _c_, and then dividing into
six vascular strands, each of which forks into two branches. The upper
part is composed of six linear segments united basally to form the
sloping surface of the cup. On the inner face of each segment is a more
or less well-defined depression covered with large elliptical spores
·29–·31 mm. long by ·18–·19 mm. broad (fig. 427, 6, 8). The presence of
a median ridge (fig. 427, 8) indicates a bilateral origin. ‘There is
no grouping of the spores or other indication of the location of the
sporangia, which were doubtless more or less completely immersed in the
tissue, the dividing wall disappearing at maturity.’ The spores are
seen in fig. 427, 2, 3, on the inner face of the lobes. Some of the
specimens have a fairly long pedicel: in the example shown in fig. 427,
1, the fleshy part of the basal portion is not preserved, only the more
resistant vascular core. In a later account of these organs Sellards
speaks of several lying by the side of a central stalk to which he
thinks they were originally attached by slender pedicels. In view of
Dr Benson’s interpretation of the morphology of _Telangium_ it is
permissible to suggest that if a central sporangium in such a synangium
as that of _Codonotheca_ developed a megaspore and the peripheral
sporogenous lobes were sterilised, the result would be an arrangement
not unlike the apical region of the seed _Physostoma_, the tentacles
of which have been homologised with the canopy of _Lagenostoma_.
There are obvious difficulties in the way of this, perhaps strained,
comparison: the larger size of the spore-bearing linear segments of
_Codonotheca_ led Sellards to regard each as a synangium rather than
a single sporangium. But precise information as to the structure of
the American fossils is not as yet available. If the association of
_Codonotheca_ with _Neuropteris_ fronds has any significance it would
favour a reference of these organs to the Medulloseae. In the absence
of anatomical data it is impossible in some cases to distinguish
microspore-bearing organs of the _Codonotheca_ type from small seeds
enclosed in a lobed cupule or even seeds with a lobed integument:
a case in point is the New Brunswick species _Pterispermostrobus
bifurcatus_ Stopes[350].
[Illustration: Fig. 427. _Codonotheca caduca_. 1. The vascular tissue
preserved as a conical base which was originally surrounded by fleshy
tissue (_cf._ 5); I–III, VI, vascular bundles (× 2). 2, 3. Segments
with spores; _c_, floor of cup (nat. size). 4. Vascular tissue as a
cone in the middle of the fleshy base (nat. size). 5. Plan of the whole
organ opened out flat (nat. size). 6. Spores (× 28). 8. A single spore
showing median slit (× 84). (After Sellards.)]
A Spitzbergen, Culm, fossil recently described by Nathorst[351] as
_Codonotheca (?) pusilla_ is briefly referred to under the genus
_Pterispermostrobus_.
=SCHÜTZIA.= Geinitz.
This generic name was instituted by Geinitz[352] for some Permian
fossils obtained by Bergmeister Schütz and regarded by the author of
the genus as probably fertile branches of some Conifer. A more complete
account was published by Goeppert[353] in his ‘Permian Flora,’ where
the name _Anthodiopsis Beinertiana_ occurs on the Plates, printed
before the publication of Geinitz’s description, but in the text the
specimens are referred to _Schützia anomala_.
_Schützia anomala_ Geinitz.
The type-species, recorded from Bohemia and Silesia, is represented
by fertile shoots consisting of a thick main axis bearing apparently
two-ranked though probably spirally disposed short lateral branches,
each of which terminates in a receptacle with numerous crowded
linear-lanceolate bracts superficially resembling a partially expanded
inflorescence of a Composite. Goeppert believed that the branches
bore seeds and he refers to this species a number of detached,
longitudinally striated and bluntly terminated, seeds. The same author
describes other specimens from the same localities associated with
_Schützia anomala_, which he names _Dictyothalamus Schrollianus_[354]:
in habit these agree closely with _Schützia_ but the receptacles,
the reticulate appearance of which suggested the generic name
_Dictyothalamus_, bear a large number of small bodies regarded as
seeds. The preservation of the fossils is not such as to enable us to
determine their true nature but it is probable that _Schützia_ and
_Dictyothalamus_ are not generically distinct. In his description of
_Dictyothalamus_ Goeppert suggests that the two associated types may
be the male and female shoots of one plant, but he speaks of seeds in
both cases. Schimper[355], who unites _Dictyothalamus_ with _Schützia_,
regards the latter as female and the former as male.
_Schützia Bennieana_ Kidston.
This species, described by Kidston[356] from the Calciferous series of
Scotland, differs from _S. anomala_ in its much more slender axis and
in the relatively narrower and less globular clusters of bract-like
appendages. The principal axis bears three lateral branches with
terminal clusters of acute and narrow linear scale-leaves. No seeds
were found in association with the specimens.
_Schützia permiensis_ (Renault).
Renault founded this Permian species as _Antholithus permiensis_[357]
on a specimen from Lodève; it consists of an incomplete inflorescence
6·4 cm. long bearing four lateral branches with stalks 1·5 to 2 cm.
long terminated by clusters of small oval bracts 5 mm. long. Renault
compares the fossil with the recent Conifers _Glyptostrobus_ and
_Tsuga_, but it exhibits a much closer resemblance to _Schützia
anomala_.
The genus _Schützia_, originally described from Permian strata, is
recorded also from Westphalian strata in North Africa[358] as well
as from Lower Carboniferous rocks in Scotland. The data at present
available are insufficient to determine the morphological nature of the
fertile branches: the evidence adduced by Goeppert in support of the
occurrence of seeds is not convincing and the interpretation of the
bract-like appendages is still an open question; they may have formed
a cupular investment to seeds, but in the Scotch species the general
appearance rather suggests that they may be microspore-bearing organs
comparable with those of _Codonotheca_[359]. There are no adequate
grounds for supposing _Schützia_ to belong to the Coniferales, a view
advanced by some authors; it is much more likely to represent the
fertile shoots of a Pteridosperm.
=WHITTLESEYA=. Newberry.
The genus _Whittleseya_, referred by many authors to the _Ginkgoales_,
has no substantial claim to be regarded as allied to that group: its
position is still uncertain, but the recent discovery of fertile
specimens suggests the probability of a relationship to _Potoniea_ and
an identification of Whittleseya as another form of microsporophyll of
a Pteridosperm.
[Illustration: Fig. 428.
A, C. _Whittleseya elegans_. A, single leaflet, from Pennsylvania.
(After Lesquereux; ⅚ nat. size.) C, diagrammatic sketch of part
of a leaflet showing the teeth, the striated texture of the
carbonised layer, and the vein-like markings on a lower
surface. (After Thomas; enlarged.)
B. _Whittleseya brevifolia_. (After White; the smaller figure ⅚ nat.
size.)]
The generic name was given by Newberry[360] to some leaves, or possibly
leaflets, originally described by C. Whittlesey from the Coal Measures
of Ohio. Whittleseya is represented by species from several North
American localities[361] in Ohio, Pennsylvania, Arkansas, Nova Scotia,
and New Brunswick[362]; it occurs in Silesia and has recently been
found in the English Coal Measures[363]. The genus is confined to Upper
Carboniferous strata.
The leaves are fairly thick; the lamina is oblong, cuneate, broadly
triangular or linear, usually rounded and truncate (fig. 428, A,
C), generally dentate at the distal end, the proximal portion being
gradually or abruptly contracted and occasionally prolonged into a
short pedicel. The veins or ribs are parallel to the sides of the
lamina and except near the base unbranched.
_Whittleseya elegans_ Newberry.
The type-species, from North America and Europe, is characterised
by its shovel-like lamina from 3 to 6 cm. long closely resembling
in shape some lepidopterous scales; the surface is ribbed, each rib
corresponding to a tooth on the distal margin; on each of the parallel
ridges are 4–5 longitudinal lines indicating either veins or stereome
strands (fig. 428, A, C). The examination of preparations made by
Dr Kidston from a leaflet of this species enables me to add a few
facts with regard to the microspores. The spores, which cover almost
the whole surface of the lamina, show a tendency to a more or less
definite arrangement in longitudinal rows. Two types of cuticularised
membrane are represented among the associated fragments: in some
pieces of cuticle the cells are short and have straight walls while
in others the preservation is inferior and the cells appear to be
longer and narrower. One or both of these membranes probably belong
to the sporangia. The oval slit, which is a striking feature on
several of the spores (fig. 429), points to their bilateral nature
and dehiscence along the major axis. A comparison of these spores
with those obtained by Kidston from the English species _Whittleseya
fertilis_ reveals a very close agreement both in size and shape and
confirms the identification of the Staffordshire specimens as leaflets
of _Whittleseya_. The large size of the microspores and the gaping oval
aperture in some of them are features in which they agree closely with
the spores of _Dolerophyllum fertile_ described by Renault[364]. In
both cases the spores tend to be arranged in long groups and they are
practically identical in form and in the nature of the exine; those
of _Dolerophyllum_ are 280μ long while those of _W. fertilis_ reach
a length of 220μ. In some of the _Whittleseya_ spores the exine has
split as in the specimen shown in fig. 429, but in others there are two
curved lines along which dehiscence has begun, a character in which the
spores appear to be identical with those of _Dolerophyllum_ described
by Renault who speaks of dehiscence by means of an operculum. There
is, I venture to think, little doubt as to the very close affinity
of the two types. The systematic position of _Dolerophyllum_ is not
certainly established; if the generic identity of the leaves described
as _D. Berthieri_ Ren. and the petrified specimens named _D. fertile_
is assumed, it is a legitimate inference that the genus is founded on
fertile pinnules of a Pteridosperm with foliage of the _Neuropteris_
or _Cyclopteris_ form. It would seem probable that both _Whittleseya_
and _Dolerophyllum_ _fertile_ are microspore-bearing leaflets of
Pteridosperms, possibly of some Medullosan plants. The leaflets of
_Whittleseya_ agree in form fairly closely with those of _Potoniea
adiantiformis_ Zeill. described on a previous page[365] as the male
organs of a Pteridosperm.
[Illustration: Fig. 429. Microspores of _Whittleseya elegans_. A, a
group of spores; B, a single spore. (Preparations made from an American
specimen, No. 2314, in Dr Kidston’s Collection.)]
The specimens described by Lesquereux from Pennsylvania as _W.
integrifolia_ and _W. undulata_ are less satisfactory than _W.
elegans_. The Arkansas species _W. microphylla_[366], characterised
by the obcuneate form of the lamina, is said to occur not only as
detached leaflets but in loose bunches at the ends of slender axes,
a circumstance favourable to the suggestion, based on the recently
described English specimens, that the _Whittleseya_ leaves may
be fertile pinnules of a Pteridosperm frond. Among other species
attributed to Newberry’s genus is _W. brevifolia_ Wh. from Nova
Scotia[367] with much smaller broadly triangular leaves 7 mm. long
exclusive of the petiole and 8 mm. broad at the distal end (fig.
428, B). Dr Matthew[368] has also described a Canadian species _W.
concinna_ from New Brunswick in beds assigned by Dr Stopes[369] to the
Westphalian series.
_Whittleseya fertilis_ Kidston.
Since the discovery of _Whittleseya elegans_ in the Coal Measures
of Staffordshire recorded by Mr Thomas, Dr Kidston has published
an account of some specimens from the same district under the name
_Whittleseya (?) fertilis_[370]: these consist of smaller cuneate
scale-leaves or leaflets 1·4–2·4 cm. long and 8–9 mm. broad; the
lamina has a dentate upper margin and is longitudinally ribbed. The
scales occur in superposed pairs, closely fitting but not organically
connected, at least in the state in which they are preserved; each
pair forms a sporangium-like case enclosing numerous spores but the
actual sporangia or synangia have not been preserved. Kidston describes
the spores as 210–222μ in length, elliptical, and characterised in
many cases by an oval slit; they are practically identical with the
microspores of _W. elegans_.
=DOLEROPHYLLUM=. Saporta.
This name was proposed[371] primarily for a large ovoid petrified
bud composed of rolled Cyclopteroid leaves from Permian rocks in the
Ural Mountains, which had been previously described by more than one
writer under different names and regarded as a young shoot of a Palm
or other Monocotyledon. Eichwald[372], who published good drawings,
called the fossil _Noeggerathia Goepperti_. Saporta connected with this
species some leaf-impressions from the Permian of Bohemia described
by Goeppert[373] as _Noeggerathia cyclopteroides_: in his family
Dolerophylleae[374] the French author included other leaves which
are probably not closely related to the type-species, _Dolerophyllum
Goepperti_. The Dolerophylleae are spoken of by Saporta and Marion[375]
as Progymnosperms. Before the publication of Saporta’s note Grand’Eury
had instituted the genus _Doleropteris_[376] and the family
Doleropteroideae; in the former he included several forms of leaves
agreeing generally with Goeppert’s _Noeggerathia cyclopteroides_.
Zeiller[377] adopts Grand’Eury’s designation for the Russian fossil in
preference to _Dolerophyllum_, a choice justified by considerations of
priority; but the latter name is retained in this account as it was
assigned by Saporta to the specimen of greatest botanical interest,
namely _Dolerophyllum Goepperti_, and because it does not suggest
affinity to Ferns.
_Dolerophyllum Goepperti_ (Eichwald).
The type-species is from the Zechstein of Orenburg in the Urals and
no specimens having precisely the same structure have been found
elsewhere. Eichwald assigned it, with leaf-impressions of various
kinds, to the Noeggerathieae and named it _Noeggerathia Goepperti_: it
had previously been described by Kutorga[378] as _Aroides crassispatha_
and Unger[379] included it among the Palms as _Palaeospathe aroidea_.
The species has been described also by Saporta and Marion and by
Renault[380]. The following account is based on sections cut from a
specimen in the British Museum[381] which, though assigned on the label
(within a query) to France and named _Dolerophyllum Berthieri_, is
undoubtedly Eichwald’s species from East Russia.
[Illustration: Fig. 430. _Dolerophyllum Goepperti_. Bud in
surface-view, A; in longitudinal section, B, C; in transverse section,
D. (British Museum; A, ⅔ nat. size.)]
The specimen (fig. 430) is 9 cm. long and 4·2 cm. broad: at the
slightly contracted and broken base is a piece of immature axis (fig.
430, B, _a_) 12 mm. in diameter overtopped by a mass of closely packed
leaves encircling one another like the bulb-scales of an Onion (fig.
430 A, C)[382]. Most of the leaves included in the bud were attached
to the axis below the broken base. The curved, dichotomously branched,
veins are seen on some of the pieces of lamina on the surface of
the bud (fig. 430, A). The considerable breadth of the leaves is
demonstrated by the longitudinal and transverse sections. In fig. C
most of the laminae can be traced through the whole height of each
of the steep-sided arches: a few overlapping margins are seen in
fig. D. The veins are for the most part imperfectly preserved and
appear as clear spaces at regular intervals in the brown mesophyll.
The axis of the shoot consists of homogeneous parenchyma except near
the sloping sides where narrow dark bands (fig. 430, B, _a_) mark
the position of desmogen-strands of thin-walled elongated elements
representing an early stage in the development of vascular bundles
some of which have already produced spiral tracheids. Short secretory
cells accompany the immature conducting elements. The lamina slightly
exceeds 2 mm. in thickness in the broadest part: the mesophyll is
composed of large parenchymatous cells of elliptical or spherical
form often loosely attached owing to the well-developed system of
intercellular spaces. The lower epidermis, assuming that the outer
face of the rolled leaves is the morphologically lower surface, forms
a uniform layer of palisade cells characterised by their free conical
ends (fig. 431, A, B, _e_) which in some oblique sections appear as
sharply pointed papillae with almost filiform apices; but while the
cells were doubtless papillose like those of the epidermis of a velvety
petal, the pointed form is due in part to the greater distinctness of
the dark contents as compared with the lighter cell-walls. The upper
epidermis is much less distinct; it consists of smaller flattened
cells with occasional stomata. Renault[383] figures a specimen with
stomata in a better state of preservation. The vascular bundles are
rendered conspicuous by large secretory cells on the lower side, in
the larger veins in the form of an arc or irregular group (fig. 432,
A), but in the finer veins as single cells (fig. 431, A, B). These
sacs resemble the tannin cells accompanying the veins in a leaf of
_Ginkgo_ (_cf._ fig. 631, G). The xylem-elements are of two kinds,
(i) elongated spiral and scalariform conducting elements, forming a
vertical plate of a few rows in the larger veins (figs. 431, 432) or a
small compact group in the more slender veins (fig. 413, A, B); (ii)
much larger isodiametric cells with reticulate or spiral thickening
resembling the transfusion-tracheids of Conifers or, perhaps more
closely, similar elements in the leaves of _Lepidodendron_. These short
tracheids are especially abundant on the flanks of the conducting
tracheids (figs. 431, _t_; 432, A, _t_), but they sometimes form a
complete investment. In the obliquely cut vein reproduced in fig. 431,
D, the transfusion-tracheids are abundant: a few are enlarged in fig.
431, E. In the smaller veins (fig. 431, A, B) they are represented by
the larger elements, _t_, on the sides of the conducting strands. The
protoxylem lies close to the upper edge in the middle line (_px_, figs.
431, A; 432, A); it is difficult to determine its precise position,
but it would seem to be slightly internal, the bundle being not quite
endarch. No phloem was recognised in the British Museum specimen, but
it presumably occurred, if present, where the black patch is shown
in fig. 432, A. Renault describes some phloem in sections which he
examined.
[Illustration: Fig. 431. _Dolerophyllum Goepperti_. Transverse
sections of leaves. _e_, epidermis; _s_, secretory cells; _t_,
transfusion-tracheids; _px_, protoxylem. (British Museum.)]
The mesophyll next the upper surface is in most cases represented
by spaces between the veins which give a crenulated outline to the
parenchyma (fig. 430, C, D); in some places the spaces contain remains
of very loose and crowded cells suggesting the original presence of
very lacunar tissue or possibly of thin-walled storage-cells. The
confinement of stomata to what is assumed to be the upper surface may,
as Renault and others have suggested, indicate leaves which floated on
water, an inference opposed to the view that the gaps in the mesophyll
mark the position of water-tissue.
No specimens have been described which enable us to correlate with
certainty mature leaves or foliage-shoots with the petrified bud.
It is, however, not improbable that the impression from Mount Pelé
near Epinac named by Renault _Dolerophyllum Berthieri_[384] may be
correctly referred to the same genus. The type-specimen consists of
an axis, whether a rachis of a compound leaf or a shoot with simple
leaves cannot be determined, bearing partially overlapping more or less
orbicular leaves 18–20 cm. in diameter, with a _Cyclopteris_ venation.
Among other leaves of unknown affinity referred to the same genus
attention is drawn to _Dolerophyllum pseudopeltatum_ (Grand’Eury)[385]
with an orbicular lamina reaching in some specimens 22 × 19 cm.
Specimens of _Dolerophyllum pseudopeltatum_ are figured by Renault
from the Commentry coal-field[386], some of which reach a diameter
of 12 cm. The only British specimen of a leaflet of this type which
I have seen is one in Dr Kidston’s collection from the Stephanian
series, Glamorganshire. It is probable that some at least of the
impressions assigned to _Dolerophyllum_ or _Doleropteris_ would be more
appropriately included in _Cyclopteris_ or _Cardiopteris_ and may have
been borne on the axis of large Pteridosperm fronds. Grand’Eury[387]
has also called attention to the difficulty of distinguishing the
larger _Cyclopteris_ leaflets from _Dolerophyllum_. Some of the
Cyclopteroid leaflets figured by Roehl[388] on _Neuropteris_ fronds
differ but slightly from those of _D. pseudopeltatum_. The shoot
showing large leaf-scars figured by Saporta and Marion[389] as probably
the axis of a _Dolerophyllum_ may well be a piece of _Cordaites_.
[Illustration: Fig. 432. A. _Dolerophyllum Goepperti_, section of
vein. B. _Dolerophyllum Berthieri_; _s_, microspores. C. _Dolerophyllum
fertile_. D. _Dolerophyllum fertile_, microspores. (After Renault.)]
_Microsporophylls assigned to_ Dolerophyllum.
Certain problematical fossils found in association with the sterile
leaves of _Dolerophyllum Berthieri_ have been described by Renault as
the male organs of that species. These are elliptical discs, 6 × 5 cm.,
with an excentrically placed stalk: embedded in a carbonised lamina are
numerous rows of elliptical bodies, 410μ × 280μ, characterised by two
curved longitudinal grooves on the surface and regarded by Renault as
pollen-grains. The chains of these microspores radiate outwards from
the neighbourhood of the stalk and cover most of the surface of the
disc (fig. 432, B). Some silicified pieces of similar spore-bearing
discs from Grand’ Croix named _Dolerophyllum fertile_[390] afford
additional information as to these remarkable reproductive organs.
The earlier account of this species by Renault is confirmed by
Solms-Laubach[391] who examined the original sections. The peltate
fleshy discs preserved as incomplete specimens consist of lacunar
parenchyma 15–18 mm. thick traversed at right angles to the surface
by numerous loculi (fig. 432, C), circular or oval in transverse
section, containing large numbers of microspores, _s_, similar in size
and form to those on the carbonised discs of the Mt Pelé specimen.
Vascular strands occur between and parallel to the spore-chambers.
The spores contain 8–10 cells (fig. 432, D) and Renault believes that
dehiscence of the exine occurred along the two deep grooves which mark
the limits of an operculum. He emphasises the peculiar structure of
the microspores by speaking of them as prepollinia: in size and in
the presence of internal cells (? male prothallus) they resemble the
spores found in the pollen-chamber of a seed described by Renault as
_Aetheotesta elliptica_[392] which he thinks may belong to a member of
the Dolerophylleae. It has also been suggested that _Codonospermum_ may
be a seed of _Dolerophyllum_[393]. An unconvincing specimen described
by Saporta and Marion[394] as a seed-bearing bract is regarded by them
as referable to _Dolerophyllum_, but the evidence for any connexion is
far from satisfactory.
There is nothing definite to be said with regard to the affinity of
_Dolerophyllum Goepperti_ or the microsporophylls represented by _D.
fertile_ and the specimens associated with _D. Berthieri_. Renault
considers that both sterile and fertile specimens belong to the same
genus, which he assigns to a position between Pteridophytes and
Cycads. As Solms-Laubach says, the evidence supplied by the structure
of the veins of _D. Goepperti_ in favour of a cycadean alliance is
not convincing. The type of vernation is unlike that of any known
Cycad or indeed of any Gymnosperm: the large size of the leaves is
another though weaker objection to this comparison, as the pinnae of
_Bowenia_ (fig. 391) and especially those of some species of _Zamia_
(fig. 388), are of equal or larger dimensions. If, as seems probable,
the xylem-strands are mesarch that is a point of contact with recent
Cycads, but the bundle as a whole bears but a remote resemblance to
that of a cycadean leaf and is much more like the veins of _Ginkgo_.
The bud shown in fig. 430, A, is probably a young shoot and not merely
a large compound leaf. If it were an unexpanded frond of _Neuropteris_
bearing _Cyclopteris_ pinnules we should expect to find indications
of scattered desmogen-strands such as would occur in the _Myeloxylon_
type of rachis. The resemblance to most forms of _Cordaites_ is by no
means close though a few leaves referred to that genus (_e.g._ _C.
circularis_, fig. 468, B) are similar to those of _Dolerophyllum_[395].
The male organs are unlike those of any other plant: they may be
described as sporophylls with microsporangia or perhaps synangia
embedded in the mesophyll and containing microspores similar to those
of some Pteridosperms or true Gymnosperms. Attention has been called to
the close resemblance of the spores shown in fig. 432, C, D to those
recently discovered by Kidston and referred to the genus _Whittleseya_
(fig. 429), and it is very probable that the striking similarity is an
index of affinity.
=Ottokaria.= Zeiller.
_Ottokaria bengalensis_ Zeiller. A specimen of doubtful affinity
from the Lower Gondwana (Karharbari beds) of Passerabhia, India, was
originally described by Zeiller[396] as _Feistmantelia bengalensis_,
but in a postscript he substituted the name _Ottokaria_ on the ground
that _Feistmantelia_ had previously been employed by Lester Ward.
Fig. 433 is drawn from the original specimen: it consists of a stalk
attached in a slightly excentric position to an almost orbicular
lamina, 2·5 cm. in diameter, with subacute marginal teeth and traversed
by numerous radially disposed striations. Zeiller compares the fossil
with _Whittleseya elegans_ and _Rhipidopsis ginkgoides_ and assigns
it with some hesitation to the Salisburieae. An examination of the
type-specimen led me to form the opinion that it may be a cupular organ
of a Pteridosperm that enclosed a seed. The lamina is slightly concave
and has the form of a shallow cup; moreover the surface-features
resemble those of a bract rather than the regularly veined lamina of
a foliage-leaf. The specimen bears a very close resemblance to one
figured by Bertrand[397] as the cupule of _Hexapterospermum modestae_
which he connects with fronds of _Neuropteris gigantea_.
[Illustration: Fig. 433. _Ottokaria bengalensis._ (Nat. size; drawn
from the type-specimen.)]
_Ottokaria_ occurs in association with fronds of _Glossopteris indica_
and with the large seeds described by Zeiller[398] as _Cardiocarpus
indicus_. I have lately obtained some evidence in favour of
assigning Feistmantel’s seeds _Carpolithes Milleri_[399] to the genus
_Glossopteris_: among several specimens from the Lower Gondwana rocks
of India I found an example showing a seed partially covered by a
scale-leaf in its natural position which appears to be identical with
scale-leaves of _Glossopteris_. It may be that the specimen represented
in fig. 433 belongs to _Cardiocarpus indicus_, though this is a mere
guess: my belief is that _Ottokaria_ is a cupular organ that enclosed
the base of a seed borne on a Pteridosperm. There is little doubt that
as additional data are obtained it will be found that Pteridosperms
played no inconsiderable part in the vegetation of Gondwana Land.
[Illustration: Fig. 434. _Strobilites Milleryensis._ (Natural size;
after Renault.)]
=Strobilites.= Schimper and Mougeot.
_Strobilites Milleryensis_ (Renault).
This species, from the Permian of France, was placed by Renault in
_Cycadospadix_[400], but having regard to the fact that it differs
essentially in habit from Mesozoic examples of that genus the
provisional name _Strobilites_[401] is suggested. The type-specimens
are long and narrow spikes or loose strobili, 8–16 cm. long and 2–2·6
cm. broad; a stout axis bears spirally disposed bracts 8–10 mm. long
attached by a slender decurrent pedicel expanded distally into a
fan-shaped laciniate lamina with a convex upper face, and there are
said to be two seeds attached to the sides of each pedicel (fig. 434).
The oval seeds appear to be platyspermic and resemble _Samaropsis
fluitans_ Daws. Two of the strobili figured by Renault are attached
at right angles to a second axis, a habit suggesting comparison with
that of a large compound frond. Renault is inclined to regard these
fertile shoots as cycadean and suggests a possible connexion with the
Permian stems _Ptychoxylon_ or _Poroxylon_, both of which are known
to have produced fairly numerous branches. In habit the spikes are
similar to some of the longer examples of _Cordaianthus_, but their
preservation is not sufficiently good to afford accurate information as
to the relation of seed to sporophyll. _Strobilites Milleryensis_ is,
perhaps, more likely to be the fertile branch of a compound frond of a
Pteridosperm, and it is significant that the seeds have been found in
association with _Callipteris_ leaves.
=COLPOXYLON=. Brongniart.
_Colpoxylon aeduense_ Brongniart. The genus was founded by
Brongniart[402] on a piece of stem 15 cm. in diameter from the
Permian of the Autun district and regarded by him as a distinct
type, with certain resemblances to recent Cycads. A thick section in
the British Museum, 13 cm. in diameter (fig. 435, A), illustrates
the main anatomical features described by Renault[403], to whom our
knowledge of the genus is chiefly due. There are two large steles of
irregular outline closely resembling those of _Medullosa Leuckarti_
(_cf._ fig. 416, H); each consists of a band of secondary xylem with
broad medullary rays and a narrow zone of phloem enclosing a central
region composed of parenchyma, in which strands of primary tracheids,
both reticulate and spiral, pursue a more or less horizontal course,
associated with a few small groups of vertical xylem-strands at the
inner edge of the secondary wood. The manoxylic nature of the wood is
clearly shown in fig. 436; the continuous ink-line marks the position
of the cambium and the dots show the internal protoxylem. Homogeneous
parenchyma surrounds the steles and beyond this is crushed tissue
containing large secretory canals and nests of stereome fibres either
as separate groups or in contact with the canals (fig. 435, C). In the
same peripheral tissue occur scattered collateral vascular bundles
(fig. 435, D) identical with those of _Myeloxylon_. The outer cortex
of the stem is marked off from the more homogeneous inner region by
a fairly distinct line where there is some indication of periderm.
The anatomical features are clearly shown in fig. 436, a photograph
from a section in Dr Kidston’s collection. At _a_ is an imperfectly
preserved vascular bundle with a crescentic group of secondary xylem
which is probably a leaf-trace that has just emerged from the secondary
cylinder. Renault speaks of these more or less circular strands as
possibly connected with reproductive shoots, but it is more probable
that they are homologous with the strands in the pericycle and inner
cortex of _Medullosa_ and represent leaf-traces before division into
smaller collateral strands. Renault describes the stem as possessing
seven vascular cylinders in the apical region and suggests branching
of the main axis as the cause of the increase in number: there is,
however, no evidence to support such correlation. The two steles seen
in fig. 435, A, become merged at a lower level into a single stele of
sinuous form (fig. 435, B).
[Illustration: Fig. 435. _Colpoxylon aeduense_. A. Diagram of a
transverse section of a specimen in the British Museum. (V. 9393.
½ nat. size.) B. Stele; much reduced from Renault’s figure. C, D.
Secretory canal with fibres and collateral vascular bundle (Kidston
Coll., 1946).]
[Illustration: Fig. 436. _Colpoxylon aeduense_. Transverse section
of half of the stem; _a_, stele of branch. Slightly enlarged. (Kidston
Coll., 1946.)]
Beyond the facts furnished by the leaf-trace bundles in the outer
cortex and the occurrence of two large scars about 5 cm. in breadth on
a stem figured by Renault, we have no positive information as to the
form of the leaves or the structure of the reproductive organs. There
is little doubt that the fronds were large and compound like those of
most species of _Medullosa_. There is, however, some slight evidence
that _Alethopteris Grandini_ Brongn. and seeds of the _Pachytesta_ type
(fig. 497) were borne on _Colpoxylon_ stems; this rests solely on the
association in the Loire coal-basin[404] of _Alethopteris_ fronds with
stems presenting structural resemblances to _Colpoxylon aeduense_.
The striking resemblance between _Colpoxylon_ and _Medullosa
Leuckarti_ has led certain authors[405] to propose the substitution of
_Medullosa_ for _Colpoxylon_. The resemblances though close are hardly
sufficient to warrant this course. In _Colpoxylon_ the stelar system
is simpler; there is no central region with star- or plate-rings as
in _Medullosa Leuckarti_ but, as in _Medullosa anglica_, the vascular
tissue consists only of large steles without a medullary system.
_Colpoxylon_ differs from _M. anglica_ in the reduction in some parts
of the stem of the vascular system to a single stele and, moreover, the
primary portion of the steles is much more parenchymatous in structure
and contains more irregularly anastomosing tracheal strands than is the
case in _M. anglica_.
The alteration in the pattern formed by the vascular system at
different levels in some Medullosan stems, especially in _Colpoxylon_,
may be compared with the varying disposition of the vascular strands
in the thick dorsiventral rhizomes of _Polypodium heracleum_ Kunz. and
_P. quercifolium_ L. In the rhizome of _P. heracleum_ there are two
vascular systems, an outer, cortical, system in the form of a hollow
cylinder composed of a lattice-work with polygonal meshes from which
branches are given off to the roots, and a more complex medullary
system that is concerned with the emission of leaf-traces. As shown
by a series of drawings reproduced in an account by Klein[406] of the
anatomy of these species of _Polypodium_, the inner system of steles
consists of two cylinders connected towards the upper surface of the
stem by a rounded arch of vascular strands; nearer the leaf-base the
two cylinders meet and eventually a larger cylinder is produced partly
from the upper halves of the two cylinders of the previous section
and in part from the connecting arch: the remains of the two smaller
cylinders become connected with the outer vascular system. These and
other changes suggest comparison with _Colpoxylon_ as also with the
stelar changes in the stem of _Ptychoxylon_. The comparison cannot
be carried beyond the grosser features and is chiefly interesting as
affording a further illustration of a similarity in plan between some
recent Ferns and extinct Pteridosperms and other Palaeozoic genera.
[Illustration: Fig. 437. _Rhexoxylon africanum_. Transverse section
of stem showing peripheral steles and central (_c_) stele. (After
Bancroft; × 2.)]
=RHEXOXYLON=. Bancroft.
_Rhexoxylon africanum_ Bancroft. The genus _Rhexoxylon_ was instituted
for a new type of stem represented by a single incomplete specimen
from the Karroo series of South Africa: its precise geological
horizon is not known but it may be referred provisionally to the
lower or Palaeozoic portion of the series. Though our knowledge of
the morphological features of the type-species is far from complete
owing in part to the method of preservation of the specimen and in
part to the destruction of the outer portion of the vascular tissue
and the whole of the cortex, Miss Bancroft’s careful description[407]
demonstrates the existence of characters which justify the employment
of a new generic name. _Rhexoxylon_ is more nearly related to the
Medulloseae than to any other group and is particularly interesting as
the first recorded example of this group from the Southern Hemisphere.
Fig. 437 shows a transverse section (7 × 5 cm.) of the stem. The
ground-tissue consists of fairly large-celled parenchyma with sclerous
nests and a few bands of periderm. At the periphery of the stem are
radially disposed groups of vascular tissue varying in size and to
some extent in shape. Unfortunately the stem is incomplete and it is
impossible to say how much vascular or other tissue originally existed
beyond the present corroded edge. The vascular groups, or steles as
they may legitimately be called, follow a vertical course through
the length of the block (6·5 cm.) and afford only slight evidence of
branching or anastomosing. A close examination of the steles shows that
they consist of portions of two series, an inner and outer set; there
is also a curved vascular band in the central ground-tissue (fig. 437,
_c_) and some isolated and scattered patches of vascular elements.
Each stele of the inner series is made up of two parts, an outer
smaller and normally orientated group of secondary xylem and a larger
inversely orientated inner group of identical structure. A single
stele of the inner series is shown in fig. 438, B, C; the larger inner
portion consists of slightly divergent rows of tracheids and uniseriate
medullary rays and is separated from the smaller portion by a narrow
space, _a_, occupied by crushed tissue which may correspond to the
‘partial pith’ or primary xylem of a Medullosan stele. The two groups
of xylem are no doubt the products of two cambium arcs, the protoxylem
of each group being situated on the flat inner face. The cambium and
phloem are represented only by crushed brown cells on the curved outer
edge of the xylem. The separate individuality of the two portions of
each stele is indicated not only by the presence of the ‘partial pith’
but by the discontinuity of the tissue at the ends of the narrow space.
The tracheids seen at _b_, fig. C, are in oblique longitudinal section
and are probably being detached to form a leaf-trace. This type of
stele may be compared with the steles of _Medullosa Solmsi_ (fig.
416, L) but those of _Rhexoxylon_ differ in the lack of continuity of
the secondary xylem round the narrow band of crushed primary xylem.
The other steles of the inner ring exhibit the same dual nature though
with local modifications. In the stele seen in fig. 438, B, there is a
close approach to a continuous cylinder of secondary xylem especially
on the right-hand side. External to the inner series are several
portions of normally orientated secondary xylem-groups (fig. 437):
these probably represent a second series of steles separated from the
inner series by a narrow crushed arc of tissue on which the protoxylem
strands of the outer groups abut. The xylem of the outer steles agrees
in its normal orientation with the outer and smaller part of the inner
steles and, as there is no accompanying group of inversely orientated
xylem corresponding to the larger mass of secondary xylem of the inner
series, the outer strands are designated partial steles. The central
stele consists of two curved irregular bands composed of vertically and
obliquely running tracheids: the central part of this stele consists of
crushed tissue that probably represents primary xylem like that between
the two parts of each of the peripheral steles.
[Illustration: Fig. 438. _Rhexoxylon africanum_. A, radial wall of
tracheid; B, C, peripheral steles; _a_, primary portion of stele; _b_,
lateral strand. (After Bancroft.)]
_Rhexoxylon_ differs from the usual Medullosan type in the structure
of the secondary xylem which is composed of tracheids with an
Araucarian form of pitting: there are usually two alternate rows of
contiguous pits (fig. 438, A) and occasionally one or three rows.
The medullary rays are uniseriate and 3 to 15 cells in depth, a
feature characteristic of coniferous wood and not of the wood of the
Medulloseae.
In the absence of more complete information as to the anatomical
structure of this stem and of all information as to the leaves
or reproductive organs it is impossible to fix with precision
the systematic position of the genus. It is, however, clear that
_Rhexoxylon_ is closely connected with _Medullosa_ in certain
features though in the structure of the wood it exhibits important
peculiarities. The imperfectly known stem _Cladoxylon Kidstoni_[408]
shows a fairly close agreement with the African plant in the form of
the steles (fig. 460) but the pitting is scalariform.
=SUTCLIFFIA.= Scott.
_Sutcliffia insignis_ Scott.
The generic name _Sutcliffia_ was given by Dr Scott[409] to a peculiar
type of stem from the Lower Coal Measures of Shore, Lancashire, in
recognition of the valuable services rendered to Palaeobotany by the
late Mr W. H. Sutcliffe, the owner of the colliery from which several
new types of plants have been obtained. Two stems are recorded, the
type-specimen and a more recently discovered stem, also from Shore,
described by Miss de Fraine[410], which differs in several particulars
from Dr Scott’s species. In view of the well-marked peculiarities of
the second stem it is convenient to speak of it as _forma β_ instead of
including it without a distinctive epithet in _Sutcliffia insignis_. We
know nothing of the reproductive organs of the genus.
1. _Sutcliffia insignis, forma α._
This consists of a piece of stem approximately 12 × 7 cm. in diameter
characterised by a broad cortex of parenchyma with secretory sacs
and ducts and strands of mechanical tissue (fig. 439, A). Decurrent
and massive leaf-bases form a prominent feature as in the stem of
_Medullosa anglica_. The stele, though compressed before petrifaction,
was probably not quite cylindrical but more or less polygonal or
broadly triangular in section; it consists of groups of large primary
tracheids (350μ in diameter) with numerous bordered pits (fig. 439,
B, and fig. 440) embedded in an anastomosing system of parenchyma
containing scattered secretory sacs, a type of protostele like that
of _Heterangium_ and _Medullosa_ _anglica_ except in the possession
of exarch protoxylem strands. The metaxylem tracheids contiguous to
the external protoxylem elements have a dense spiral or scalariform
type of pitting. In the lower part of the stem the primary xylem is
enclosed by a cambium which has added a few secondary tracheids (120μ
in diameter), but in the upper part of the specimen the cambium is only
partially developed and the addition of secondary xylem has hardly
begun (fig. 440). A narrow band of secondary phloem was recognised in
places consisting of small-celled parenchyma with some sieve-tubes and
medullary rays continuous internally with the parenchyma of the primary
stele. In close association and occasionally in organic connexion with
the surface of the stele are several tangentially elongated and large
groups of vascular tissue associated with smaller oval strands varying
considerably in size. These groups, designated meristeles (Fig. 439,
A), are identical in structure with the main stele and are occasionally
invested by a feebly developed zone of secondary xylem and phloem. The
meristeles are detached at intervals from the parent stele around which
they form by anastomoses an irregular network: the larger meristeles
give off smaller strands and from these the actual leaf-traces are
produced by subdivision. It appears, however, that in this type
the meristeles are not completely used up in the production of the
leaf-traces, portions of them behaving as cauline vascular strands.
A protoxylem of a meristele still attached to the central protostele
occupies an internal position, and at a higher level, as separation
of the meristele is effected, the spiral tracheids occur on the inner
face. New meristeles are given off at intervals from the main stele
‘to compensate for those parts of the reticulum which were used up in
the formation of leaf-trace strands[411].’ The meristeles form the
starting-point for the leaf-traces, an intermediate system between the
main stele and the actual leaf-traces; they differ, therefore, from the
parent leaf-traces of _Medullosa anglica_, which are completely used up
by repeated subdivision. Moreover in _Sutcliffia_ the leaf-bundles are
concentric and not collateral.
[Illustration: Fig. 439. _Sutcliffia insignia_ Scott. A, B, E, forma α.
(After Scott.) C, D, forma β. (After de Fraine.)
A. Transverse section of stem; _a_, _b_, double rows of leaf-trace
bundles. The larger black masses are the meristeles; the smaller
patches represent bundles derived from the meristeles.
B. Longitudinal section of a radially symmetrical bundle from a
leaf-base.
C. The junction between the primary and secondary xylem.
D. Vascular system in transverse section: _m_, _m′_, meristeles; _s_,
main stele; _e_, extrafascicular strands; _lt_, leaf-traces; _c_,
secondary cortex.
E. Vascular bundle from petiole showing a ring of large sieve-tubes
surrounding the xylem, and portions of stereome strands.]
[Illustration: Fig. 440. _Sutcliffia insignis._ Part of the primary
xylem showing on the upper edge the beginning of secondary growth.
(Cambridge Botany School, 560.)]
A conspicuous feature of the stem of _forma α_ is the occurrence of two
double rows of vascular strands stretching across the cortex (fig. 439,
A, _a_, _b_). These are interpreted by Scott as downward continuations
in the stem of the inner surface of leaf-bases. The outer cortex of the
stem and leaf-bases has hypodermal strands of stereome which remain
separate or rarely anastomose, and form a superficial zone exactly
like that of some species of _Medullosa_. The leaf-trace bundles may
be radially symmetrical or unilateral in the arrangement of the xylem
which is in all cases completely surrounded by phloem. Fig. 439, B,
shows part of a longitudinal section of a large leaf-trace bundle:
spiral protoxylem elements (_px_) abut on the phloem (_ph_) and are
succeeded to the left by narrow scalariform and large reticulately
pitted tracheids. In the larger and radially constructed traces there
are several protoxylem-strands distributed over the surface of the
xylem, while in the smaller unilateral traces there may be one or
two protoxylem strands. A characteristic feature of the xylem of the
leaf-traces is the admixture of parenchyma with the tracheids (fig.
439, B, E) and another noteworthy character is the occurrence of large
thin-walled tubes in the phloem described by Scott as sieve-tubes and
compared with the large sieve-tubes in Marattiaceous leaf-bundles.
Immediately internal to the hypoderm is a row of leaf-bundles (fig.
439, A) each of which is accompanied by stereome strands.
The petioles, which reach a diameter of 12 cm., contain numerous,
occasionally anastomosing, concentric bundles. Nothing is known of the
fronds as a whole beyond the fact that they are spirally disposed and
had decurrent bases of large dimensions in proportion to the stem.
_Sutcliffia insignis, forma β._
This form, described by Miss de Fraine as _Sutcliffia insignis_[412],
is represented by a stem rather smaller than the type-specimen
described by Scott, though it is probably an older example of the same
species. It is distinguished by a greater development of secondary
xylem and phloem both on the main stele and the meristeles; it differs
also in the absence of the greater part of the cortex and leaf-bases
which have been cut off as the result of the formation of a deep-seated
periderm (fig. 439, D, C). The meristeles are smaller and fewer than in
the larger form and are distinguished by some other peculiarities. At
_m′_ in fig. 439, D, a meristele is seen attached to the main stele.
In _Sutcliffia insignis forma α_ the main stele is enclosed by an
irregular network of subsidiary steles or meristeles and these form
the points of departure of the leaf-traces, but the meristeles are not
completely used up in the process of conversion into leaf-traces. In
_Sutcliffia insignis forma β_ the meristeles agree in structure with
the main stele except in the smaller amount of secondary tissue: they
do not, however, form a network as in _forma α_ but occur as strands
parallel to the central stele, ‘giving off leaf-traces and ultimately
dividing up into smaller strands, often unequal in size, the primary
wood of the meristeles being entirely used up in the production of
radially symmetrical or unilateral bundles.’ In this respect, as Miss
de Fraine points out, _forma β_ agrees more closely than _forma α_ with
_Medullosa anglica_ and supports Scott’s view that _Sutcliffia_ is a
primitive type of Medullosan stem. The leaves were given off at fairly
long intervals as in some species of _Medullosa_: the leaf-traces are
exarch and similar to those of the type-species. The most striking
feature of the new stem is the presence of a vascular network (fig.
439, D, _e_) which encloses both the main steles and the meristeles;
it consists of extrafascicular strands composed of normally orientated
bands of secondary xylem and phloem often assuming a fan-like
arrangement and occasionally almost concentric or inversely orientated.
These strands are always accompanied by short, usually reticulate,
tracheids on the inner margin of the xylem: similar isodiametric
tracheids also occur in the pericyclic region. The extrafascicular
strands are believed to be secondary structures phylogenetically
independent of the meristeles and main stele, comparable with the
successive cylinders or arcs of secondary xylem and phloem in some
recent Cycads and in some species of _Medullosa_[413].
The stele is exarch and roughly triangular; except in the broader
zone of secondary tissue it agrees with the protostele of _forma α_.
The secondary xylem (fig. 439, C) possesses numerous medullary rays
3–4 cells broad and of considerable depth: the secondary phloem is
characterised by the presence of thick-walled elements, presumably
sieve-tubes, like those of _Medullosa Leuckarti_ and _M. anglica_.
The ground-tissue is rich in secretory tissue and the stem-surface,
from which the leaf-bases have been detached, is limited by a wide zone
of secondary tissue produced by a phellogen.
_Sutcliffia Williamsoni_ (Seward).
1876. _Myelopteris_ (_pars_) Williamson, Phil. Trans. R. Soc. Vol.
166, Pl. +ii.+ figs. 7, 8; Pl. +iv.+ fig. 17.
1893. _Rachiopteris Williamsoni_ Seward, Ann. Bot. Vol. +vii.+ p. 1.
1894. _Rachiopteris Williamsoni_ Seward, _ibid._ Vol. +viii.+ p. 207,
Pl. +xiii.+
1906. _Sutcliffia Williamsoni_ Scott, Trans. Linn. Soc. Vol. +vii.+
pt +iv.+ p. 62.
In an account of _Myelopteris_ (= _Myeloxylon_) published in 1876
Williamson included some sections of petioles from the Lower Coal
Measures which I afterwards with his concurrence transferred to the
genus _Rachiopteris_ as _R. Williamsoni_. In _Rachiopteris Williamsoni_
the vascular bundles are concentric and not collateral, and are
further distinguished from those of _Myeloxylon_ by the association of
parenchyma with the tracheids. In the arrangement of the bundles and
in the structure of the ground-tissue the petioles of _Rachiopteris
Williamsoni_ agree with those of _Myeloxylon_. An outstanding feature
of the vascular strands of the former is the occurrence at fairly
regular intervals in the peripheral part of the phloem of comparatively
large tubes described by me as secretory canals on the ground that
evidence was furnished of their development by the schizogenous
separation of cells to form a central canal. The canals were compared
with the large sieve-tubes of the Marattiaceae, but with the
qualification that ‘their mature form and their manner of development
are strongly suggestive of small secretory canals.’ Nothing was known
as to the stem which bore these petioles until Scott’s discovery of
_Sutcliffia_ with vascular bundles in the cortex and leaf-bases of the
same type as those of _Rachiopteris Williamsoni_. As Scott says, there
are a few distinguishing features which suggest that _R. Williamsoni_
is not specifically identical with the petioles of _Sutcliffia
insignis_, though the agreement is such as to justify the substitution
of _Sutcliffia_ for _Rachiopteris_. The tubular elements in the phloem
which I considered to be secretory canals are regarded by Scott and
Miss de Fraine as sieve-tubes.
Miss de Fraine gives an excellent summary of our knowledge of the genus
and discusses, in the light of the additional facts furnished by the
second stem, the position of _Sutcliffia_ in a phylogenetic series.
The new form strengthens the comparison instituted by Scott between
_Sutcliffia_ and _Medullosa_ and gives support to a close connexion
between the Medulloseae and the Cycadaceae. Scott lays stress on the
fact that the Medulloseae, except _Sutcliffia_, are polystelic, while
the Cycads, at least the adult stems, are monostelic. The views of
Worsdell and other botanists who believe that the Medulloseae and
the Cycads are intimately related are discussed on another page: in
reference to these views Miss de Fraine adds a caveat as to the danger
of attaching excessive importance to evidence based on seedling anatomy
when we are concerned with broad phylogenetic questions. The stele of
a cycadean stem may be derived from a protostelic type such as that of
_Sutcliffia_ by the gradual disappearance of the internal tracheids:
in _Sutcliffia_, as in some Cycads and species of _Medullosa_,
extrafascicular strands and cylinders are a characteristic feature
and these may well have arisen independently of the central stele
in response to physiological requirements. From such a type as
_Sutcliffia_ evolution may have proceeded along two lines; in one
direction new types were produced in which increasing complexity,
as represented by a multiplication of steles, was an outstanding
feature. These forms, illustrated by _Medullosa anglica_ and other more
elaborate species, proved inefficient and were unproductive. Along
another line the protostelic condition was maintained though in some
cases extrafascicular strands or cylinders and cortical steles were
superadded: it was this line that led to the recent Cycads.
_General considerations suggested by the anatomical features
of_ Medullosa.
A comparison of the stems described under the generic name _Medullosa_
reveals a considerable range in the grosser anatomical features
superadded to certain fundamental characters denoting a common
origin[414]. If additional data were available giving us a fuller
knowledge of individual plants differences between species would
be more clearly defined and would provide adequate grounds for the
institution of new genera for some of the types now included in
the comprehensive genus _Medullosa_. A proposal by Dr Lotsy[415]
to adopt the names _Pecopteromedullosa_ and _Neuropteromedullosa_
rests primarily on the relatively unimportant difference between the
fronds associated with certain Medullosan stems and, as Scott[416]
points out, this two-fold division if applied to such a species as
_Medullosa Leuckarti_ would result in its separation from species
which anatomically are clearly of the same generic type. In _Medullosa
anglica_, one of the oldest and simplest types, there are three
steles of equal importance, and each of them is practically identical
with the single stele of _Heterangium_. Each stele—as indeed all
Medullosan steles—consists of a strand of primary xylem enclosed by
secondary xylem and phloem, and it is obvious that the development
of a constantly increasing cylinder of secondary conducting tissue
about three centres would lead to serious mechanical difficulty: a
stem constructed on the plan of _Medullosa anglica_ or the smaller _M.
pusilla_ could not increase the thickness of its secondary vascular
tissue beyond a certain point without detriment to its efficiency. In
some types this difficulty is partially overcome by the production of
complete concentric cylinders of centrifugally developed conducting
tissue external to an inner system of concentric steles agreeing
individually with those of _Heterangium_ (fig. 415, B). _Medullosa
anglica_, regarded from the point of view of the architectural
efficiency of its vascular system, affords a much less promising _point
d’appui_ for further evolution than some of the forms described under
_Medullosa stellata_ in which the mechanical impasse is avoided by the
adoption of the cycadean plan as represented by such genera as _Cycas_
and _Macrozamia_. The English species _Medullosa centrofilis_ (fig.
417) affords the first example of a characteristic Medullosan feature,
namely the presence of a small concentric stele in the central region
of the stem: this so-called star-ring differs not only in its smaller
dimensions but in its more cylindrical form from the larger peripheral
steles. In the later Permian species, _e.g._ _Medullosa porosa_ and
_M. Solmsi_, the single star-ring of the older _M. centrofilis_ is
replaced by a large number of precisely similar conducting strands.
These star-rings are structurally comparable with the cortical steles
of _Cycas_ and, in position, with the medullary system of bundles in
a _Macrozamia_; they are essentially cauline and take no part in the
emission of leaf-traces. _Medullosa Leuckarti_ (fig. 416, H) resembles
in its vascular plan _M. centrofilis_, but in this larger stem there
are several star-rings and the enlarged peripheral steles are more or
less sinuous. In _Medullosa Solmsi_ (fig. 416, E) the star-rings are
still more numerous and the main vascular system consists of a double
series of concentric steles, each agreeing with the larger peripheral
steles of _M. Leuckarti_.
Some of the forms included in _Medullosa stellata_ appear to be very
different from _M. anglica_ and _M. Leuckarti_ (_cf._ fig. 416, F, A,
H), but their similarity is apparent if we imagine _Medullosa anglica_
with only one main stele (with the addition of star-rings) which is
stretched tangentially until it becomes a long and narrow plate-ring
and is then rolled into a hollow cylinder like that in fig. 416, D.
A modification of the tubular type of stele is seen in _Medullosa
stellata_ var. _corticata_ (fig. 416, G) in which the vascular
cylinder is broken up into two or more curved plate-rings, a change
superficially similar to that by which a dictyostele is produced from a
solenostele, but in _Medullosa_ it is not the overlapping of leaf-gaps
that is the cause of the change. A striking feature in the stem of
_Medullosa stellata_ represented in fig. 416, D and F (especially
F), is the inequality in breadth of the centrifugal and centripetal
xylem: this inequality is an expression of the difficulty caused by the
presence of an internal as well as an external addition of secondary
conducting tissue. A limit is set to the production of secondary
centripetal xylem by the space available for extension, whereas there
is unlimited room for increase in the case of the centrifugal tissue.
This tendency to a greater development of xylem and phloem on the
outer side of the primary portion of the steles is illustrated also in
_M. Solmsi_ var. _lignosa_ (fig. 416, L) where the outer of the two
series of peripheral steles has a much larger proportion of centrifugal
xylem. In _M. anglica_ the secondary xylem on the inner side of the
steles is not infrequently broader than the corresponding tissue on
the abaxial side[417]; but this is exceptional in the genus. A further
development of centrifugal conducting tissue without any corresponding
development of centripetal tissue is effected in certain cases (fig.
416, K, L) by the addition of concentric cylinders of centrifugal
xylem and phloem beyond the original concentric steles. The type
illustrated by _Medullosa Solmsi_ var. _lignosa_ and _M. stellata_ var.
_gigantea_ (fig. 416, L, K) at once suggests comparison with stems of
_Cycas_, _Macrozamia_, and _Encephalartos_, the chief difference being
the presence in _Medullosa_ of an inner series of concentric steles
and a central ground-tissue containing star-rings, though the latter
may be regarded as corresponding to the medullary system of bundles
in _Macrozamia_. In _Macrozamia_ the central region of the stem is
considered to be the pith of a monostelic stem, whereas in _Medullosa_
the stem is polystelic. In recent cycadean stems it is not uncommon
to find patches of inversely orientated xylem and phloem internal to
one or more of the cylinders of centrifugal vascular tissue. These
abnormal developments are considered by Worsdell[418] to be relics of
the inner portions of concentric steles possessed by the Medullosan
ancestors of recent Cycads. This interpretation affords a means of
bringing into closer relationship the polystelic Medulloseae and the
monostelic Cycadaceae, the apparent simplicity of the latter being
the result of the progressive loss of centripetal xylem and phloem,
the normal cycadean cylinder being therefore regarded as a one-sided
remnant of a concentric Medullosan stele. In other words, the Cycads
are descended from polystelic ancestors. As further evidence in support
of this view Worsdell points to the occurrence of concentric steles
in the cortex of _Cycas_ and their occasional presence in the pith
of other genera. Matte[419] has shown that in the seedling stem of
_Encephalartos Barteri_ (fig. 396, K) there are three concentric steles
each similar to a normal Medullosan stele: at a higher level in the
axis the steles become ‘unrolled’ and assume the form of one-sided
cylinders of centrifugal xylem and phloem.
In the peduncles of some recent Cycads, _e.g._ _Stangeria_, there is
a tendency towards a somewhat irregular orientation of the collateral
bundles that constitute the vascular cylinder, and tracheids
occasionally occur internal to the protoxylem of the individual
bundles[420]. Worsdell regards these features as evidence of a
Medullosan ancestry. If the sinuous plate-rings of a stem of _Medullosa
Leuckarti_ (fig. 416, H) were broken up into separate portions and
wholly or in part deprived of the centripetal xylem, the result would
be an arrangement of bundles comparable with that in a _Stangeria_
peduncle[421]. The scattered centripetal tracheids discovered by Scott
in _Stangeria_ and other cycadean peduncles are interpreted by Scott
and by Worsdell as relics of some ancestral centripetal xylem, but
with this important difference in the point of view; Scott believes
that they represent the almost completely aborted centripetal xylem
of a single stele like that of _Lyginopteris_, while Worsdell sees
in them fragmentary vestiges of the central primary xylem of two or
more Medullosan steles. An abnormal seedling of _Araucaria Bidwillii_
described by Shaw[422] exhibits features analogous to those in some
cycadean seedlings: within the normal stele a cambium forms an
inversely orientated vascular cylinder which at a lower level becomes
continuous with the outer centrifugal tissue, the whole vascular system
being eventually represented by two concentric steles. The polystelic
stage is a development of a monostelic condition, and the inner or
inverted portion of each of the two concentric steles is derived from
an inversely orientated cylinder in the central region of the root.
This abnormal root does not, however, supply an argument in favour
of the derivation of a monostelic type of stem from one that was
polystelic, but it shows a close relation between the two plans in
one organ. The seedling is not altogether normal in form apart from
structure and it is not improbable that the anatomical abnormality is
connected with some pathological cause.
It has been suggested[423] that Worsdell attaches too much phylogenetic
significance to the irregularities in the disposition and form of the
vascular bundles in the peduncle of _Stangeria_, and the criticism that
insufficient allowance is made for the possible reaction on structure
of the special physiological requirements of reproductive shoots is
well founded. Granting an overestimate of the arguments drawn from the
occasional occurrence of concentric vascular strands, a considerable
body of evidence remains in favour of Worsdell’s main contention.
Mrs Thoday[424] has drawn attention to certain features exhibited by
the inflorescence-axes of _Welwitschia_, particularly the occurrence
of concentric and inversely orientated bundles, similar to those
characteristic of the seedling of _Cycas siamensis_[425] described
by Matte, and to anatomical characters occasionally present in adult
cycadean stems and normally represented in _Medullosa_. She is of
opinion that the occurrence in _Welwitschia_ of certain Medullosan
features has a phylogenetic significance. The differences between the
Gnetales, Medulloseae, and Cycadales are considerable, and it would
seem unlikely that the anatomical resemblances described by Mrs Thoday
are of great value as criteria of close relationship. The comparison
of _Lagenostoma_ with gnetalean seeds is alluded to elsewhere. To the
statement that the presence of concentric and inversely orientated
steles in _Welwitschia_ are reminiscent of the polystelic Medulloseae,
Mrs Thoday adds the qualifying remark that the occurrence of four
concentric groups of vascular tissue in the hypocotyl of _Welwitschia_
is not sufficient to justify the conclusion that the ancestral type was
polystelic. This reservation accords with the contention of Scott and
other botanists, that the occasional occurrence in cycadean seedlings
and adult stems of anatomical features suggestive of polystely does
not in itself furnish an adequate reason for doubting that the
apparent monostely of Cycads is phylogenetically what it seems to
be, namely, an indication of monostelic ancestry. This brings us to
the question of a possible monostelic ancestor. It may be that the
Upper Carboniferous genus _Sutcliffia_ affords a clue to the problem
of the origin of the polystelic type illustrated in various forms by
_Medullosa_. The protostele of _Sutcliffia_ bears a close resemblance
to each of the three steles of _Medullosa anglica_; the fact that
_Sutcliffia_ is exarch and that _Medullosa anglica_ has mesarch xylem
is of secondary importance, particularly as exarchy is represented
within the genus _Medullosa_. The extrafascicular strands of xylem
and phloem and the accessory strands are points in which _Sutcliffia_
and _Medullosa anglica_ agree and, as Miss de Fraine[426] adds, the
meristeles of _Sutcliffia_ may be homologous with the leaf-trace
strands of _Medullosa_. Scott[427] gave expression to the characters
shared by these two types by describing _Sutcliffia_ as the most
primitive of the Medulloseae. It is suggested that the protostelic
axis of _Sutcliffia_ may be regarded as the starting-point of the
monostelic Cycads, the central mass of tracheal tissue being replaced
by a parenchymatous pith, while the extrafascicular and accessory
strands arose independently of the central stele in response to
increased physiological demands consequent on the increase in size of
the stem. From the same starting-point evolution may have progressed
along another line through such a type as _Medullosa anglica_ leading
to the more complex Permian species of _Medullosa_. Chodat’s view[428]
that the Medulloseae are Protocycadaceae, if we include _Sutcliffia_
as well as _Medullosa_ in the Medulloseae, is probably correct. There
is clear evidence of a close bond of union between recent Cycads and
the Medulloseae, and _Sutcliffia_ offers a possible means of deriving
complex polystelic types from a monostelic ancestor.
Worsdell’s opinion[429] that the stele of _Lyginopteris_ affords
evidence of derivation from a polystelic ancestor and is not
homologous with the true monostele of _Heterangium_ is opposed to
the undoubted signs of intimate connexion exhibited by these genera.
The Lyginopterideae are, as Scott[430] says, a less advanced group
than the Medulloseae and, it may be added, they are more remote from
the modern representatives of the Cycadales. The Lyginopterideae and
the Medulloseae are probably offshoots of a common stock, but the
Medulloseae occupy a position farther removed from the filicinean
ancestry than _Heterangium_ and _Lyginopteris_[431].
The relative meagreness of our knowledge of the reproductive organs of
the Medulloseae gives precedence to anatomical data in phylogenetic
considerations, but the evidence furnished by _Trigonocarpus_ and other
seeds that may fairly be assigned to Medullosan plants is in harmony
with the conclusions based on vegetative characters with regard to a
close affinity between the Medulloseae and Cycads.
The comparative examination of recent Cycads naturally suggested by any
attempt to compare the group as a whole with Palaeozoic types leads to
some apparently contradictory results. The habit of the megasporophyll
of _Cycas_ is usually quoted as a primitive attribute: the close
resemblance in plan and in manner of occurrence on the stem between
megasporophylls and foliage leaves recalls both Ferns and Medullosan
fronds. On the other hand the production of eight body-cells in the
pollen-tube of _Microcycas_[432] in place of the usual single cell may
also be regarded as a primitive character. It is perhaps possible, as
Miss Dorety[433] says, that the polyspermy may be a case of recurrence
and not a direct inheritance. _Microcycas_ differs from _Cycas_ in
having only one vascular cylinder, and if the presence of several
concentric cylinders in _Cycas_ be interpreted as an indication of
a closer connexion with a Medullosan ancestry, the _Microcycas_ type
would represent a more advanced stage in evolution. Attempts to arrange
plants according to a natural sequence are frequently frustrated by
instances of unequal progress in the development of vegetative and
reproductive organs; one or other set of members lags behind; some
characters point to the retention of primitive traits while others
indicate a marked progressive tendency. It is noteworthy that the
Mesozoic Bennettitales are characterised by a greater simplicity of
stem-structure than is the rule in recent Cycads, and both in their
vegetative features and in the structure of the seeds they are further
removed from the Medullosan type.
[Illustration: Fig. 441. _Steloxylon Ludwigii._ A, surface-view
(longitudinal) of stem showing leaf-bases. (After Solms-Laubach.) B,
longitudinal section showing anastomosing vascular strands. C, steles,
the lower one showing the emission of a leaf-trace. (B, C, after
Schenk.)]
II. A. =STELOXYLEAE.=
=Steloxylon.= Solms-Laubach.
_Steloxylon Ludwigii_ (Goeppert and Stenzel). The genus is founded
on a piece of stem from Siberia, possibly of Permian age though
not improbably older, which was originally described as _Medullosa
Ludwigii_[434]. It is characterised by numerous cylindrical and
band-like vascular strands forming an irregular anastomosing system
(fig. 441, B) and by crowded spiral leaf-scars on the exposed face.
The appearance presented by the transverse section figured by these
authors, while suggesting comparison with _Medullosa_, reveals a
distinctive character, namely the absence of a definite peripheral
system of vascular rings such as forms a striking feature of the
continental Medulloseae. A more complete description was afterwards
published by Schenk[435] who recognised more fully the peculiar
features and hinted at the possibility that the species might
more appropriately be regarded as a member of a distinct group.
Solms-Laubach[436] went a step further and instituted the generic name
_Steloxylon_, and in a later publication gave a fuller account of the
anatomical characters. The complete stem must have reached a diameter
of approximately 13 cm. The homogeneous ground-tissue forms a matrix
enclosing an anastomosing vascular system of cylindrical or oval steles
(fig. 441, C). Each strand consists of a band of secondary xylem
tracheids with one or several rows of circular or oval bordered pits
on the radial walls and narrow medullary rays usually 1–2 cells broad
and 1–4 cells deep, though occasionally deeper. No phloem is preserved.
The tissue in the centre of each stele is very imperfectly preserved,
but it is clear that the secondary xylem enclosed a central region
(‘partial pith’) like that in the steles of a _Medullosa_, doubtless
consisting of primary xylem and conjunctive parenchyma.
The stem is covered with leaf-bases of oval or circular section and
between them are small organs, probably multicellular hairs (fig. 441,
A). A leaf-base consists of an outer zone of strengthening tissue
and a parenchymatous ground-tissue traversed by two or more small
vascular strands which assume various forms. These petiolar strands
are simply portions of the main vascular system which bend outwards
at the periphery of the anastomosing network. The more noteworthy
features in which _Steloxylon_ differs from _Medullosa_, particularly
such species as _M. stellata_ and _M. Leuckarti_, are (i) the crowded
and comparatively small leaf-bases in place of the massive decurrent
petioles of _Medullosa_; (ii) the supply of the leaves by compact
branches of the stelar network instead of the bundles detached
as leaf-traces from a stem-stele of _Medullosa_ (the origin of a
leaf-trace in _Steloxylon_ is shown in fig. 441, C); (iii) the absence
of a peripheral system of vascular plate-rings and the irregular
distribution of cylindrical and plate-steles in the ground-tissue.
Nothing is known of the reproductive organs or leaves beyond the
structure of the attached leaf-bases. The opinion expressed by P.
Bertrand[437] that the fossil described by Stenzel as _Asterochlaena_
(_Clepsydropsis_) _kirgisica_ is the petiole of _Steloxylon_ was
abandoned after the additional facts published by Solms-Laubach.
As regards the affinities of _Steloxylon_: the structure of the steles
agrees closely with that of the star- and plate-rings of a _Medullosa_,
while the pitting of the tracheids is more like that in _Medullosa_
than _Cladoxylon_. In the tendency to a more radial than tangential
disposition of the band-like steles _Steloxylon_ recalls _Cladoxylon_
rather than _Medullosa_, but in _Cladoxylon_ the vascular system does
not form an irregular network as in _Steloxylon_. The information as
to the structure of the primary xylem is very meagre, but it points
to a closer connexion with _Medullosa_ than with _Cladoxylon_. On the
whole _Steloxylon_ may perhaps be defined as a genus allied to the
Medulloseae in the anatomical features of the stem more closely than
to other genera, but sufficiently distinct to be excluded from the
Medulloseae as at present understood[438].