PTERIDOSPERMEAE.
I. LYGINOPTERIDEAE.
LYGINOPTERIS.
The genus _Lyginopteris_ is selected for the first place in this
chapter simply on the ground that we have a fuller knowledge of its
morphology than in the case of other types. It is not regarded as the
most primitive member of its class. _Lyginopteris_ may be described
in a few words as a plant having the habit and to a large extent the
anatomical features of a Fern, but differing from existing ferns in the
possession of integumented megasporangia or seeds and in the power of
secondary growth in thickness by means of a cambium in both stem and
root. The seed (_Lagenostoma_) agrees with those of recent Cycads and
Gnetales more closely than with the corresponding organs in Conifers
or any other group, while the structure of the secondary wood is
practically identical with that of Cycads. The microsporangia occur
as groups of small bilocular sporangia, or synangia, at the tips of
fertile pinnae of highly compound fronds.
_Nomenclature and Historical Summary._ In 1866 E. W. Binney[93] of
Manchester published a short description of a small petrified stem
from the Lower Coal Measures of Lancashire and named it _Dadoxylon
oldhamium_, employing Endlicher’s term _Dadoxylon_ which that author
substituted for _Pinites_ as previously used by Witham[94]. Three
years later Williamson[95] drew attention to certain features in which
Binney’s type differs from the genus _Dadoxylon_ and substituted a new
name _Dictyoxylon_, suggested by the reticulate pitting on the walls
of the tracheids. In a subsequent paper Williamson[96] gave a fuller
description of Binney’s species and spoke of it as ‘one of the most
common plants in the calcareous nodules of the Lower Coal Measures’ of
Lancashire and Yorkshire. He connected certain casts of arborescent
dimensions with Binney’s type on the ground that the surface-features
of the casts are such as would be produced by partially decorticated
stems having a hypodermal reticulum of mechanical tissue like that
preserved in the small petrified specimen described by Binney (fig.
402). Mr Carruthers called Williamson’s attention to a paper by Mr
Gourlie[97] in which the generic name _Lyginodendron_ is instituted
for stem-casts identical in surface-features with the fossils figured
by Williamson. In spite of the much larger dimensions of the reticulum
on the casts described by Gourlie as compared with that in the outer
cortex of Binney’s stem, Williamson concluded that _Lyginodendron_ is
‘undoubtedly an inorganic cast of the prosenchymatous layer of the
bark of _Dictyoxylon_.’ It is but fair to add that Williamson was
influenced in coming to this conclusion by a discovery by Mr Nield of
a piece of a large petrified stem believed to be generically identical
with Binney’s type, but subsequently referred to a distinct genus[98],
which was comparable in size with the stems responsible for Gourlie’s
_Lyginodendron_ casts. The type-specimen of Gourlie’s _Lyginodendron
Landsburgii_[99], from Carboniferous rocks at Stevenston in Ayrshire,
Scotland, is represented in fig. 401. The convex areas represent casts
of depressions in a reticulum of cortical tissue, originally occupied
by comparatively delicate cells, which decayed or shrunk more quickly
than the enclosing framework of stronger fibrous elements that remained
as a prominent reticulum and produced the depressions bounding the
raised portions of the cast. Such a cast would undoubtedly be formed
by the stem on which Binney founded his species: the radially disposed
bands of thick-walled cells seen in the outer part of the section (fig.
402) are portions of an irregular anastomosing mechanical system, the
reticulate arrangement of which is seen in the impression of a rachis
of a _Lyginopteris_ frond shown in fig. 405, E, and indicated in the
more slender axis reproduced in fig. 404, A, _b_. This reticulate
form of cortical stereome on which Brongniart founded the genus
_Dictyoxylon_[100], a term since applied by Solms-Laubach and other
authors to a certain type of cortex not confined to a single genus of
plants, occurs also in some Palaeozoic lycopodiaceous stems[101] and in
itself cannot be regarded as a safe criterion of botanical affinity.
The largest example of Gourlie’s _Lyginodendron_ that has come under my
notice is an incomplete sandstone cast from Upper Carboniferous strata
near Harrogate reaching a length of 100 cm. and with convex areas 13
cm. long. A similar cast, 36 cm. broad, has recently been figured by
Nathorst[102] from the Culm of Spitzbergen, and from the Upper Devonian
of Ellesmere Land the same author has described impressions of a
cortical reticulum under the name _Lyginodendron Sverdrupi_[103]. These
specimens are interesting as pointing to the former occurrence in the
Arctic regions of stems—probably Lepidodendroid—reaching the dimensions
of a fairly large tree. As Potonié[104] pointed out, Gourlie’s generic
name serves a useful purpose for casts of stems of the type shown in
fig. 401 that cannot be assigned to a definite systematic position.
The genus was first used for a specimen which has nothing to do with
the plant usually spoken of as _Lyginodendron oldhamium_ (Binney).
Though loath to give up a name by which Binney’s type has long been
known, in spite of its retention in the second volume of this work I
feel compelled so far to conform to the recognised principles governing
nomenclature as to adopt Potonié’s generic term _Lyginopteris_.
[Illustration: Fig. 401. _Lyginodendron Landsburgii_. (Kidston Coll.
¾ nat. size.)]
_Lyginopteris oldhamia_ (Binney).
i. _Stem_.
1866. _Dadoxylon oldhamium_, Binney, Proc. Lit. Phil. Manchester, vol.
+v.+ p. 113.
1869. _Dictyoxylon oldhamium_, Williamson, Monthly Micros. Journ. vol.
+ii.+ p. 66.
1873. _Lyginodendron oldhamium_, Williamson, Phil. Trans. Roy. Soc. vol.
+clxiii.+ p. 404.
1899. _Lyginopteris oldhamia_, Potonié, Lehrbuch der
Pflanzenpalaeontologie, p. 171.
ii. _Leaf_.
1828. _Sphenopteris Hoeninghausi_, Brongniart, Prodrome, p. 51.
1872. _Edraxylon_, Williamson, Proc. R. Soc. vol. +xx.+ p. 438.
1874. _Rachiopteris aspera_, Williamson, Phil. Trans. R. Soc. vol.
+clxiv.+ p. 684.
1877. _Calymmatotheca Hoeninghausi_, Stur, Culm Flora, +ii.+ p. 266.
1905. _Crossotheca Hoeninghausi_, Kidston, Proc. R. Soc. vol. +lxxvi.+
p. 358.
iii. _Seed_.
1877. _Lagenostoma_, Williamson, Phil. Trans. R. Soc. vol. +clxvii.+ p.
234.
1903. _Lagenostoma Lomaxi_, Oliver and Scott, Proc. R. Soc. vol. +lxxi.+
p. 477.
iv. _Root_.
1876. _Kaloxylon Hookeri_, Williamson, Phil. Trans. R. Soc. vol.
+clxvi.+ p. 23.
[Illustration: Fig. 402. _Lyginopteris oldhamia_. Transverse section
of the type-specimen in the Binney Collection, Sedgwick Museum,
Cambridge. (After Arber.)]
i. _Stem_.
The petrified stem on which Binney founded the species was first
figured by Dr Arber[105] from a section in the Binney collection in
the Sedgwick Museum, Cambridge: this section (13 mm. in diameter)
is reproduced in fig. 402. The most striking features are: (i) the
pith consisting of an unusually large and irregular group of dark
thick-walled parenchyma, (ii) the broad cylinder of manoxylic secondary
xylem characterised by multiseriate medullary rays, (iii) the outer
cortex composed of dark radially disposed and oblique bands of
mechanical tissue separated from one another by partially destroyed
and tangentially elongated parenchymatous elements. It is this outer
cortex that Williamson aptly compared with the Roman numerals on a
clock-face. In the perimedullary region and in contact with the inner
edge of the secondary-xylem cylinder are six strands of primary xylem
representing the xylem halves of collateral bundles separated from the
primary phloem strands by the intervening cylinder of secondary wood.
Two of the primary xylem strands in lateral contact are seen in fig.
404, C; the other four occur as separate bundles. Each primary xylem
strand contains a small group of spirally thickened protoxylem elements
(_px_) associated with a few parenchymatous cells. The large primary
tracheids internal to the protoxylem are characterised by multiseriate
bordered pits on their walls, while those external to the protoxylem,
which are in contact with the innermost secondary tracheids, have
scalariform pitting. The dark patch _s_ (fig. 404, C) is a portion of
the large group of sclerenchymatous cells, shown in figs. 402, 403.
The perimedullary xylem strands of mesarch structure are the lower
portions of leaf-traces and, as Scott points out, ‘each of the bundles
surrounding the pith is, in fact, a sympodium, composed of the united
lower ends of successive adjacent leaf-traces.’ The larger of the two
bundles shown in fig. 404, C, is on the point of passing out to a leaf,
while the smaller strand is on its way to a higher level before bending
outwards through the secondary wood. Slightly beyond the middle of the
secondary xylem there is an arc of narrower tracheids comparable with
an incomplete annual ring. Although zones or arcs of narrow tracheids
are not uncommon in the wood of _Lyginopteris_ there is no satisfactory
evidence of regularly recurring seasonal changes. On the outer face of
the secondary wood are a few leaf-trace strands pursuing a vertical
course in the pericycle region; but the structure and behaviour of
these bundles are more clearly illustrated in the stem reproduced in
fig. 403. The tissue between the crushed phloem and pericycle and the
outer cortex (fig. 402) consists of radially compressed parenchyma with
scattered secretory cells separated from the more internal tissue by a
narrow band of periderm formed by a phellogen in the outer part of the
pericycle.
A larger and better preserved stem, 3·7 cm. in diameter, is seen
in fig. 403. In this stem the pith of parenchyma and scattered
sclerenchymatous nests is larger in proportion to the stele than
in Binney’s type-specimen. From the inner edge of the secondary
xylem several primary xylem-strands project as rounded wedges
or tangentially elongated groups where two traces are laterally
united in the perimedullary zone. The cylinder of secondary wood
is partially interrupted at _r_ by the bending outwards of the stele
of an adventitious root cut across transversely as it bends down
after emerging from the outer cortical region. In more or less close
association with the outer surface of the secondary xylem are four
pairs of leaf-trace bundles and one larger trace at _d_ containing
two widely separated protoxylem strands and faced externally with
an arc of secondary xylem: this is a leaf-trace which shows by the
slight constriction on the outer edge of its primary xylem that it is
beginning to divide into a pair of equal strands. A precisely similar
strand is shown on a larger scale in fig. 404, D. The twin bundles
seen at _b_, fig. 403, represent a divided leaf-trace at a slightly
higher level than the partially severed trace at _d_, and the arcs of
secondary xylem are narrower. The appearance of the double leaf-trace
at a still higher level is shown at _c_: the two strands are farther
apart and the secondary xylem has almost disappeared, while those at
_e_, nearer their entrance into the leaf-stalk, consist exclusively of
primary xylem and phloem. At _a_ the two strands of a leaf-trace, still
nearer to the petiole, are inclined towards one another preparatory
to reunion after reaching the leaf-stalk. A slender root is seen in
transverse section at _r′_ immediately outside the two leaf-bundles.
As Williamson and Scott[106] have pointed out, there are always five
leaf-traces beyond the xylem cylinder of a _Lyginopteris_ stem as seen
in transverse section, and these traces in the pericycle, separated
from one another by ⅔ of the circumference, alternate in position with
the lower portions of leaf-traces in the perimedullary region of the
same stem. The phyllotaxis is thus seen to be ⅖.
[Illustration: Fig. 403. _Lyginopteris oldhamia._ _a–e_, foliar
bundles; _f_, decurrent base of petiole; _r_, _r′_, roots; _s_, seed
(_Lagenostoma_). (× 3. Kidston Coll. 592, B.)]
The secondary wood is succeeded by a cambium of normal structure
passing gradually into a narrow band of secondary phloem which in
well-preserved stems is seen to consist of sieve-tubes and parenchyma
with medullary rays rather broader than those in the xylem. Beyond the
phloem is the comparatively broad pericycle consisting of parenchyma
with nests of sclerenchyma like those in the pith and scattered
secretory cells. In the outer layers of the pericycle a phellogen
was formed at an early stage in the growth of the plant, producing
several layers of secondary tissue, which is regarded as periderm and
forms a conspicuous feature in _Lyginopteris_ stems; it appears as a
comparatively dark sinuous band where it bends outwards to wrap round
the leaf-traces in their almost vertical course through the pericyclic
region (fig. 403). The periderm is clearly seen at _p_ close to the
crushed secondary phloem of the dividing leaf-trace in fig. 404, D. All
the leaf-traces seen in fig. 403 beyond the secondary wood are still
within the deep-seated periderm and, as Williamson and Scott showed,
each leaf-trace after emerging from the secondary wood remains in the
pericycle-zone for a length of five internodes as it very gradually
inclines outwards. Once free from this region the twin bundles bend
more sharply towards the petiole. Stated briefly, the history of
each leaf-trace from the perimedullary region to the leaf-base is as
follows: at the outer edge of the pith a single trace consists of a
mesarch xylem bundle with one protoxylem strand; it passes vertically
through five internodes and then bends out through a foliar gap in
the xylem-cylinder, and the primary tracheids receive additions from
the cambium of the stele on their outer face. As the trace leaves the
secondary xylem it bends upwards and, as seen at _d_, fig. 403, begins
to divide into twin bundles. As the trace passes higher the bisection
of the protoxylem and metaxylem is completed and the secondary
xylem-arcs are gradually lost until the separate strands of each pair
are reduced to single mesarch bundles composed wholly of primary
tracheids. As the trace bends outwards through the cortex the phloem
gradually encircles each xylem-strand until a concentric structure is
substituted for the collateral disposition of the conducting tissue.
At the same time the protoxylem strands divide and occupy a position
near the inner edge of the metaxylem. On reaching the petiole or after
passing some distance up the axis of the frond, the twin bundles unite
and usually form a V-shaped vascular strand (figs. 404, E; 405, A).
The single meristele subsequently divides into two equal portions
preparatory to the bifurcation of the petiole (fig. 406).
The inner cortex, consisting of parenchymatous tissue and many
secretory cells with an occasional group of sclerenchymatous elements
in place of the abundant nests of this tissue in the pericycle, has
been invaded in the stem shown in fig. 403 by numerous rootlets of
_Stigmaria_ and _Lyginopteris_, some of which are seen interrupting
the continuity of the outer cortex. The greater width of the cortical
region at _f_, fig. 403, is due to the decurrent base of a petiole
the meristele of which is not included in the section. The lighter
and broader bands between the cross-sections of the stereome-network
in the outer cortex are occupied by remains of tangentially stretched
parenchymatous cells, and beyond this zone in a younger stem there are
a few layers of parenchyma forming the superficial tissue, but there
appears to be no well-defined epidermal layer.
[Illustration: Fig. 404. _Lyginopteris oldhamia._
A, B. Frond fragments; _a_, pinnule; _b_, reticulum of sclerenchyma.
C. Portion of stele of the stem reproduced in fig. 402.
D. _px_, protoxylem; _s_, sclerenchyma; leaf-trace close to the edge
of the secondary xylem; _p_, periderm.
E. Petiole; _m_, meristele.
(A, Kidston Coll. 664 B; B, Camb. Botany School 508; C, Binney Coll.
179; D, E, Camb. Botany School, 93, 159.)]
Young stems have been recognised in which there is very little
secondary xylem and phloem: in these the stereome bands in the outer
cortex are closer together than in the stretched hypodermal tissue
of older shoots and the scattered sclerous nests are represented by
unthickened cells. In addition to young stems Williamson and Scott
described a distinct type of small stem in which the primary xylem
forms an almost complete ring[107] comparable with the primary xylem of
some adult Sigillarian stems (vol. +ii.+ p. 220) but distinguished by
its mesarch structure and by the reticulate pitting of the centripetal
xylem.
A characteristic feature of the stem is the occurrence of emergences
from the outer cortex which have the structure either of spinous
processes, broadly linear or flask-shaped, or of stalked glands[108]. A
portion of a glandular emergence is shown in fig. 405, B: the group of
small cells immediately below the blunt apex is in this instance still
intact though showing signs of disorganisation in the centre; but in
many cases the secretory tissue has not been preserved and the head of
the emergence is occupied by a space. A single stoma is seen at _s_ in
longitudinal section. Further reference to the emergences is made in
the description of the leaf.
It occasionally happens that a meristematic layer is formed in the
parenchymatous tissue immediately internal to some of the perimedullary
xylem strands of a _Lyginopteris_ stem from which either secondary
parenchyma is produced or a zone of secondary xylem and phloem, the
phloem facing the centre of the pith. An example of such internal
xylem was figured by Williamson[109] and similar occurrences are
more fully dealt with by Williamson and Scott[110] who consider that
the perimedullary cambium may represent an internal extension through
a leaf-gap of the normal cambial cylinder. In the stem represented
in fig. 403 there are two perimedullary xylem strands to the left
of the bottom of the V-shaped gap in the secondary xylem-cylinder,
_r_, and on the inner face of one of these, as shown in fig. 405, C,
there is a narrow arc of internal secondary xylem, _c_, between the
xylem-strand and the outer edge of one of the sclerous nests. The
sporadic occurrence of arcs of inversely orientated secondary vascular
tissue affords an interesting parallel with a similar morphological
feature in some recent Dicotyledonous genera such as _Tecoma_ and
_Iodes_. As Williamson and Scott point out, this similarity affords ‘a
striking warning against the _indiscriminate_ use of even conspicuous
anatomical characters[111].’ While admitting the necessity of guarding
against the danger of attaching importance to occasional and abnormal
characters they may have some significance as collateral evidence in
comparisons of different types of stems. It is conceivable that these
anomalous arcs of secondary tissue on the inner side of the primary
xylem strands may, as Worsdell[112] maintains, be reversions to an
ancestral character and in this sense comparable with the strands of
inverted vascular tissue in some recent Cycadean stems. The question of
relationship of _Lyginopteris_ and allied types to recent Cycads and
the Palaeozoic Medulloseae is considered in a later chapter.
In 1902 Lomax[113] described two branching specimens of _Lyginopteris_,
and more recently two others have been discovered at a locality
near Bacup in Lancashire which have been thoroughly investigated by
Miss Brenchley[114] who constructed models from drawings of serial
sections[115]. One specimen shows six leaf-bases in a length of 4¾
inches and branches spring from the axils of five of them: some of
the branches show secondary ramifications. The phyllotaxis of the
leaf-bases on a branch is always the reverse of that on the main stem,
a divergence to which no parallel was found in a selection of trees
and shrubs examined by Miss Brenchley. The secondary wood of the stem
swells below the point of exit of a branch and frequently a fairly
large amount of wood occurs in the pith when a branch is given off:
this anomalous wood may help to close the branch-gap.
ii. _Leaf_.
In his account of _Lyginopteris_ stems published in 1873
Williamson[116] suggested that the vascular bundles met with outside
the xylem-cylinder might be the leaf-traces of large fronds and
expressed the opinion that the ‘stems or petioles’ previously
described by him under the generic name _Edraxylon_ might belong to
_Lyginopteris_. A year later he substituted the name _Rachiopteris
aspera_ for the petioles previously referred to _Edraxylon_ and
inclined to the view that this type of _Rachiopteris_ may be
the petiole of the Carboniferous fronds known as _Sphenopteris
Hoeninghausi_ Brongn., an inference based to a large extent on the
occurrence of emergences on _Rachiopteris aspera_ (fig. 404, E)
preserved as petrifactions like those on impressions of _Sphenopteris
Hoeninghausi_ as figured by Brongniart (figs. 404, A; 405, D, D’). In
1890 Williamson was able to demonstrate the truth of the surmise that
_Rachiopteris aspera_ and _Lyginopteris oldhamia_ are respectively
the petiole and stem of the same plant, which he believed to be an
arborescent fern[117]. The petioles of _Lyginopteris_ fronds, which
may reach a diameter of 1 cm., are attached by a broad base to the
stem, and as already suggested by the number of internodes traversed by
each leaf-trace, the leaves are comparatively far apart. A transverse
section of a petiole is shown diagrammatically in fig. 405 A. The
hypodermal stereome is a prominent feature, but the narrow radial
plates of the stem-cortex tend to be replaced in the rachis by broader
and confluent masses of strengthening elements: the upper surface of
the petiole is slightly grooved. Glandular and spinous emergences
are often very abundant, as in the section reproduced in fig. 404,
E. A glandular emergence is seen at _a_ in fig. 405, A. The spinous
emergences may be compared with those of _Davallia_ (_Odontosoria_)
_aculeata_[118], a West Indian fern of climbing habit and with the
prickles on _Hemitelia_ and other recent Cyatheaceous fronds[119],
while capitate glands, though simpler than those of _Lyginopteris_,
occur on the leaf-stalks of some recent Polypodiaceous species[120].
The concentric meristele may consist in the lower part of the petiole
of two separate and slightly curved strands like those seen in fig.
404, E, _m_: sooner or later the two strands unite to form a wide-open
V or a W-shaped bundle with several slightly internal protoxylem
groups close to the lower edge. The two sections represented in fig.
406, A and B show the gradual divergence of the two meristeles of a
petiole as they approach the level where it divides into two equal
branches, a characteristic feature of _Sphenopteris Hoeninghausi_ and
allied fronds. At a lower level than that represented in fig. 406 the
vascular strand of the petiole would have the form of a W as figured
by Williamson in one of his earlier memoirs[121]. The phloem with
scattered secretory sacs and the adjacent tissue of the leaf-stalk are
occasionally preserved in wonderful perfection[122]. No endodermis has
been recognised. Sclerous nests are scattered in the ground-tissue as
are also secretory sacs (figs. 404, E; 405, A). A small root _r_ has
penetrated the parenchyma of the rachis shown in fig. 405, A.
[Illustration: Fig. 405. _Lyginopteris oldhamia_. A, petiole section;
_a_, glandular emergence; _r_, root. B, stalked gland; _s_, stoma. C,
inner edge of wood of a stem; _c_, arc of inversely orientated vascular
tissue. D, D’, part of a frond of _Sphenopteris Hoeninghausi_. E, part
of axis of D. (A, C, D, Kidston Coll.; B, Manchester Coll. R. 645.)]
[Illustration: Fig. 406. _Lyginopteris oldhamia._ Transverse section
illustrating branching of petiole. × 5. (From a drawing supplied by
Prof. Oliver.)]
_Sphenopteris Hoeninghausi_ Brongn.[123], founded on material from
English Coal Measures, was regarded by Williamson as the foliage of
_Lyginopteris_ chiefly on the ground of the occurrence of emergences on
the axes (figs. 404, A, B) and laminae of the impressions like those
on the petrified stems, and this comparison received support from the
resemblance of the fragments of pinnules associated with _Lyginopteris_
and its petioles in the calcareous nodules to the leaflets of
Brongniart’s type. This identification is supported by subsequent work.
The quadripinnate fronds, which attain a considerable size, resemble
those of recent species of _Davallia_ and other ferns, but the forking
of the rachis and branches of the frond is a striking feature: the
pinnae may reach a length of 15 cm.[124] The portion of carbonised
rachis shown in fig. 405, E, reveals the existence of a hypodermal
reticulum like that in the outer cortex of a _Lyginopteris_ stem and
the same feature is seen in the more slender axis represented in fig.
404, A, at _b_.[125] The pinnules are usually deeply lobed and the
segments may be comparatively broad and blunt or narrow[126] (fig. 290,
C, vol. +ii.+ p. 399; fig. 404, A, _a_, B; fig. 405, D’). The lamina
has a well marked dorsiventral structure: the palisade-tissue next
the upper surface is separated from the epidermis by small hypodermal
cells, possibly functioning as a water-storage layer, and the central
part of the mesophyll consists of loose aerenchyma: the veins are
collateral as in recent ferns and stomata occur in the lower epidermis.
Emergences are seen both on impressions (fig. 405, D’) and on petrified
specimens. A striking feature of the pinnules is the rounded surface
caused by the revolute edge of the lamina as seen in the section
reproduced in fig. 404, B. This character coupled with the occasional
occurrence of groups of short tracheal elements at the termination of
the veins denotes a tendency to a xerophilous habit.
On the strength of a very close resemblance between _Sphenopteris
Hoeninghausi_ and _Calymmatotheca Stangeri_ (fig. 408, E,
F)—characterised by fertile pinnules bearing stellate groups
of small linear valves, regarded by Stur as the open lobes of
an indusium—Zeiller included Brongniart’s type in the genus
_Calymmatotheca_. The resemblance in general habit between the two
species extends to the presence in their rachises of the _Dictyoxylon_
form of cortex[127]. The view formerly held by some authors that the
valve-like appendages to the fertile segments of _Calymmatotheca_ are
sporangia is incorrect: a re-examination of Stur’s specimen (fig.
408, E, F) has confirmed the original description[128]. The stellate
lobes are now regarded as portions of a cupular investment of a seed
similar to _Lagenostoma Lomaxi_, the female reproductive apparatus of
_Lyginopteris oldhamia_. The axes of the fertile pinnae bear small
thorn-like emergences probably identical with those on the cupule of
_Lagenostoma_ and on the petioles of _Lyginopteris oldhamia_. It was
stated in vol. +ii+ that the fronds known as _Sphenopteris Linkii_
(Goepp.) represent, with other closely allied forms, leaves belonging
to _Heterangium_ stems. This statement was based on a misconception:
the rachis of _Sphenopteris Linkii_, as I have satisfied myself by
an examination of impressions shown to me by Dr Kidston, exhibits
the reticulate pattern characteristic of _Lyginopteris_ and not the
transverse ribs characteristic of _Heterangium_.
It is not an easy task even for those most familiar with Carboniferous
fronds to distinguish clearly between species agreeing generally with
_Sphenopteris Hoeninghausi_, a species regarded by some authors as the
type of a group of very similar and closely allied forms all of which
were probably borne on stems referable to the genus _Lyginopteris_.
The species _Lyginopteris oldhamia_ as generally understood probably
includes more than one specific type, and it is safe to assert that
in the Carboniferous period _Lyginopteris_ was represented by several
forms characterised by highly compound fronds with forked rachises like
_Sphenopteris Linkii_, _S. Hoeninghausi_, and others. The features
characteristic of fronds included in the _Sphenopteris Hoeninghausi_
group have recently been described by Gothan[129]. Stur’s generic name
_Calymmatotheca_ originally applied to the species _C. Stangeri_ was
applied to _Sphenopteris Hoeninghausi_ by Zeiller, and although the
fronds of the latter type have not been found with fertile appendages
of the _Calymmatotheca_ type there can be no doubt as to the generic
identity of these, barely distinguishable, species both of which belong
to stems of _Lyginopteris_. Prof. Johnson[130] has recently described
some impressions from the Coal Measures of Ireland, which he refers
to _S. Hoeninghausi_, bearing stellate groups of lobes like those of
_Calymmatotheca_, and in one case he describes a seed in the middle of
the carbonised remains of a stellate group of cupular segments. An
examination of the specimen in Dublin convinced me that there is no
satisfactory evidence of the seed-nature of the appearance on the rock
believed by Johnson to be an elliptical _Lagenostoma_-like seed. The
actual attachment of the stellate lobes to the pinnae of the frond is
not clearly demonstrated.
iii. _Microsporangia._
In 1905 Kidston[131] announced the discovery of microsporangia on
fronds of _Lyginopteris_: he described specimens from the Coal
Measures of Dudley identified by him with _Sphenopteris Hoeninghausi_
showing sterile and fertile pinnae in organic connexion. The fertile
pinnules (fig. 407, B) are slightly expanded distally into an oval
limb about 2 mm. long bearing 6 to 7 bilocular fusiform microsporangia
3 mm. long and 1·5 mm. broad: in the immature condition the sorus is
hemispherical, the summit being formed of the incurved apices of the
sporangia. At maturity the sporangia spread out, the sorus assuming
the form of an epaulet. Fig. 408, H, shows a sorus in transverse
section and in fig. 408, G, the limb and two pendulous sporangia are
shown. The microspores, 50–70 µ in diameter, are studded with numerous
blunt spines and each spore shows a triradiate ridge. The section
reproduced in fig. 407, A, from the Coal Measures of Oldham is probably
a bilocular sporangium of the same type as those described by Kidston
from Dudley. Dr Kidston[132] describes a second type of microsporangial
sorus as _Crossotheca Hughesiana_ which agrees closely with _C.
Hoeninghausi_, but the fertile segments are not associated with any
sterile pinnae. The generic name _Crossotheca_, founded by Zeiller[133]
in 1883, was substituted for _Sphenopteris_ on the ground that
Brongniart’s species _S. Hoeninghausi_ is shown to possess sporangia
of the _Crossotheca_ type. If Kidston’s specific determination is
correct, his discovery demonstrates that _Lyginopteris_ fronds
bore microsporangia having the characters of _Crossotheca_, a type
characteristic of several Carboniferous species belonging both to the
form-genera _Sphenopteris_ and _Pecopteris_. Reference has already
been made to the difficulty of distinguishing between impressions
of fronds of the _Sphenopteris_ _Hoeninghausi_ group, a difficulty
that is illustrated by Dr Gothan’s statement[134] that the Dudley
specimens of _Crossotheca_ are not identical in the character of the
sterile pinnules with _Sphenopteris Hoeninghausi_. An examination of Dr
Kidston’s specimens led me to agree with his determination; but, it may
be asked, have we any evidence of the association with _Lyginopteris_
fronds of sporangia other than those of the _Crossotheca_ type? Prof.
Chodat[135] believes that certain petrified fragments of pinnules
occasionally met with in the calcareous nodules bearing sessile
and apparently annulate sporangia belong to _Lyginopteris_ fronds.
These sporangia appear to be identical with those named by Scott
_Pteridotheca Butterworthi_[136] and regarded by him as filicean
sporangia that cannot be referred to any known Carboniferous genus. The
piece of lamina bearing an empty sporangium, which may or may not have
possessed an annulus, reproduced in fig. 407, C, occurs in association
with the larger specimen shown in fig. 404, B, and it would seem not
unreasonable to regard both as parts of the same frond, namely a
frond of _Lyginopteris_. As Prof. Weiss[137] points out, the accurate
determination of small pieces of petrified pinnules is exceedingly
difficult and without more decisive evidence we are hardly justified in
asserting that the sporangia figured by Chodat and Scott and that shown
in fig. 407 belong to the genus _Lyginopteris_. Although the available
data appear to favour the view generally held that Kidston’s conclusion
is correct additional evidence would be welcome.
[Illustration: Fig. 407. A, B. _Crossotheca Hoeninghausi._ C. Pinnule
with a sporangium, _s_. (A, Kidston Coll. 1277; B, after Kidston; C,
Camb. Botany School, 508.)]
_Telangium._ Reference was made in vol. +ii.+[138] to the genus
_Telangium_ instituted by Dr Benson for some petrified sporangia
from the Coal Measures regarded by her as the microsporangia of a
Pteridosperm, probably _Lyginopteris_. The sporangia of _Telangium_
are similar to those of _Crossotheca_. Scott points out that they
are borne on a flat disc or lamina ‘quite comparable to a fertile
pinna of _Crossotheca_[139],’ and he concludes that these sporangia
are not generically distinct from the impressions on which the genus
_Crossotheca_ was founded. Kidston[140] regards _Telangium Scotti_,
Benson, as the microsporangium of a Pteridosperm though not of
_Lyginopteris_, on the ground that the microsporangia described by
Miss Benson are not attached to a limb and that they have a single
loculus in place of the double loculus (fig. 407, A) of _Crossotheca_.
The presence of a limb in _Telangium_ recognised by Scott removes one
of these distinguishing features. There are, however, no adequate
reasons for regarding _Telangium Scotti_ as specifically identical with
_Crossotheca Hoeninghausi_. The synangium of _Telangium Scotti_, 5 mm.
in length, consists of 6–12 sporangia united basally and opening when
ripe by longitudinal dehiscence. Fig. 493, E, shows eight sporangia
of a synangium in transverse section: the two sporangia at the lower
end of the section are less distinct than the others, some are full
of spores and others have shed their contents by the splitting of
the thin inner walls of the loculi. The sporangial walls are composed
of an outer layer of large cells with dark contents succeeded by 2–3
layers of smaller and crushed cells. The spores, 5–6 µ × 3·5–4 µ in
diameter, have a reticulately sculptured exine: Dr Benson[141] states
that they agree closely with pollen-grains found in the pollen-chamber
of _Lagenostoma ovoides_ except in their slightly smaller size; she
notes the association of _Telangium_ with fragments of the vegetative
organs of _Lyginopteris_ and draws attention to resemblances in the
structure of the tissues; but the most interesting comparison, at least
in an academic sense, is with the seed _Lagenostoma_, the integumented
megasporangium of _Lyginopteris_. Dr Benson points out that a
transverse section of a _Lagenostoma_ in the plane of the canopy,
showing the nucellar apex surrounded by radially disposed chambers
(fig. 409), presents a certain resemblance to a synangium of _Telangium
Scotti_; and it is suggested that the chambers encircling the nucellus
may represent sterilised sister-sporangia[142]. ‘The seed in fact is
assumed to be a synangium in which all but one of the sporangia are
sterile, and form an integument to the one fertile sporangium which
has become a megasporangium with one large megaspore.’ This view,
though clearly incapable of confirmation in the present state of our
knowledge, is not merely an ingenious hypothesis but a stimulating
suggestion as to possible homologies: as an argument in favour of
associating _Lagenostoma_ and _Telangium_ as the spore-bearing organs
of the same plant it has but little weight.
iv. _The Seed._ =Lagenostoma= Williamson.
_Lagenostoma Lomaxi_, Oliver and Scott ex Williamson, +MS.+
In 1877 Williamson[143] proposed the generic name _Lagenostoma_ for
some petrified seeds from the Lower Coal Measures of Lancashire and
described two species, _Lagenostoma ovoides_ and _L. physoides_: in his
+MS.+ Catalogue a third type is referred to as _Lagenostoma Lomaxi_. It
is this third type that Prof. Oliver was the first to recognise as the
megaspore-bearing organ of _Lyginopteris oldhamia_. Its structure has
been thoroughly described by Oliver and Scott[144] and these authors
contribute a judicial summary of the evidence on which _Lagenostoma_
and _Lyginopteris_ are believed to stand for one and the same plant.
The evidence is based chiefly on the following considerations: an
agreement in the structure of the vascular bundles in the investments
of the seed with those in the leaves of _Lyginopteris_; the presence
in the outer envelope of the seed of stalked glands identical with
those on the stems and petioles. The evidence does not as yet amount
to absolute proof, as the seeds, which occur either with or without
a stalk, have not been found attached to a _Lyginopteris_ frond.
But ‘where vegetative and reproductive organs presenting identical
structural features, not known to occur in other plants, are thus found
in close and constant association, the inference that the one belonged
to the other appears irresistible.’ While most botanists believe that
a satisfactory case is established there are a few[145] who refuse to
believe in a connexion between _Lagenostoma_ and _Lyginopteris_ until
an actual union has been demonstrated. The discovery by Kidston[146]
of seeds attached to pinnae bearing _Neuropteris_ pinnules and the
demonstration of organic continuity between seeds and the pinnules of
other Palaeozoic fern-like fronds supply abundant confirmatory evidence
that leaves no doubt as to the occurrence of seeds on modified pinnae
of _Sphenopteris Hoeninghausi_ and of other closely allied fronds which
represent the foliage of different forms of _Lyginopteris_. In this
connexion it is pertinent to add that Grand’Eury[147] has found seeds
of the _Lagenostoma_ type in close association with impressions of
_Sphenopteris Dubuissonis_ and other leaves of similar habit.
A seed of _Lagenostoma Lomaxi_ reaches a length of 5·5 mm. with a
maximum diameter of 4·4 mm.; it is broadly oval or barrel-like (fig.
408, C) and when immature was invested by a loose irregularly lobed
glandular envelope (fig. 408, B) from which the seed eventually
freed itself by a natural process of abscission. The central body or
nucellus, except in the apical region, is concrescent with a fairly
stout integument or testa (fig. 408, C, _f_) the outer portion of
which is characterised by regular longitudinal rows of palisade-like
cells comparable with the broad palisade-layer in the sporocarp of
_Pilularia_. On the exposed surface of this palisade-tissue are small
dark structureless pegs[148], possibly the remains of a mucilaginous
layer such as occurs on the seed-coats of some recent Flowering
plants. At the base of the nucellus the chalazal region, fig. 408, C,
_ch_, is provided with sclerous elements and forms a hard investment
to the axial vascular strand from the pedicel. It is at the base
of this chalazal region that the seed is eventually cut off by an
absciss-layer. The integument is supplied throughout its length by nine
vascular bundles of endarch, or approximately endarch, structure. The
free portion of the integument seen from the outside (fig. 408, B) has
the form of a fluted cone with a circular opening at its summit. The
greater part of this domical apex, as seen in longitudinal section in
fig. 408, C, appears to be hollow, but in the living state the dome, or
canopy as Williamson called it, was filled with parenchyma in which the
vascular bundles were embedded and, as shown in the transverse section
in fig. 409, the canopy is divided into compartments by radial septa
which in its basal region are replaced by regular and deep furrows
on the inner face. Enclosed by the canopy, with its outer surface
fluted as the result of the partial collapse of the outer wall of each
compartment due to the decay of the filling tissue, is the flask-shaped
apex of the nucellus; between the apical cone of nucellar parenchyma
and the superficial layer is an annular cavity which Williamson[149]
called the lagenostome. The parenchymatous core tapers to a narrow
summit which slightly overtops the integument and is constricted at the
broad base (fig. 493, A, B; page 311). The bottle-shaped apical tissue
is separated by an annular space, _c_, fig. 493, B, from the limiting
layer of the nucellus: this space is the pollen-chamber formed in the
living seed by the disorganisation of the subepidermal cells of the
nucellar apex. The pollen-chamber is a feature characteristic of recent
cycadean ovules (see p. 6). In _Lagenostoma_ the annular form of the
pollen-chamber is a peculiarity distinguishing this type of seed from
those of recent Gymnosperms and most other Palaeozoic seeds. As Oliver
says, it marks an ‘advance in precision’[150] over other forms as the
microspores which fall into the chamber are brought direct to the
surface of the underlying megaspore and presumably to the archegonia
which, it is reasonable to believe, were disposed in a circle at the
base of the annular crevice. Microspores frequently occur in the
pollen-chamber and some have been discovered apparently in the act of
liberating male gametes[151].
[Illustration: Fig. 408. A, A′, _Lagenospermum Sinclairi._ B,
_Lagenostoma_, restoration. C, _Lagenostoma Lomaxi_; _c_, micropyle;
_d_, space between integument and nucellus; _e_, cupule; _f_,
integument; _ch_, chalaza. D, microspores of _Lagenostoma ovoides._
E, F, _Calymmatotheca Stangeri._ G, H, _Crossotheca Hoeninghausi._
H, section of G in line of arrow. (A, after Arber; B, C, E, F, after
Oliver; D, after Benson; G, H, after Kidston.)]
[Illustration: Fig. 409. _Lagenostoma._ Transverse section near the
micropyle, showing the pollen-chamber, _pc_, the space, _s_, between
the nucellus and integument, the fluted canopy with vascular bundles,
_v_. (After Oliver.)]
The outer wall of the nucellus is bounded externally by a similar
circular space (_d_, figs. 408, C; 493, B) which separates it from the
domical canopy. In the great majority of specimens the central tissue
of the seed is not preserved and an empty sac supported from the base
of the nucellus-apex occupies nearly the whole of the interior: the
shrunken wall of the sac is all that remains of the large megaspore.
It would seem, then, that the nucellus was almost completely destroyed
as a consequence of the growth of the megaspore or embryo-sac, which
eventually occupied nearly the whole of the seed-body.
In an exceptionally well preserved specimen recently described and
admirably illustrated by Mr McLean[152] part of the parenchymatous
tissue of the prothallus which originally filled the megaspore is
clearly seen: its surface-layer consists of small cells succeeded by
a broad band of radially elongated elements closely resembling the
alveoli in the prothalli of some recent Gymnosperms, particularly
certain Conifers. No archegonia have been discovered. The cupular
envelope of immature seeds, compared by Oliver and Scott with the
lobed and glandular husk of _Corylus colurna_ L.[153], receives
several vascular bundles of collateral and mesarch structure from the
axial strand, and these subdivide as they ascend. The glands which
occur on all parts of the cupule are sessile or stalked and identical
with those on the vegetative organs of _Lyginopteris_. Assuming that
pollination occurred at a comparatively early stage in the development
of the seed when the cupule was still intact, it is conceivable, as
Sir Joseph Hooker suggested, that the glandular secretion may have
attracted insects and so aided in the transport of pollen which were
perhaps drawn down the narrow pollen-chamber by exuded mucilage as in
recent Conifers. The evidence obtained in recent years in favour of
insect-pollination in certain Cycads and in _Welwitschia_ lends support
to this view: the dragon flies hovering over a fertile _Lyginopteris_
frond in a recent restoration[154] may be a legitimate addition.
A striking feature of _Lagenostoma_ as of other Palaeozoic seeds is
the absence of an embryo: this and other considerations have led
certain authors, notably Chodat[155], to question the justification
for the use of the term seed. Various suggestions have been offered in
explanation of this fact. In recent Cycads, as already pointed out, the
development of the embryo does not always occur before seed-fall. It
may be that these older seeds had no resting-period or there may have
been a period of rest after fertilisation and not as now at a stage
subsequent to the formation of the embryo[156]; it is also suggested
by Scott that ‘the nursing of the embryo had not yet come to be one of
the functions of the seed, and that the whole embryonic development was
relegated to the germination stage[157].’ In this connexion reference
may be made to a statement by Miss Gibbs[158] who speaks of seeds of
a _Podocarpus_ picked up from the ground in apparently a mature state
and with the associated bracts coloured and swollen as though ready
to aid in dispersal but with no embryo: the seeds had matured before
fertilisation and fell from the tree after pollination. Whatever
may be the true explanation of the absence of embryos this negative
character should not be allowed to outweigh the evidence furnished
by morphological features as to the applicability of the term seed.
As Prof. Oliver says, ‘there is a long chapter in evolution to be
deciphered before we can connect ... the seed of _Lyginodendron_ with
the sporangium of any fern at present known to us’[159].
The cupule of _Lagenostoma_ has been homologised with the outer
part of the integument of a recent cycadean seed[160] which, it is
suggested, consists of an inner and an outer envelope that have become
concrescent, and this hypothesis is supported by another author by a
comparison between _Lagenostoma_ and Gnetalean seeds[161]. A comparison
has also been made between the collar of a _Ginkgo_ seed and the much
more conspicuous cupule of _Lagenostoma_[162]. Dr Benson and Miss
Welsford[163] institute a comparison between the vascular supply of the
outer integument of the ovules of _Carpinus_ and _Morus_ and that of
the cupule of _Lagenostoma_, a comparison suggested by Miss Kershaw’s
remarks[164] on the similarity between the vascular system of the
ovules of _Myrica Gale_ and _Trigonocarpus_. In 1908 Dr Benson[165]
described some germinating microspores in the pollen-chamber of another
species of _Lagenostoma_, _L. ovoides_, and recognised what she
believed to be antherozoids. I am indebted to this author for allowing
me to make a drawing from her section (fig. 408, D). Two microspores
are seen with thick outer walls showing irregular holes probably of
secondary origin and not part of a regular reticulum as Dr Benson
suggests. Close to the upper microspore is a hemispherical body, _s_,
described as a male gamete, and a similar body is seen still enclosed
by the lower microspore. It is by no means improbable that these are
antherozoids: they were presumably ciliate like those of _Ginkgo_ and
recent Cycads (fig. 396, M). The microspores are approximately 70μ in
length and the supposed antherozoids have a maximum diameter of 45μ,
the latter being about ⅔ the size of the sperms of _Microcycas_ and ⅙
the diameter of those of _Zamia_. The smaller and more delicate cells
near the lower microspore (fig. 408, D) are no doubt fungal cells as
Miss Benson suggests. With reference to the difficulty of determining
the nature of Miss Benson’s supposed gametes it is worth calling
attention to some figures given by Zopf[166] of vesicular cells and
sporangia of the Phycomycetous genera _Rhizophidium_ and _Lagenidium_
in the pollen of Flowering plants and Pines. It has been suggested by
Burlingame[167] that the ‘gametes’ may be prothallial cells; but this
is very improbable.
_Lagenostoma ovoides_ Williamson.
In the memoir in which the genus was founded Williamson described
two species from the Lower Coal Measures of Lancashire, _Lagenostoma
ovoides_ and _L. physoides_[168]. The seeds described under the
latter name had previously been assigned by him to another new genus,
_Physostoma_, and named _P. elegans_[169]. _Lagenostoma physoides_
was afterwards figured by Butterworth[170] who recognised some new
features. For this species Prof. Oliver[171] has adopted Williamson’s
earlier name, _Physostoma elegans_. The former species, which has
recently received exhaustive treatment by Miss Prankerd[172], agrees
generally in its morphological characters with _L. Lomaxi_, but
differs in the structure of the surface-tissue of the integument and
in some anatomical features. Moreover no cupules have been found and
there is ‘very little trace of a layer of separation’ such as occurs
in _L. Lomaxi_. Over the surface of the integument is a layer of
prismatic cells, much shorter and less palisade-like than those in _L.
Lomaxi_, and there are none of the pegs which are a constant feature
in that species. There are, however, indications that mucilage was
poured out by the rupture of the distended cells. Some microspores
were found in the pollen-chamber with an average size of 72 × 53μ;
they may be as much as 90μ long. None were observed with sperm-like
contents like those described by Dr Benson. Miss Prankerd discusses
the morphology of the integument in relation to that of cycadean seeds
and makes an instructive comparison between the lagenostome (that
is the modified nucellar apex) and such fern sporangia as those of
_Angiopteris_, _Osmunda_, and _Schizaea_, but especially the sporangia
of _Senftenbergia_[173] with their multiseriate annulus.
An interesting feature is shown in the longitudinal section reproduced
in fig. 493, A (p. 311). The apex of the nucellar cone appears to be
composed of thick-walled, dark cells and it is suggested that this
may have served as a stopper blocking up the circular orifice of the
pollen-chamber (seen below the apex between the nucellar cone and the
thick surface-layer of the nucellus) and serving as a protection to the
embryo. A comparable closing-up of the micropyle occurs in the seeds of
_Gnetum Gnemon_[174] and in the beak of cycadean seeds. At the time of
pollination, when the pollen-chamber must have extended to the apex of
the lagenostome, the tip of the nucellar cone may have secreted some
sticky substance to which the microspores would adhere.
Prof. Lignier[175] has recently described some large megasporangia
from the Westphalian Coalfield of Ostrau in Austrian Silesia which he
made the type of a new genus _Mittagia_, after Herr Mittag, Director
of Mines. Two sporangia, between 2 and 3 mm. in diameter, were found
in close association as though belonging to a single sorus; one was
empty and the other contained four megaspores. The structure of the
thick wall of the sporangia is very similar to that of the testa
of _Lagenostoma Lomaxi_, but it apparently split into two valves.
Lignier refers the new type, _Mittagia seminiformis_, to some
unknown Palaeozoic group of heterosporous Filicineae, possibly the
ancestral stock of the Pteridosperms, and he thinks it probable that
the sporangia resembled seeds in their facilities for dispersal. In
the structure of the sporangial wall _Mittagia_ also resembles the
sporocarp of _Pilularia_.
_Seeds presented as impressions, without internal structure,
superficially resembling Lagenostoma._
_Lagenospermum_ Nathorst.
This generic name is adopted for seeds represented by casts or
impressions agreeing in external features with _Lagenostoma_ but
which on the available evidence cannot be confidently assigned to
that genus[176]. Two types of seed were described by Arber[177]
from the Lower Coal Measures of Scotland as _Lagenostoma Kidstoni_
and _L. Sinclairi_: the former has been removed by Oliver[178] to
_Physostoma_ and both are included by Arber[179] in a recent paper in
the genus _Radiospermum_. This new generic term is proposed by Arber
for a number of small sub-cylindrical seeds founded on impressions
including ‘small seeds which, when the structure is preserved,
are known as _Lagenostoma_, _Physostoma_, and _Conostoma_.’ The
question of nomenclature is invariably raised by cases in which
impressions resemble in their superficial characters genera founded on
anatomical characters: the seeds originally referred to _Lagenostoma
Sinclairi_ afford a good example of this difficulty. Nathorst has
recently proposed the generic name _Lagenospermum_ as preferable
to _Lagenostoma_ and _Radiospermum_ in the case of _Lagenostoma
Sinclairi_ and similar seeds which afford no proof of the possession
of such morphological characters as would justify their inclusion in
the genus _Lagenostoma_ but which may be examples of that genus. As
Nathorst points out, the adoption of _Radiospermum_ for _L. Sinclairi_
is inadvisable on the ground that it is also applied to seeds of a
different type. The type-species of _Lagenospermum_ is _L. Sinclairi_
and Nathorst describes additional species from Lower Carboniferous
rocks in Spitzbergen.
_Lagenospermum Sinclairi_ (Arber ex Kidston +MS.+).
Although it is not certain that these seeds are morphologically
identical with the genus _Lagenostoma_, a brief description is
intercalated here as the habit of the seed-bearing axes supplies a
probable key to the habit of the fertile fronds of _Lyginopteris_.
The type-specimens were collected by Mr Sinclair from the Lower
Coal Measures of Ayrshire, Scotland, and recorded by Kidston as
_Lagenostoma_ sp.: they were afterwards named by him in manuscript
_L. Sinclairi_ and handed to Dr Arber for description. The seeds are
elliptical-oblong, 4–5·5 × 1·5–3 mm., radially symmetrical and enclosed
by a loose envelope which is longitudinally ribbed and divided distally
into several linear-lanceolate lobes (fig. 408, A, A′). This covering,
though much longer than the cupule of _Lagenostoma Lomaxi_, is probably
a homologous structure. The most interesting point is the attachment
of the seeds to slender branches of a compound axis (fig. 408, A). It
is probable that the seeds were borne on a frond characterised by the
reduction or complete abortion of the sterile lamina or perhaps, as
in the recent Fern _Thyrsopteris elegans_[180], some of the pinnae of
a large compound frond were fertile. It is worthy of note that Arber
recognised pinnules of _Sphenopteris obtusiloba_[181] in association
with _L. Sinclairi_, a fact, as he says, in itself of no value but
which acquires significance in view of the discovery by Carpentier[182]
of cupules in close proximity to the same species of frond. Specimens
described by Dr Stopes from Westphalian rocks of New Brunswick as
_Pterispermostrobus bifurcatus_[183] bear a close resemblance to _L.
Sinclairi_.
_Lagenospermum oblongum_ (Kidston).
The species recently described by Dr Kidston[184] as _Lagenostoma
oblonga_ from the South Staffordshire coal-field appears to be closely
allied to Arber’s _L. Sinclairi_: it is represented by pairs of seeds
borne at the ends of forked branchlets: the seed is 2·5 mm. long by 1·5
mm. broad and is surrounded by a longer oblong cupule divided distally
into 6 free lobes.
A larger type of seed, 3 cm. long and 2·5 cm. broad, is described by
Kidston[185] from the same coal-field as _Lagenostoma? urceolaris_. A
characteristic feature is the truncate apex surrounded by a prominent
canopy formed of the expanded apical free portion of the integument.
The lack of anatomical data in both these seeds is a reason for the
substitution of some less committal term than _Lagenostoma_.
Grand’Eury[186] and Carpentier[187] have published accounts of
impressions of seeds from the Coal Measures of France compared by them
with species of _Lagenostoma_ though not assigned to new species.
These and similar seeds should be referred to Nathorst’s genus
_Lagenospermum_.
The difficulty of recognising the true nature of seed-like impressions
is illustrated by some specimens in the Goldenberg collection in
Stockholm described by Arber[188] as _Carpolithus Nathorsti_: these
consist of pieces of _Sphenopteris_ pinnae probably, as Zeiller
suggested, _Sphenopteris Schaumburg-Lippeana_ (Stur) bearing at
the ends of the segments of deeply divided pinnules what appeared
to be seeds 1 mm. long, oval and longitudinally ribbed, and
possibly enclosed in a cupule. Arber considered the ‘seeds’ to be
related to _Lagenostoma_, probably belonging to some member of the
Lyginopterideae. An examination of the specimens by Nathorst[189]
showed that the supposed seeds are collections of spores; but whether
the spores of a true Fern or the microspores of a Pteridosperm cannot
be determined.
_Pterispermostrobus_ Stopes.
_Pterispermostrobus bifurcatus_ Stopes.
Dr Stopes[190] has recently called attention to a resemblance between
specimens from the Westphalian of New Brunswick, described by her as
_Pterispermostrobus bifurcatus_, and _Lagenospermum Sinclairi_. The New
Brunswick fossil is made the type of a new genus _Pterispermostrobus_,
which is employed for fructifications of Pteridosperms that cannot
be associated with a known species of parent and may be either seeds
or complex male organs borne on a definitely branched rachis. The
type-species is represented by a slender axis bearing lateral branches
divided into two widely divergent arms each of which bears a terminal
body, 4 × 3 mm., characterised by 3–5 apical lobes extending 2 mm.
beyond the distal end of the seed-like organ and resembling a cupule.
In this as in many other cases it is impossible to determine the
true nature of the reproductive bodies, whether they are small seeds
or groups of microsporangia: the new generic name serves a useful
purpose though it is not always possible definitely to refer doubtful
fructifications of this kind to a Pteridosperm. The organs in question
may also be compared with _Codonotheca_[191].
_Pterispermostrobus pusillus_ (Nathorst).
The name, _Codonotheca pusilla_, is given by Nathorst[192] to some
doubtful specimens from the Culm of Spitzbergen representing short
stalks bearing linear-lanceolate scale-like bodies, 9–10 mm. long
by 1 mm. broad, coalescent at the base. Nathorst compares them
with Sellard’s species, _Codonotheca caduca_, but adds that they
may be cupules of some Pteridosperm and calls attention to their
resemblance to some fossils figured by Carpentier as _Calymmatotheca
acuta_. Both Nathorst’s species and the French specimens described
by Carpentier[193] as cupules may be referred to Dr Stopes’ genus
_Pterispermostrobus_ as their morphological nature cannot be determined.
+v.+ _Roots._
In 1876 Williamson[194] described some petrified vegetative organs
from the Lower Coal Measures of Lancashire under the name _Kaloxylon
Hookeri_ characterised by a division of the secondary xylem into
cuneate masses (fig. 415, C) like those in some recent Bignoniaceous
stems. Williamson at first believed _Kaloxylon_ to be a stem, but in a
later memoir he expressed the opinion that ‘it is difficult to believe
that these organs have been other than roots’[195]. Felix[196] had
meanwhile described a specimen from the Coal Measures of Westphalia as
_Kaloxylon cf. Hookeri_ and suggested that it might be a waterplant. In
1894 Williamson and Scott[197] demonstrated that _Kaloxylon Hookeri_
is the root of _Lyginopteris_, a conclusion independently reached by
Hick[198].
The roots of _Lyginopteris_ arise on all sides of the radially
symmetrical stem in the pericycle region; they are copiously
branched as is shown by the abundance of roots of various sizes in
close association. No roots have been discovered exceeding 1 cm. in
diameter nor have any been recorded with secondary periderm-tissue.
The absence of root-hairs and the more or less lacunar structure of
the cortex are indicative of swampy ground. It is seldom that the
palaeobotanist has an opportunity of investigating the growing-points
of Palaeozoic plants, and for this reason some well-preserved apices
of _Lyginopteris_ roots, attributed to that genus on the ground
of constant association with fragments of stems in the calcareous
nodules of Dulesgate, are of special interest. One of these specimens
was figured by Dr Stopes and Mr Watson[199] in their account of
plant-bearing nodules, and a description of that and other examples has
since been published by Prof. Weiss[200]. A longitudinal section of a
root-tip, ·21 mm. in diameter at its broadest part, shows a root-cap
which suggests an origin from a single cell, but an examination of
the plerome-cylinder in a slightly tangential section does not afford
conclusive evidence of the occurrence of a single initial cell. Weiss
on the whole inclines to the view that _Lyginopteris_ possessed a
single apical cell like the roots of Leptosporangiate Ferns, though he
prefers to leave the decision open. Attention is drawn to the fact that
the plate of tracheidal tissue in emerging lateral rootlets is vertical
as in Phanerogams and not horizontal as in recent Pteridophyta.
The vascular tissue of a _Lyginopteris_ root (fig. 410) consists of
from three to eight alternate strands of centripetal xylem and phloem,
and with the metaxylem is associated a small amount of conjunctive
parenchyma which does not form a central pith. The pericycle, one to
several layers broad, is succeeded by an endodermis which occasionally
shows the characteristic thickenings on the radial walls. A broad
cortex of thin-walled lacunar tissue with numerous secretory cells is
bounded externally by a superficial cylinder of two or more layers
of comparatively large and thin cells, the outermost of which are
radially elongated. This superficial tissue forms a striking feature
by which a _Lyginopteris_ root may often be recognised at a glance.
The root represented in fig. 410, approximately 2 mm. in diameter, has
a heptarch stele divided into seven xylem-groups by crushed bands of
parenchyma and a protoxylem strand occupies the apex of each projecting
angle (fig. 410, _px_). The superficial cylinder of clear cells is seen
at _a_. A very small root is seen at _r_ in fig. 410.
[Illustration: Fig. 410. _Lyginopteris oldhamia_, young root. × 30.
(Kidston Coll. 403.)]
Fig. 415, C, represents part of an older root in which the pentarch
primary xylem is enclosed by broad wedge-like groups of secondary
xylem and phloem separated by conspicuous medullary rays opposite the
protoxylem strands (_px_). Crushed primary phloem arcs, _p_, are often
clearly recognisable beyond the cambium. The secondary thickening,
as Williamson and Scott state, ‘takes place exactly in the manner
typical of roots of Dicotyledons, so that this fossil might very well
be used for purposes of demonstration as illustrating the secondary
growth of a root with diagrammatic clearness’[201]. The young roots
of _Lyginopteris_ resemble in many respects those of Marattiaceous
Ferns, though the presence of a single apical cell, if such occurs, is
a distinguishing feature; but in the presence of secondary conducting
tissue they agree with those of Phanerogams.
_Distribution of Lyginopteris._
The frequency with which petrified fragments of _Lyginopteris_
stems occur in the calcareous nodules of the English coal seams
shows that the genus must have been plentifully represented in the
Upper Carboniferous vegetation, and the occurrence in both North
American[202] and European localities of fronds identical with or
closely resembling _Sphenopteris Hoeninghausi_ affords evidence of wide
geographical range. Petrified specimens were recorded by Felix[203]
from Westphalia in 1886, and Zalessky[204] has recently discovered
_Lyginopteris_ in the Donetz coal-basin of Russia. An investigation
by Kubart[205] of the calcareous nodules, to which attention was
first drawn by Stur, in the Ostrau Coal Measures led to the discovery
of several examples of _Lyginopteris_ stems. The descriptions and
figures so far published are hardly sufficient to enable us to
estimate the degree of relationship to the English type, but some of
the stems appear to be new species and Kubart considers them all to
be specifically distinct from _Lyginopteris oldhamia_. _Lyginopteris
heterangioides_ contains scattered tracheids in the pith and thus
affords an interesting transitional type between _Lyginopteris_ and
_Heterangium_. In _L. lacunosum_ the inner cortex is lacunar and
the primary xylem bundles pursue an independent course in the stele
in contrast to the anastomosing arrangement in _L. oldhamia_ and in
another Hungarian species _L. tristichum_. The species recorded by
Kubart occur in the Millstone grit and the Coal Measures.
The geological range of _Lyginopteris_ as represented by petrified
stems does not extend beyond the limits of the Carboniferous system.
=HETERANGIUM.=
The generic name _Heterangium_ was first used by Corda[206] for a
piece of stem from the Coal Measures of Radnitz, Bohemia, represented
by part of the vascular axis of a stem consisting of strands of
large reticulately pitted tracheids intermixed with parenchyma and
exhibiting structural features differing apparently from those of any
known type. Corda’s material has been re-examined by Kubart[207] who
figures a section from it. _Heterangium_ is a genus closely allied to
_Lyginopteris_ both in habit and in general anatomical characters. The
stem is monostelic; the vascular cylinder prior to secondary thickening
resembles the protostele of certain recent species of _Gleichenia_ and
may be compared also with _Trichomanes scandens_[208]. It agrees with
that of _Lyginopteris_ in the possession of primary mesarch bundles
but differs in the substitution of a cauline axial mass of metaxylem
for the pith of _Lyginopteris_. The secondary vascular tissue agrees
closely with that of recent Cycads and _Lyginopteris_. A characteristic
feature is the occurrence of numerous horizontal bands of sclerous
cells in the cortex (fig. 412) of the stem and in the ground-tissue of
the rachis and larger branches of the fronds. The stem was erect and
rarely branched ‘giving off large foliar appendages at somewhat distant
intervals and from its entire circumference’[209]. Our knowledge of the
reproductive organs is less precise than in the case of _Lyginopteris_;
but we are justified in asserting that _Heterangium_ is a Pteridosperm
which in all probability bore fern-like microsporangia and seeds
similar in general plan to _Lagenostoma_.
The association of some seeds included in Williamson’s genus
_Conostoma_ with _Heterangium Grievii_ in the Pettycur beds and their
resemblance to _Lagenostoma_, the seed of _Lyginopteris_, suggested
the possibility of actual connexion: further evidence in support of
this view has recently been brought forward by Dr Benson[210] in the
case of a species of _Conostoma_ which she transfers to a new genus
_Sphaerostoma_.
The two species _Heterangium Grievii_ and _H. tiliaeoides_ are
described in illustration of the genus and reference is made to a few
other types.
_Heterangium Grievii_ Williamson.
(_Stem_.)
1872. _Dictyoxylon Grievii_, Williamson, Brit. Ass. Rep. (Edinburgh
Meeting), p. 112.
1873. _Heterangium Grievii_, Williamson, Phil. Trans. R. Soc. Vol. 162,
p. 404.
(Leaf.)
[1720. _Fumaria officinalis_, Volkmann, Silesia subterranea, p. 111, Pl.
+xiv.+ fig. 2.]
1822. _Filicites_ (_Sphenopteris_) _elegans_, Brongniart, Class. Vég. p.
233.
1828. _Sphenopteris elegans_, Brongniart, Hist. Vég. Foss. p. 172.
1836. _Cheilanthites elegans_, Goeppert, Foss. Farnkr. p. 233.
1877. _Diplothmema elegans_, Stur, Culm Flora, +ii.+ p. 130.
i. _Stem_.
At the Edinburgh meeting of the British Association Williamson[211]
gave a brief account of some petrified stems from the Lower
Carboniferous strata of Burntisland on the Firth of Forth which he
included in the genus _Dictyoxylon_. One of these was named _D.
Grievii_ after Mr Grieve the discoverer of the specimens. In a later
and more complete description Williamson adopted Corda’s generic name
on the ground of the close resemblance of the Scotch stem to the
Bohemian fragment _Heterangium paradoxum_. In 1873[212] Williamson
added new facts in regard to _H. Grievii_ and in 1890[213] he described
a very closely allied type from the Lower Coal Measures of Lancashire.
Five years later his descriptions were considerably extended and
modified in the joint memoir with Dr Scott[214].
The great difference in age between the English Upper Carboniferous
stem and the Scotch specimens from the Lower Carboniferous beds of
Burntisland suggests a probable specific difference. Dr Scott has
recently adopted the name _Heterangium Lomaxi_, proposed but not
published by Williamson, for the English type. Though in the following
account the species _Heterangium Grievii_ is treated in the broader
sense it should be recognised that the geologically younger stem
is worthy of specific recognition; it is characterised, to quote
Scott[215], by ‘the great distinctness of the primary xylem strands,
by their nearly exarch structure, with little primary centrifugal
wood, by the abundant secretory sacs of the stele, and by the rather
scattered leaves.’
[Illustration: Fig. 411. _Heterangium Grievii_. _lt_, _a_, _b_, _c_,
leaf-traces; _p_, pericycle with periderm; _ph_, phloem; _x_¹, _x_²,
primary and secondary xylem; _px_, protoxylem; _r_, root; _s_, sclerous
tissue.]
_Heterangium Grievii_ has a radially symmetrical stem bearing compound
leaves with decurrent petioles which give to the otherwise cylindrical
axis an angular outline as seen in transverse section (fig. 411, A).
The phyllotaxis appears to be ⅜. The stem rarely exceeds 1·5 cm. in
diameter: in the centre is a comparatively large stele consisting in
young stems of primary xylem and phloem, but in older stems these
are separated by a cylinder of secondary vascular tissue which in
this species is always narrower than in _Lyginopteris oldhamia_ and,
as Williamson pointed out, often of unequal thickness on different
radii. The medullated stele of _Lyginopteris_ is replaced by a solid
xylem-cylinder consisting mainly of groups of large tracheids, reaching
·3 mm. in diameter, with multiseriate bordered pits (fig. 411, D)
embedded in an anastomosing parenchymatous tissue-system. In the stele
reproduced in fig. 411, B, which with the exception of a very narrow
zone of secondary xylem, _x_², consists entirely of primary xylem,
_x_¹, the parenchyma is represented by a darker reticulum (_cf._ fig.
415, B) dividing the metaxylem into islands as in _Gleichenia_. In
the peripheral portion of the xylem the tracheids are rather narrower
and arranged in more definite groups in many of which is a single
strand of narrow spiral elements (fig. 411, A′, _px_) close to the
outer margin. These peripheral primary bundles in which protoxylem
is recognisable may be described as leaf-traces of mesarch structure
consisting of centripetal xylem and, to a much less extent, of smaller
centrifugal elements for the most part with dense spiral bands in place
of the multiseriate pits of the rest of the metaxylem. The structure
of these leaf-traces is practically identical with that of the primary
bundles of _Lyginopteris_. There is, however, a difference to which
attention is drawn by Williamson and Scott. While in _Lyginopteris_ in
any transverse section the primary bundles in the stele are equal in
number to the leaf-traces in the pericycle and cortex, in _Heterangium_
the peripheral groups in the stele may be as many as twenty, a number
considerably in excess of the leaf-traces beyond the limits of the
primary xylem of the stele. It may be that the leaf-trace of each
leaf, which joins the stele at a distance of 6–10 internodes below
its entrance into the cortex from the leaf-stalk, may branch in its
descent in the axial region, or some of the primary groups of xylem may
be confined to the axial region and independent of the leaf-traces.
Portions of the peripheral region of the stele may be occupied by
metaxylem groups without protoxylem and identical with those which make
up the bulk of the metaxylem.
Scott[216] has recently published a note in which he states that most
of the British Coal Measures Heterangiums were polydesmic. Two bundles,
and not a single strand as in the Scotch _H. Grievii_, leave the stele
for each leaf, and these divide into four, in some cases at least,
before entering the petiole.
The secondary xylem is continuous at its inner edge with the outermost
primary tracheids (fig. 411, A′) and consists of rows of tracheids,
1–3 elements broad, alternating with numerous broad medullary rays
of radially elongated parenchyma. Beyond a typical cambium-zone the
secondary phloem consists of parenchyma and sieve-tubes bounded by
crushed arcs of primary phloem. Abutting on the phloem is a pericycle
composed of several layers of small parenchymatous cells (fig. 411,
A, _p_) and in the outer layers of this tissue a phellogen (fig. 411,
C, _p_) and some periderm are usually present though, as Williamson
and Scott point out, the periderm is less regular and narrower than
in _Lyginopteris_. The inner cortex, composed of short parenchymatous
cells, is traversed by numerous narrow bands of dark, thick-walled
cells similar in the structure of the elements, though peculiar in
the horizontal elongation of the groups, to the sclerous nests in the
pericycle and pith of _Lyginopteris_. These characteristic bands are
chiefly seen in the oblique longitudinal section of a stem represented
in fig. 412. In this section, 25 mm. in length, the lighter band, _p_,
is the pericycle and in it a few obliquely cut leaf-traces are shown as
dark patches. The horizontal bands are similar in structure and shape
to the diaphragms of thick cells in the pith of _Abies magnifica_[217],
and in both plants they probably serve as supports to the softer
parenchyma. There may be as many as 46 bands in a vertical length of
cortex of 1 inch (about 19 per centimetre). It was the occurrence of
precisely similar transverse lines on the carbonised impressions of the
rachis of _Sphenopteris elegans_ that led Kidston[218] to suggest a
connexion between that species and the stem of _Heterangium Grievii_.
The outer cortex, consisting of alternate strands of parenchyma and
stereome similar to that of _Lyginopteris_, is much narrower and a less
conspicuous feature than in _Heterangium_; the stereome bands do not
form so regular a hypodermal network and extend much further vertically
without anastomosing. The epidermis has been described as a layer of
fairly thick cells showing in one case an appearance of a depressed
stoma[219]. There are no secretory canals like those of Cycads but,
as in _Lyginopteris_, scattered cells with dark contents in the
stem-tissues probably represent secretory sacs.
The leaf-traces on leaving the stele pursue a very gradually ascending
course to the petiole; they retain their collateral structure in the
pericycle and cortex and have no secondary xylem, but become concentric
as they enter the base of a leaf.
[Illustration: Fig. 412. _Heterangium Grievii_. Oblique longitudinal
section of stele and part of cortex; _p_, pericycle. × 3. (Kidston
Coll. 529.)]
Before passing to the description of the leaves, the more striking
features in the stem may be summarised with reference to the
diagrammatic sketches shown in fig. 411. Fig. 411, A, represents a
section of _Heterangium Grievii_ approximately 2 cm. in its maximum
diameter; at the periphery of the primary xylem, _x_¹, and close to its
outer margin are several protoxylem groups, not shown in the drawing,
each of which marks the position of a mesarch trace. The zone of
secondary xylem, _x_², is interrupted by the exit of leaf-traces and
one of these is seen at _a_ in fig. 415, A, separated from the central
primary xylem by a foliar gap filled with parenchyma. The pericycle
is shown at _p_ in fig. 412 and its outer boundary at _p_ in fig.
411, A. Beyond the pericycle is the broad parenchymatous cortex with
leaf-traces, _lt_, and some sclerenchymatous patches, _s_. The vascular
strand _a_ is passing into the base of a leaf-stalk. In the stem shown
in fig. 411, B, 1·4 cm. × 7 mm., a decurrent petiole is seen at the
upper end with its single vascular strand, _a_, and two sclerous nests;
a similar though detached leaf-base occurs at the opposite end of the
long diameter. Other leaf-traces are seen at _b_ and _c_. From the
left-hand side of the stele a curved strand of tracheids is passing out
to supply a root, _r_.
ii. _Root_.
Adventitious roots of endogenous origin are occasionally met with in
_Heterangium_ stems, but we have less information as to their anatomy
than in the case of _Lyginopteris_. In a specimen of _Heterangium
Lomaxi_ figured by Williamson and Scott[220] three roots are seen in
a vertical series growing outwards through the cortex of a stem. The
roots agree generally with those of _Lyginopteris_ but the outermost
cortical layers possess no special features.
iii. _Leaf_.
The large compound fronds long known as _Sphenopteris elegans_
were recognised by Kidston as the leaves of _Heterangium_ by the
closely arranged transverse striae or narrow ribs on the rachis and
pinnae which are the expression on the carbonised impressions of
the horizontal plates of sclerous tissue in the petrified stems and
petioles of _Heterangium_. The dichotomously branched fronds are
included by Stur in his genus _Diplotmema_ and that author figures
several typical examples in his ‘Culm Flora’[221]. Fig. 413, A,
shows a forked axis with the bases of more slender branches and the
characteristic transverse bands and in fig. 413, B, part of a pinna
is reproduced. In general appearance, except in the bifurcating
pinnae, the fronds resemble those of _Davallia tenuifolia_ with which
Brongniart compared the Palaeozoic species. There is little doubt
that _Sphenopteris dissecta_ and some other species were also borne
on _Heterangium_ stems. The rachis and petioles differ from those of
_Lyginopteris_ fronds in the absence of emergences (_cf._ fig. 404,
E). The petioles (fig. 411, B, _a_) have a single concentric vascular
bundle with internal protoxylem.
[Illustration: Fig. 413. _Sphenopteris_ (_Diplotmema_) _elegans_. A.
Forked rachis with bases of lateral branches (Kidston Coll.); B, pinna.
(After Stur.)]
iv. _Reproductive organs_.
As yet no satisfactory evidence has been published with regard to
the nature of the microsporangia but in all probability these were
constructed on the same plan as those of _Lyginopteris_. There is a
strong _prima facie_ case for assigning the seed _Sphaerostoma_ to
_Heterangium_: absolute proof of organic connexion is still lacking
though Dr Benson’s recent account of the seeds associated with
_Heterangium Grievii_ almost amounts to demonstration of continuity
between vegetative organs and seeds.
_Sphaerostoma ovale_ (Williamson).
[Illustration: Fig. 414. _Sphaerostoma ovale_. A. Longitudinal
section showing the cupule, _c_, integument, _e_, _f_, with vascular
bundle, _v_, the upper part of the nucellus, _n_, and megaspore,
_m_; _a_, archegonia. B. Transverse section through the roof of the
pollen-chamber, _pc_, and the summit of the nucellus, _n_. (After
Benson.)]
In 1877 Williamson described some detached petrified seeds from the
Lower Carboniferous rocks of Fifeshire, Scotland, as _Conostoma ovale_
and _C. intermedium_. Dr Benson’s investigation[222] of these two
forms leads her to confirm Williamson’s doubts as to the validity of a
specific separation and she assigns the single type to the new genus
_Sphaerostoma_. The seeds are always associated with the vegetative
organs of _Heterangium Grievii_. In 1909 Oliver[223] expressed the
opinion that the Burntisland species of _Conostoma_ (= _Sphaerostoma_)
is probably the seed of _Heterangium_. The seed consists of a central
body representing the nucellus, an inner integument, and an enveloping
cupule or outer integument: most specimens have lost the cupule and in
this condition they are 3·5 mm. long with a maximum breadth of 2·2 mm.
In the middle the seed is circular in transverse section and octagonal
near the base and apex. The free apical part of the integument forms
a frill (canopy) round the micropyle and extends beyond the nucellar
apex which consists of a relatively flat plinth surmounted by a central
dome or lagenostome (fig. 414). The lagenostome is surrounded by an
annular pollen-chamber on to the lower surface of which abuts the
large embryo-sac, and remains of archegonia were noticed below the
pollen-chamber. The roof of the chamber in the young state consists of
a layer of thin-walled cells extending across the flattened apex of
the nucellus, _n_, but as the pollen-chamber becomes differentiated
from the nucellar tissue by the disorganisation of the zone of cells
its roof-cells thicken their vertical walls and assume the structure
of a multiseriate annulus, which acts as a mechanism for opening the
pollen-chamber by a circular dehiscence in such a way that the edge of
the ruptured roof of the pollen-chamber slightly overlaps the periphery
of the central column of nucellar tissue after it has returned to its
original position subsequent to the entrance of the microspores. The
micropylar region is surrounded by eight lobes of the integument and
each is characterised by a crest of radially elongated cells, fig. 414,
_f_, especially prominent on the outer side. External to this is the
slightly longer cupular sheath (fig. 414, _c_) which may also have been
lobed. The surface of the integument below the terminal crests consists
of a layer of cells with small papillae which eventually ruptured and
discharged mucilage. Both integuments have a vascular supply, that of
the inner integument being represented by eight vascular bundles, some
of which were found to have mesarch xylem, given off from the single
strand in the pedicel. Fig. 414 shows the apical region of a seed
of _Sphaerostoma_: the flat-topped nucellar cap, _n_, is surrounded
by the annular pollen-chamber, _pc_, below which are indicated the
archegonia: the wall of the megaspore (embryo-sac) is seen at _m_ and
external to this vascular bundles, _v_, run up the inner portion of
the integument accompanied by some large cells (aqueous tissue). The
elongated epidermal cells at the apices of the lobes of the integument
form the frill, _f_, and at a lower level the cells of the same layer
are much smaller and papillate (_e_): the outer integument, _c_, forms
the so-called cupule. The transverse section shown in fig. 414, B,
is taken at the level of the roof of the pollen-chamber and of the
nucellar cap; it illustrates the contrast between the ‘multiseriate
annulus’ and the central column of small parenchyma.
_Sphaerostoma_ differs from _Lagenostoma_ in the whorl of crests around
the micropyle, in the nearly hemispherical form of the lagenostome
and in the relatively wider pollen-chamber with its peculiar form of
dehiscence. Miss Benson, while regarding _Sphaerostoma_ as similar to
_Lagenostoma_ in general plan, believes the distinguishing features of
the former to be such as are consistent with a more primitive form.
An important argument in support of connecting this seed with
_Heterangium_ is derived from the juxtaposition of some seeds and
portions of _Heterangium_ petioles, a juxtaposition that is believed to
demonstrate original continuity.
Grand’Eury[224] has recorded the association of two species of leaves,
_Sphenopteris elegans_ and _S. dissecta_, with small seeds compared by
him with _Lagenostoma_. In the absence of petrified specimens it would
be practically impossible to distinguish between _Lagenostoma_ and
_Conostoma_ or _Sphaerostoma_.
Carpentier[225] has described some impressions from French Westphalian
beds as _Conostoma_ and he records cupules without seeds on fronds of
_Sphenopteris obtusifolia_ which he speaks of as having transverse
striations like those of _Heterangium_. Dr Kidston pointed out to me
that the surface-features of the _Sphenopteris_ rachis are probably
due to ramental scales and not to the presence of horizontal sclerous
bands. Carpentier’s seeds may be compared with _Lagenospermum
Sinclairi_.
_Heterangium tiliaeoides_ Williamson, Phil. Trans. R. Soc. Vol. 178, p.
289.
This species, founded by Williamson on material from the Lower Coal
Measures of Halifax, Yorkshire, while agreeing in the structure of
the primary stele and in the general features of the cortex with the
older _Heterangium Grievii_, is clearly distinguished by certain
well-marked characters. Sclerous groups occur in the inner cortex as
in _H. Grievii_ but they are present also in the pericycle. The
peripheral leaf-traces in the stele show the mesarch structure rather
more distinctly than in _H. Grievii_, and the secondary xylem, which
forms a much broader cylinder than in the Scotch type, is divided by
broad medullary rays into characteristic cuneate masses each of which
rests at its base on the centrifugal tracheids of a leaf-trace strand
of xylem (fig. 415, B). The most striking distinctive feature is
afforded by the secondary phloem, which is often preserved in wonderful
perfection; this is unusually thick and owing to the tangential
expansion of the principal medullary rays the secondary phloem is
divided into separate masses which decrease in breadth towards the
external arcs of primary phloem. The triangular form of the phloem
rays, composed of tangentially stretched parenchyma, suggested the
specific name _tiliaeoides_ on account of their striking resemblance
to the rays of _Tilia_. The leaf-traces are nearly always in pairs as
they pass out through the cortex; they subsequently divide and appear
as four vascular strands in the petiole. The portion of stem reproduced
in fig. 415, _B_, 8 mm. broad, shows clearly the separation of the
secondary xylem and phloem into wedge-shaped groups: in each group
there are several narrow medullary rays. The extrastelar tissues are
represented by a few fragments only. Several layers of crushed periderm
occur in the pericyclic region but the more external tissues have been
almost completely exfoliated[226].
[Illustration: Fig. 415. A, B, _Heterangium_. A, _H. Grievii_, _a_,
leaf-traces. B, _H. tiliaeoides_. C, root of _Lyginopteris_.
(A, B, Kidston Coll., 529, 294; C, Williamson Coll., 1631.)]
Reference has already been made to _Heterangium Lomaxi_, the English
type originally included by Williamson in _Heterangium Grievii_. The
provisional species _Heterangium cylindricum_ Williamson and Scott[227]
differs, as Scott says, in no important respect from _H. Lomaxi_ and
should not be retained. A new species, _H. minimum_ Scott[228], has
been founded on a very small stem from the Coal Measures of Dulesgate
in which the leaf-traces leave the stele as single bundles as in the
Scotch _H. Grievii_.
The French species _Heterangium Duchartrei_[229] Ren. from Permian
rocks was originally referred by Renault to the genus _Poroxylon_: it
is represented by little more than the xylem of the stele and bears
a close resemblance to _H. tiliaeoides_. _Heterangium punctatum_ Ren.
and _H. Renaulti_[230] (Brongn.) also from the Permian of France were
originally placed in the genus _Lycopodium_ and afterwards recognised
as stems of _Heterangium_. A fourth French Permian species, _H.
bibractense_[231], is peculiar in the possession of a very small
primary stele encircled by deep wedges of secondary xylem, but without
more information it is impossible to speak with confidence as to
its systematic position. Kubart[232] has recently published brief
descriptions of some stems from the Ostrauer coal-basin in Moravia all
of which he regards as specifically distinct from the English types.
In _Heterangium Sturi_ the primary xylem is almost exarch and the
peripheral xylem groups are not very clearly defined: in _H. alatum_,
so called from the presence of lateral wings on the petioles, the
leaf-trace strands are more sharply differentiated from the rest of
the stele. _H. polystichum_ is a similar type, and _H. Andrei_, with a
relatively larger amount of parenchyma in the stele and thicker stems
forms an additional link between _Heterangium_ and _Lyginopteris_[233].
Prof. Johnson[234] has described a species of _Heterangium_, _H.
hibernicum_, from Upper Devonian and Lower Carboniferous beds in Co.
Cork, Ireland, based on some impressions of frond fragments without any
pinnules. The occurrence of numerous transverse striae on the rachis
and lateral branches suggests comparison with _Heterangium_ fronds,
but an examination of the specimens led me to suspect that some at
least of the striae are cracks and not original features. The presence
of spur-like appendages from the lower surface of the pinnae near
their origin from the rachis is recorded as a peculiar character, and
some obscure oval bodies, the nature of which is extremely doubtful,
are considered to be seeds. The imperfection of the material hardly
justifies the institution of a new species of _Heterangium_.
_Heterangium_ ranges from the Lower Carboniferous to the Permian strata
and is thus older than _Lyginopteris_ which in the form of petrified
stems is not recorded from the Lower beds of the Carboniferous system.
_Heterangium_ has been described as having a ‘great preponderance
of fern-like characters,’ but having regard to the resemblance of
the primary xylem of the latter to that of the Osmundaceae it would
seem doubtful whether in their relation to the Ferns there is any
important difference. _Heterangium_ may safely be spoken of as the
more primitive genus. The polydesmic character of the petioles of most
species is particularly interesting as it brings the genus nearer to
the Medulloseae and to _Rhetinangium_[235].