=Seed-bearing plants closely allied to members
of the Lycopodiales.=
i. _Lepidocarpon._
In 1877 Williamson[662] published an account of some fossil seeds which
he referred to Brongniart’s genus _Cardiocarpon_[663], a generic title
for certain Gymnospermous seeds. Some of these he identified, on the
authority of the author of the species, with _Cardiocarpon anomalum_
Carruthers[664]. Several years later Wild and Lomax described a new
type of strobilus from the Lower Coal-Measures of Lancashire[665].
The result of this discovery and of the subsequent examination by
Scott of additional material, was to establish the fact that the seeds
described by Williamson and generally accepted as Gymnospermous, are
in reality sporangia belonging to a Lycopodiaceous cone. The seeds to
which Carruthers gave the name _Cardiocarpon anomalum_ are, however,
distinct from those described under the same name by Williamson and
are those of a true Gymnosperm. For this seed-bearing strobilus
Scott[666] instituted the generic name _Lepidocarpon_, which he thus
defined: “Strobili, with the characters of _Lepidostrobus_, but each
megasporangium was inclosed, when mature, in an integument, growing
up from the superior face of the sporophyll-pedicel. Integument,
together with the lamina of the sporophyll, completely enveloping
the megasporangium, or nucellus, leaving only an elongated, slit-like
micropyle above. A single functional megaspore or embryo-sac developed
in each megasporangium, occupying almost the whole of its cavity.
Megaspore ultimately filled by the prothallus or endosperm. Sporophyll,
together with the integumented megasporangium and its contents,
detached entire from the axis of the strobilus, the whole forming a
closed, seed-like, reproductive body. Seed-like organ horizontally
elongated, in the direction of the sporophyll-pedicel, to which the
micropylar crevice is parallel.”
_Lepidocarpon Lomaxi_, Scott. Fig. 218.
An immature cone of _L. Lomaxi_ is practically identical with a
_Lepidostrobus_; its sporangia are naked and only acquire their
integuments at a later stage. A mature strobilus has a diameter of at
least 3 cm. and is about 4 cm. in length. As in typical _Lepidostrobi_,
the axis bears spirally disposed sporophylls, and each sporophyll has a
long narrow pedicel approximately at right angles to the cone axis with
its distal end expanded into a broad and thick lamina (fig. 218, B).
At the distal end the pedicel has a thin marginal wing (fig. 218, C,
right-hand half) continuous with the upturned protective lamina. To the
upper face of each sporophyll is attached along the whole length as far
as the ligule, a single large sporangium; on each side of the base of
the sporangium the sporophyll forms a supporting cushion. The relation
of the sporangium to the ligule, _l_, is shown in fig. 218, B, and in
the tangential section, C, which illustrates the triangular form of the
sporangium near its distal end.
In mature cones, the sporangia assumed the form of seeds, the change
being due to the growth of an investing integument from the upper
face of the sporophylls on each side of the sporangia. Fig. 218, A,
illustrates the form of a sporangium as shown in tangential sections;
the vascular bundle is seen below the base of the sporangium and the
gaps right and left of it probably mark the position of parichnos
strands. On each side of the sporangium, _b_, a fairly thick wall of
tissue has grown up from the sporophyll, forming an integument which
overtops the apical ridge of the sporangium, leaving a narrow micropyle
in the form of a long crevice (_m_, fig. 218, B). At the proximal
end of the sporangium the integument forms an enclosing wall; at the
distal end it abuts on and is continuous with the upturned end of the
sporophyll. It is clearly established by Scott that the tissue which
invests the sporangia is not the upturned margins of the sporophyll,
but a new formation fully entitled to the designation integument. It
is noteworthy that the integument is not developed until a late stage
in the ontogeny of the strobilus; it is not formed until after the
production of the prothallus[667]. The diagrammatic sketch, fig.
218, B, shows the relation of the integument to the sporophyll and
sporangium, the outline of the latter being indicated by a broken
line. The columnar wall of the sporangium (fig. 218, A, _b_) forms a
closed beak within the micropylar crevice, and in the interior of the
sporangial cavity the slightly shrivelled membrane, _a_, represents the
single megaspore; traces of the aborted sister-cells of the megaspore
are occasionally met with. Scott describes a specimen in which the
megaspore is filled with tissue agreeing in appearance with the
prothallus in a megaspore of _Isoetes_ or _Selaginella_; no undoubted
archegonia or female organs have been discovered, nor has any spore
been found containing an embryo.
[Illustration: FIG. 218. _Lepidocarpon Lomaxi_, Scott.
A and C. After Scott.
B. Diagram of a single sporophyll: _m_, micropyle; _St_, stele.]
The axis of _L. Lomaxi_ has a medullated stele constructed on the same
plan as that of some species of _Lepidodendron_ and _Lepidostrobus_;
the vascular bundles supplying the sporophylls pass obliquely upwards
and outwards from the stele, _St_, fig. 218, B, and bend slightly
downward just before entering the pedicel of a sporophyll.
Dr Scott has also described a strobilus containing microsporangia
partially enclosed by a rudimentary integument. It is, however, of
considerable interest to find a partial development in the case of a
male flower of an integumentary outgrowth, which it would seem could
only be of real functional importance in the female shoot.
It is important to notice that specimens of a second species of
_Lepidocarpon_, _L. Wildianum_, are recorded from Lower Carboniferous
beds of Scotland, a fact which points to a considerable antiquity for
this seed-bearing Lycopodiaceous type[668].
The most important question to consider in regard to _Lepidocarpon_
is—are we justified in applying to the integumented sporangia the term
seed? The megaspore was not set free as it is in recent Pteridophytes,
such as _Azolla_ and other genera with which _Lepidocarpon_ may be
compared; it was on the other hand retained in the sporangium, as
may sometimes happen even in recent species of _Selaginella_ (cf.
fig. 131, D). Moreover, the megaspore is characterised by a thin
enclosing membrane in contrast to the thick coat of a spore which
is destined to be shed. The peculiar slit-like form of the micropyle
is a distinguishing feature, but this may be readily explained as
a convenient form in the case of a radially elongated sporangium.
The absence of an embryo, though a distinguishing feature of
_Lepidocarpon_, cannot be held to be a serious obstacle to the use of
the term seed; in recent Cycads the embryo, as Scott points out, may
not begin to develope until the seed has been shed. It is possible that
the seeds of _Lepidocarpon_ were not pollinated on the parent plant.
The lesson which this extinct type teaches, is that certain
Lycopodiaceous plants of the Palaeozoic era had reached an important
stage in the evolution of a seed. The morphological essentials of true
seeds had been acquired; but we do not know the biological conditions
under which pollination and fertilisation were effected. Another point
of considerable interest is the value of this discovery as an argument
in favour of the view that some Gymnosperms are derived from Lycopod
ancestors. Leaving the general question until later, it may at any rate
be stated that in _Lepidocarpon_ we have a demonstration of the fact
that the Lycopodiales were not always distinguished from Gymnosperms
by the absence of seeds. There are certain features in _Lepidocarpon_
shared by the seeds of Araucarieae[669] which may well mean something
more than mere parallel development in two distinct phyla of the
plant-kingdom[670].
ii. _Miadesmia._
In 1894 Prof. Bertrand[671] published an account of certain fragments
of petrified leaves and twigs of a small herbaceous Lycopodiaceous
plant, under the name _Miadesmia membranacea_, which he discovered
in English material in association with _Lepidodendron Harcourtii_.
Subsequently Scott recognised the megasporophylls of the same plant,
and microsporophylls have also been discovered. The most complete
account of _Miadesmia_ so far published we owe to Dr Benson[672],
whose description is based on specimens from several sources.
_Miadesmia membranacea_, Bertrand. Fig. 219, A–D.
[Illustration: FIG. 219. A–D _Miadesmia_; E _Bothrodendron_.
A. Radial section of megasporophyll: _s_, sporangium; _m_,
megaspore; _l_, ligule. (From a drawing kindly lent by Mrs D. H.
Scott.)
B, C. Leaf with ligule. (From a section in Dr Kidston’s Collection.)
D. Transverse section of sporophyll. (After Scott.)
E. Radial section of microsporophyll of _Bothrodendron_. (From a
section in the Manchester Museum; Hick Collection R. 406.)]
The slender stem, characterised by unequal dichotomy, has a single
protostele composed of scalariform tracheids with 3–6 peripheral
protoxylem groups. A zone of delicate tissue surrounds the xylem; this
is described as phloem, but it is not clear whether the designation
is based on histological characters or primarily on its position.
The cortex consists of an inner lacunar tissue and an outer region
limited by a small-celled superficial layer sharply contrasted with
the underlying layers of larger cells. The stem of _Miadesmia_ is not
uncommon in sections of the Lancashire calcareous nodules, and may be
recognised by the delicate crushed tissue of which it mainly consists
and by large hypodermal parenchyma. The spirally disposed leaves bear
a conspicuous and relatively large ligule, 3 mm. long, in a deep pit
(fig. 219, B and C) roofed over by a few layers of tissue corresponding
to the velum in _Isoetes_ (cf. fig. 133, E, _v_). The fairly thick
central region of the lamina is expanded laterally into thin wings,
which in the living state probably bore delicate hairs. These delicate
leaves, apparently without stomata, were attached to the stem at an
acute angle, and Miss Benson suggests that their form and arrangement
may have enabled them to hold water by surface-tension. As seen in
fig. 219, B, C, which represents part of a transverse section near the
leaf-base, the ligule is a very characteristic feature, and the size
of the single vein is in keeping with the almost filmy nature of the
lamina.
In addition to the sections in British collections, I have been
enabled by the kindness of Prof. Bertrand to see photomicrographs of
the sections on which he founded the genus. One of these sections,
transverse to the stem and leaves, illustrates in a striking manner the
relatively large size of the leaves and ligules in proportion to the
delicate axis of the shoot.
The megasporangiate cone has an axis which agrees in its structure
with that of the vegetative stem and bears several megasporophylls
approximately at right-angles. As in the foliage leaves, the ligule
is prominent and large, and lies in a groove which contains also
the megasporangium; both ligule, _l_, and sporangium, _s_, as seen
in the transverse section represented in fig. 219, D, are covered
by an integument or velum which arises in the proximal part of the
leaf and leaves a circular micropylar opening at the beak-like apex
of the sporangium. The circular micropyle is surrounded by numerous
hairs borne on the integument and which presumably played the part
of a feathery stigma. A single megaspore with a thin membrane, _m_,
abuts on the fairly strong sporangial wall, _s_; in some cases the
sporangium and megaspore walls may be indistinguishable, a feature
suggesting comparison with seed-structure. Some megaspores have
been found filled with a prothallus. The longitudinal section shown
in fig. 219, A, illustrates the characteristic horizontal position
of the megasporophyll, as also the relation of the ligule, _l_, to
the sporophyll with its single vascular bundle, and to the hairy
integument, which overarches both sporangium and ligule; the line
_m_ shows the position of the megaspore-membrane, detached from the
sporangial wall on the upper side but in contact with it below.
The microsporophyll shown in 219, E, was originally referred to
_Miadesmia_ but has since been recognised by Watson[673] as that of a
_Bothrostrobus_.
_Miadesmia_ affords an example of a Palaeozoic plant comparable with
_Isoetes_ and _Selaginella_; it agrees also with _Lepidocarpon_ in
possessing true seeds, and with Watson’s _Bothrodendron_ cone in the
shape of the sporangia, which are more like those of _Selaginella_
than the radially elongated sporangia of _Lepidostrobus_. _Miadesmia_
agrees with _Selaginella_, e.g. _S. spinosa_, in its stelar structure,
in the form of the sporangia, and in the presence of a ligule. It
is distinguished by having only one instead of four megaspores in a
sporangium, in the possession of an integument which formed a close
investment to the spore and served as a stigma (comparable with the
stigma-like integument of the male flower of _Welwitschia_), and in the
shedding of the megasporophylls, which have been aptly compared with
winged seeds.
[Sidenote: LEPIDOCARPON]
On the ground of their general anatomical features _Lepidocarpon_
and _Miadesmia_ are clearly entitled to be included among extinct
representatives of the Pteridophyta. These plants had, however,
crossed what it has been customary to regard as the boundary between
Pteridophytes and Phanerogams: they possessed megasporangia with the
attributes of seeds. It has been suggested by Lester Ward[674] that
Pteridophytic seed-bearing plants shall be recognised as a distinct
phylum for which he proposes the name Pteridospermaphyta, a designation
implying exclusion from the Spermatophyta as usually understood. For
seed-bearing Lycopodiaceous genera he suggests the name Lepidospermae.
As knowledge of the Palaeozoic seed-plants increases revision of
existing classifications and group names will become necessary, but as
yet we are hardly in a position to draw up a satisfactory scheme of
grouping; we know little of _Lepidocarpon_ as a whole and it would be
premature to commit ourselves, even provisionally, to a classification
which is based on such meagre evidence as we possess. Moreover the
value to be attached to the seed-habit as a basis of classification can
hardly be estimated until fuller information is obtained.