PALAEOZOIC GYMNOSPERMOUS SEEDS.
Seeds are abundantly represented as fossils from Carboniferous to
Post-Tertiary deposits. The importance of fossil and sub-fossil
species in the later geological series has been demonstrated by the
investigations of Mr and Mrs Clement Reid and a few other workers in
this neglected field. In cases where it is possible to assign seeds
to their parent-plants the descriptions of casts, impressions, or
petrifactions are added to the account of vegetative organs; but
it frequently happens that seeds are preserved only as detached
specimens many of which have little or no value as botanical records,
while others that afford striking examples of the possibilities of
petrifaction as a means of preserving the most delicate structures,
are of great importance. In Volume +ii.+ an account was given of such
Palaeozoic seed-bearing organs as _Lepidocarpon_ and _Miadesmia_, and
the genera _Lagenostoma_, _Sphaerostoma_, and _Trigonocarpus_ are
dealt with in this volume under _Lyginopteris_, _Heterangium_, and
_Medullosa_. Certain seeds afford some evidence as to the systematic
position of the parent-plants though insufficient to warrant more than
a surmise as to the nature of the vegetative organs: in several cases
it is only from the resemblance of detached seeds to types that on the
strength of more or less convincing evidence are referred to definite
parent-plants that any conclusions can be drawn with regard to precise
systematic position. In view of the occurrence of several different
types of seeds that retain their morphological features, but cannot be
assigned with any degree of certainty to genera founded on vegetative
organs, a special chapter is devoted to a comparative study of selected
examples with the object of directing attention to data bearing on
evolutionary problems. The chief interest of Palaeozoic seeds to the
botanist lies in the facts they contribute towards the elucidation
of questions connected with the promotion of the megasporangium and
megaspore of the Pteridophyta to the higher stage represented by the
integumented megasporangium (nucellus) and single megaspore that in
the main fulfil the definition of a seed[790]. ‘With the evolution
of the seed,’ as Oliver says, ‘the plant rose at a bound to a higher
plane, and this structure in its perfected form has become the very
centre of the plant’s existence[791].’ We can as yet form a very
partial conception of the successive stages in the adoption of the
seed-habit, but since 1855, the year in which Hooker and Binney[792]
published their paper on the structure of _Trigonocarpus_, ample
proof has been furnished of the importance of Palaeozoic seeds from
the standpoint of affinity between recent Gymnosperms and extinct
seed-bearing plants, and of the _modus operandi_ of evolutionary
tendencies. A cursory examination of Palaeozoic seeds suffices to
demonstrate their resemblance to those of recent Cycads and the seed
of _Ginkgo biloba_; but while it is clearly with these Gymnosperms
that the majority of the seeds described in the following pages are
most closely allied, the extinct types possess many distinguishing
features that throw light on some at least of the factors concerned in
the production of the modern type. In many of the Palaeozoic seeds the
nucellus stands free within the integument, to which it is attached
only in the chalazal region, in contrast to the lateral union between
integument and nucellus in the ovules of recent Cycads. It has been
suggested by Oliver[793] that the seed of the Conifer _Torreya_ affords
a clue to the interpretation of this difference and that the lower part
of the seed in Cycads and _Torreya_ represents a later intercalation
in the basal portion of the ovule, the ancient type having a free
nucellus in contrast to the nucellus of modern seeds which is free
only at the apex. It has been objected[794] that there are no adequate
grounds for assuming the addition of an intercalated zone or of the
elongation of the ovule that this implies, the more probable view
being that the lateral union of nucellus and integument represents
congenital fusion in the ancestral type, a fusion comparable with that
of the coherent petals of a gamopetalous corolla. In the presence of a
pollen-chamber most of the Palaeozoic seeds agree with those of recent
Cycads, but in the extinct forms it is usually a more highly developed
structure. The name pollen-chamber was given by Brongniart[795] to
the pollen-containing cavity in the free region of the nucellus in
the petrified seeds from St Étienne in ignorance of the use of the
same term by Griffith[796] in a posthumous work published in 1852 for
the nucellar chamber in _Cycas_[797]. The genus _Stephanospermum_
(fig. 494, A) illustrates the means by which the pollen-chamber was
liberally supplied with water and thus adapted to the requirements of
fertilisation by motile gametes. The pollen-chamber and its vascular
supply paved the way for siphonogamy, that is the development of a
pollen-tube for the more direct transmission of the male sperms. The
highly developed mantle of tracheal tissue at the periphery of the
nucellus in _Stephanospermum_, represented on a reduced scale by the
separate vascular strands of other seeds, may be compared with the
tracheal investment to the nucellus in the recent Dicotyledonous genus
_Cassytha_[798]. The presence of a nucellar vascular system in several
Palaeozoic seeds is a feature in which they differ from those of recent
Cycads with the exception of _Bowenia_. The retreat of the vascular
supply from the immediate neighbourhood of the pollen-chamber in recent
Cycads may, as Oliver points out, be correlated with the evolution of
the pollen-tube—the substitution of siphonogamy for zoidiogamy. The
diagram reproduced in fig. 492 represents a synthetic type based on
such seeds as _Stephanospermum_ and _Cardiocarpus_ which illustrate an
arrangement of conducting tissue frequently found in Palaeozoic seeds:
the main strand gives off a pair of bundles in the sarcotesta in the
principal plane, as in _Cardiocarpus_[799]; from the tracheal mass in
the chalazal region numerous bundles pass up the nucellus as far as the
floor of the pollen-chamber. The nature of the vascular supply in this
generalised type and in individual genera should be compared with that
in the seeds of recent Cycads[800] described in Chapter +xxviii.+
[Illustration: Fig. 492. ‘A conjectural synthetic type of seed
embodying the characters of such a seed as _Stephanospermum_ with those
of a _Cardiocarpus’_; _n_, nucellus; _pc_, pollen-chamber. (After
Oliver.)]
Recent research has revealed the not unexpected fact that in such
Upper Carboniferous petrified seeds as have been investigated—a
small proportion of the large number produced in the Palaeozoic
forests—there was a remarkable range in the mechanism connected with
pollination and the maturation of the microspores. A comparison of
the genera _Physostoma_, _Lagenostoma_, _Conostoma_, _Trigonocarpus_,
_Stephanospermum_ and others reveals the occurrence of very different
though not unrelated structural features especially in the apical
region of the seed. These seeds, including _Physostoma_, probably the
most archaic type, represent a stage in evolution already far removed
from the starting-point: the diversity of plan recalls the variety in
the form of the chloroplasts in the Green Algae, and in both cases
we are in touch with an experimental phase representing a tentative
advance towards greater efficiency.
In its differentiation into an outer fleshy region, the sarcotesta, a
stony layer, the sclerotesta, and in many cases an inner flesh, the
Palaeozoic seeds resemble recent Cycads: in both extinct and modern
seeds the balance of evidence would seem to be in favour of attributing
a single rather than a double origin to the integument.
Among the numerous types of Palaeozoic seeds are several which
invite comparison with the fruits or carpels, apart from the seeds,
of Angiosperms. Impressions of _Samaropsis_ seeds (figs. 502, B–K;
503; 504) bear a close resemblance to the laterally expanded fruits
of the common Crucifer _Thlaspi arvense_; the ribbed testa of
_Hexagonocarpus_ (fig. 506, H) and other genera recalls the fruit-wall
of _Alstroemeria_; the recently described Lower Carboniferous seed
_Thysanotesta sagittula_ Nath. (fig. 506, F) simulates a carpel of
_Erodium_. These and similar instances of a close parallelism in
external features between organs that are not homologous, though in
themselves of no morphological significance, are at least interesting
as illustrating the plasticity displayed by reproductive structures,
which in the Palaeozoic period marked a morphological achievement
comparable in its importance with the still greater achievement
represented by the highly specialised fruits of the modern Flowering
plants. The range in form and surface-features of Angiospermous
fruits was foreshadowed by Palaeozoic seeds. Structural types and
in some cases, superadded to these, features which may reasonably
be supposed to have facilitated dispersal had been acquired by the
seeds of Palaeozoic plants in forms that in a much later period were
adopted by fruits even to a greater degree than by seeds. Characters
useful in seed-dispersal, that are now shared by fruits and seeds,
are illustrated by the fleshy and possibly edible seeds of extinct
Gymnosperms, the plumes and hairy beak of _Gnetopsis_ (fig. 494,
E) and _Thysanotesta_ (fig. 506, F) suggestive of feathery stigmas
and other appendages. The lacunar sarcotesta of _Aetheotesta_, the
thick endotesta of _Pachytesta_ (fig. 497), and the air-chamber of
_Codonospermum_ (fig. 498), are strictly comparable with aids to
buoyancy in fruits of existing Flowering plants. The mucilage-hairs and
superficial cells in _Physostoma_ (fig. 494, I) and _Stephanospermum_
may be compared with the thick mucilaginous investment of the
megaspores of recent water-ferns and with similar tissues of some
Angiospermous seeds.
The bionomics of Palaeozoic plants is a subject worthy of more
serious attention than it has so far received. The search for
morphological characters that may have facilitated the wanderings
of widely distributed genera and species and a closer investigation
of physiological-anatomical problems presented by the vegetative
organs of petrified plants would not only extend our knowledge of the
morphology of ancient types but would stimulate comparative study
and, incidentally, relieve the dullness of pure description. It may
be argued that we should first establish a more solid foundation
by further observations on living plants; but even at the risk of
allowing speculation too free a hand the attempt is worth making, and
it may be urged that, as in phylogenetic enquiries so in other branches
of botany, facts obtained from plants of other ages may serve to
supply deficiencies in knowledge based only on existing forms. One of
the difficulties inseparable from the study of fossil plants, namely
the identification of impressions and casts with specimens exhibiting
anatomical characters, is particularly well illustrated by seeds. The
description of a genus based on mere external form may sometimes be
extended without great risk of error to include species founded on
anatomical characters, but on the other hand, there are many instances
in which—despite a general resemblance in form and size between
petrifactions and impressions—lack of evidence of generic identity
requires the employment of distinctive names. The determination of
impressions is, as Lesquereux recognised, ‘subject to a great deal of
uncertainty,’ and many of the genera founded on external features are
purely artificial and include species that have no essential features
in common. Moreover in the case of petrified specimens the apparent
absence of an external fleshy layer is often due to destruction before
preservation: as Solms-Laubach[801] points out, it is obviously
impossible to be certain as to the number of integumental layers
in seeds that are not well preserved in all their parts. Goeppert
founded a new genus, _Acanthocarpus_, on a Permian seed described as
_A. xanthioides_[802], because of the occurrence of spinous processes
attached to an obcordate kernel: these apparent spines are in all
probability the remains of a very imperfectly preserved sarcotesta. The
preservation of the central portion of a seed, that is the seed-cavity
with the enclosing shell, conveniently called the nucule, has often led
to an unnecessary multiplication of generic terms. Other examples of
confusion resulting from different states of preservation are quoted in
the accounts of some of the selected types.
Williamson in 1877 pointed out that we learn from the large number
of different kinds of Palaeozoic seeds that ‘there were in the
Carboniferous forests many gymnospermous stems clothed with foliage
of which we have not yet discovered any traces, probably because
these Gymnosperms did not flourish upon the low swampy grounds which
were the homes of the great mass of the coal-producing plants[803].’
Prof. Zeiller[804] has also drawn attention to the numerical excess of
seeds over vegetative organs. This discrepancy has to a large extent
been explained by the discovery that many of the supposed Ferns were
seed-bearing plants, and a further explanation is suggested by the
superiority of seeds over stems and leaves in their adaptation to
dispersal by water.
In 1874 Brongniart[805] described several petrified seeds from material
discovered by Grand’Eury in the St Étienne coal-field, and seven years
later his descriptions were republished[806], with the addition of
several beautifully executed drawings, as a posthumous volume edited by
his distinguished pupil Renault. Williamson’s researches supplied much
additional information, and in recent years the more detailed study of
French and English seeds by Bertrand and particularly by Oliver and
his pupils has further emphasised the interest and importance of this
field of work. Brongniart proposed a two-fold classification of French
seeds: (i) bilaterally symmetrical seeds, more or less flattened in
section, which he believed to be Cordaitean; (ii) radially symmetrical
seeds, circular in transverse section: the latter group he considered
to be less closely allied to recent types. The employment of the
terms Platyspermeae and Radiospermeae, proposed by Oliver[807] for
Brongniart’s divisions, serves a useful purpose if due regard is paid
to the adequacy of the evidence as to symmetry and if it is recognised
that this classification cannot be rigidly employed in all cases.
It was pointed out by Brongniart that the occasional occurrence of
tricarinate seeds of _Ginkgo_ (fig. 631) and _Taxus_ is an exception
to the general rule of bilateral symmetry: seeds of _Cycas_ are
normally bilateral, but radially symmetrical forms also occur[808].
The genus _Conostoma_ (fig. 494, B) represents an intermediate
type which, though almost radially symmetrical, exhibits a slight
tendency towards platyspermy. Evidence recently brought forward by
Nathorst[809] renders probable a connexion of a presumably radiospermic
seed _Lagenospermum Arberi_[810] with the Lower Carboniferous fronds
_Adiantites bellidulus_ Heer, and this furnishes an interesting
illustration of the association of both platyspermic and radiospermic
seeds with the same generic type of foliage. While retaining Radiosperm
and Platysperm as convenient descriptive terms, I have not adopted them
as group-designations on the ground that they do not in themselves
serve as trustworthy criteria of relationship. Attention is called
by Salisbury[811] to the occurrence of bilaterally and radially
symmetrical fruits among British Carices and to a similar mixture in
the family Polygonaceae.
The acquisition of more detailed and accurate knowledge of Palaeozoic
seeds led to an extension of the two-fold division of Brongniart
and Oliver which is based on such characters as the position of the
vascular tissue in relation to the integument and nucellus, the form of
the pollen-chamber, and other features. The division Lagenostomales has
been instituted for _Lagenostoma_ and some other Radiosperms connected
by certain important characters: these seeds may be referred to the
Pteridospermeae though it is only in the case of _Lagenostoma_, and to
a less extent _Sphaerostoma_, that a correlation between vegetative
organs and seeds has been rendered sufficiently probable to justify
an assumption of generic identity. The name Trigonocarpeae[812]
has recently been used for a section of Radiosperms represented by
_Trigonocarpus_, _Stephanospermum_, and other genera. Although the
genus _Stephanospermum_, as Oliver[813] says, may be regarded as
the type-genus of a group of seeds, it is more fitting, as the same
author[814] insists, to adopt a divisional term based on the generic
name of the much more widely spread and more familiar _Trigonocarpus_.
For the sake of uniformity in nomenclature it is proposed to adopt
the name Trigonocarpales instead of Trigonocarpeae to rank with
Lagenostomales.
The Platyspermeae comprise such seeds as _Cardiocarpus_,
_Mitrospermum_, and _Rhabdospermum_, genera characterised by
well-marked anatomical features and probably Cordaitean; it has,
however, been shown that typical Platysperms were also borne on leaves
of Pteridosperms and, as Mrs Arber[815] says, the notion that every
member of the Platyspermeae was necessarily a Cordaitean seed has been
discredited by the discovery of the seeds of _Aneimites_ (_Wardia_)
and _Pecopteris Pluckeneti_[816]. For general purposes it is hardly
necessary to adopt the subdivisions of the Lagenostomales used by
Oliver and Salisbury[817], though as facts accumulate we shall no
doubt be able to make further advances towards a natural system of
classification. The following three divisions of Permo-Carboniferous
seeds include genera founded on petrified specimens and thus
afford valuable morphological data. The groups Lagenostomales and
Trigonocarpales include types belonging to closely related plants, a
relationship clearly expressed in the seed-characters.
I. +Lagenostomales.+
The seeds included in this group are for the most part Radiosperms, but
in its slightly developed bilateral symmetry _Conostoma oblongum_ is a
type transitional between Radiosperms and Platysperms. The testa may
be ribbed and the ribs vary in number. The nucellus (megasporangium)
is united to the integument not only at the base but laterally as
far as the shoulder of the seed up to a level corresponding to the
base of the pollen-chamber (lagenostome) as in all recent Cycads
and in the majority of Conifers. The seeds proper apart from the
cupule are supplied with a single set of vascular bundles; there is
no vascular tissue in the nucellus, a feature no doubt correlated
with the fusion of nucellus and integument[818]. The free portion of
the integument is more or less deeply lobed or, in _Lagenostoma_, it
forms a pyramidal canopy of fused lobes enclosing the lagenostome. The
presence of a tapetal zone surrounding the megaspore is believed to
be a feature characteristic of the group[819]. The testa, wholly or
partially ribbed, is relatively thinner than in the Trigonocarpales and
Cardiocarpales, and in its differentiation agrees less closely with
the testa of recent Cycadean seeds. In _Lagenostoma_ and possibly in
other genera a loose sheath or cupule surrounded the ovule, while in
_Gnetopsis_ a similar envelope enclosed two to four seeds.
The microspores are multicellular and smaller than those of
Trigonocarpales, the average dimensions (_Conostoma_, _Physostoma_,
_Lagenostoma_) being 67μ × 52μ.
Genera. _Physostoma_; _Conostoma_; _Sphaerostoma_; _Lagenostoma_;
_Gnetopsis_.
_Lagenostoma_ may safely be referred to _Lyginopteris_, and
_Sphaerostoma_ with but little risk of error to _Heterangium_: the
parent-plants of the other genera are unknown, but all may be regarded
as the seeds of Pteridosperms and probably of genera more nearly
allied to the Lyginopterideae than to the Medulloseae. The genus
_Lagenospermum_, recently instituted by Nathorst[820], is dealt with in
Chapter +xxxi.+
=Physostoma.= Williamson.
_Physostoma elegans_ Williamson.
The generic name _Physostoma_[821] was applied by Williamson[822] to
a seed from the Lower Coal Measures of Lancashire which he named _P.
elegans_; he afterwards described it as _Lagenostoma physoides_, but
the original name has been revived by Oliver[823] to whom our knowledge
of this type is chiefly due. The specimens figured by Williamson[824]
as _Sporocarpon ornatum_ also belong to _Physostoma elegans_. The
seeds are circular in section, approximately 6 mm. long with a maximum
diameter of 2 mm. The testa has about 10 longitudinal ribs passing in
the apical region into a ring of free lobes or tentacles surrounding
and considerably overtopping the nucellar apex: these tentacles take
the place of a micropylar tube (fig. 494, I; fig. 493, D) and are a
feature ‘in which this seed differs from all other known seeds, fossil
or recent[825].’ A single vascular strand passes through the chalazal
region and divides into 10 bundles, one to each rib and tentacle.
The single integument consists of a few layers of cells, those of
the epidermis being prolonged into clavate mucilaginous hairs, fig.
494, I, _h_, that may reach a length of ·5 mm. and in the living seed
almost covered the whole surface of the testa, being especially long
on the ribs and tentacles. There is no special development of sclerous
tissue, the vascular bundles, _v_, being embedded in parenchyma in
the inner portion of the integument. The nucellus is represented by
a zone rich in secretory cells, and internal to this is a tapetum.
Integument and nucellus are coalescent up to the apical region where
the former splits into 10 tentacles. The nucellar apex has the form of
a tall dome surrounded by a bell-shaped pollen-chamber (fig. 494, I,
_pc_; fig. 493, C, D, _c_) into which it projects like the base of a
wine-bottle. The circular opening of the pollen-chamber overtops the
roof of the dome formed of the secretory tissue of the nucellus and the
carbonised remains of the tapetum: this dark band surrounds the large
megaspore-cavity (fig. 494, I). _Physostoma_ is the only member of the
Lagenostomales in which the megaspore projects into the free nucellar
apex: in other genera intercalary growth has produced a more or less
prominent plinth, the name given to the free portion of the nucellus
between the megaspore and the pollen-chamber. Williamson[826] described
the mammillated apex of the nucellus as pushed up into the base of the
lagenostome which ‘looks like a bladder half full of fluid resting
upon and overhanging the end of a soda-water bottle’: it was this
appearance that suggested the name _Physostoma_. The section reproduced
in fig. 493, D, shows in the centre the limiting tissue of the nucellus
surrounded by the pollen-chamber, _c_, and external to this are the
tentacles with their groups of long hairs: the vascular bundles are
represented by spaces in the more internal small-celled tissue (see
also fig. 494, I). A characteristic feature is the presence of a
tapetum or megaspore-jacket[827] in the nucellus: immediately internal
to the vascular bundles stretching from the chalaza to the apex of the
megaspore-cavity is a layer of delicate cells with secretory sacs,
and this is succeeded by a broad black layer of rather larger cells,
a tissue which was probably in full activity in a younger stage of
development.
[Illustration: Fig. 493. A, _Lagenostoma ovoides_, longitudinal
section of the lagenostome. B, _Lagenostoma Lomaxi_, transverse
section of the lagenostome and canopy; _c_, pollen-chamber; _d_, space
between the nucellus and integument. (A, after Miss Prankerd; B, from
a section in the Manchester Museum, R 1048.) C, _Physostoma elegans_,
longitudinal section showing microspores in the pollen-chamber, _c_.
(From a section in the University College Collection.) D, _Physostoma
elegans_, transverse section of the apical region showing the
tentacles, pollen-chamber, _c_, etc. E, _Telangium Scotti_, transverse
section of sporangia (Manchester Coll. R 1047). F, G, _Stephanospermum
akenioides_. F, section of sclerotesta. G, tracheids from the nucellus.
(University College Coll.)]
A comparable tapetal layer is described by Lang[828] in the ovule of
_Stangeria_: the majority of the sporogenous cells surrounding the
megaspore become disintegrated and are absorbed, but the outermost
zone forms a more definite tapetal layer: as already suggested, this
tissue in _Physostoma_ may be a group-character. No archegonia have
been found, but in a few cases some of the delicate prothallus-tissue
occurs in the interior of the seed. Microspores are often abundant
in the pollen-chamber (fig. 493, C, _c_); in one seed 80 are
recorded. The occurrence of so many microspores suggested to Oliver
that insect-agency may have been responsible for the precision in
pollination that is greater than one would expect in anemophilous
plants. The spores are smaller than those of _Lagenostoma_ (55μ × 45μ)
and in several of them the remains of a cellular tissue are preserved
(fig. 494, N), also some sub-reniform bodies (fig. 494, M) similar to
those described as spermatozoids by Dr Benson in _Lagenostoma_ (fig.
408, D).
The most interesting features of _Physostoma_ are: the absence of
a continuous micropylar tube and its replacement by a circle of
integumental lobes; the apical prolongation of the nucellar apex into
the pollen-chamber, and the presence of long mucilaginous hairs on the
integument. The large pollen-chamber is a character which distinguishes
_Physostoma_ from _Conostoma_ and its form is very different from that
in _Lagenostoma_.
The tentacles of the integument and the form of the nucellar apex are
features consistent with Oliver’s view that _Physostoma_ is the most
primitive of Palaeozoic seeds though, as Burlingame[829] says, the
elaborate form of the encasing envelope marks a considerable advance
beyond the earliest type of megasporangium integument.
A new type of _Physostoma_ has been briefly described by Gordon[830],
without a specific designation, from the Lower Carboniferous beds of
Pettycur (Fife): it was found in association with _Heterangium_ and
_Sphaerostoma ovale_.
We have no knowledge of the plant to which _Physostoma_ belonged,
but the general plan of organisation of the seed points to a near
relationship to _Lagenostoma_ and presumably, as regards the
parent-plant, to a genus related to _Lyginopteris_.
=Conostoma.= Williamson.
This name[831], suggested by the funnel-like pollen-chamber or
lagenostome, was applied by Williamson[832] to some seeds from the
Lower Coal Measures of Lancashire and Yorkshire and from the Lower
Carboniferous beds of Burntisland. The Burntisland seeds, referred by
Williamson to two species, have recently been united and described
by Miss Benson as _Sphaerostoma ovale_[833]. The English species has
been thoroughly investigated by Oliver and Salisbury[834] who have
also described a second species, _C. anglo-germanicum_, from the Coal
Measures of Lancashire and Germany.
_Conostoma oblongum_ Williamson.
This rare type is represented by approximately cylindrical seeds with
an average length of 5 mm. and a maximum breadth of 2·3 mm. borne on
a relatively stout stalk and tapering to a blunt apex characterised
by a canopy of six short lobes (fig. 494, B, C) in marked contrast to
the long tentacles of _Physostoma_. In the basal region the integument
has six prominent ribs which soon die out when traced upwards: a
transverse section through the body of the seed is hexagonal (fig.
494, D), the angles corresponding to the basal ribs, and there is a
slight tendency to platyspermy. The testa has an epidermal mucilaginous
layer which becomes exfoliated through the lifting-up of the cuticle
by the underlying mucilage: below this, at the apex of the seed, is
a cap of fleshy tissue (fig. 494, B, _sa_) which, it is suggested,
may have had a secretory function in connexion with a drop-mechanism
in pollination like that in recent Conifers. No microspores have
been found in the pollen-chamber. The epidermis, called by Oliver
and Salisbury the blow-off layer (fig. 494, B, _m_), together with
the cap of soft tissue constitute a feebly developed sarcotesta. A
sclerotesta consisting of a palisade-layer and a fibrous hypoderm
extends over the main body of the seed below the epidermis; it forms
the basal ribs and increases considerably in breadth at the apical
region to form a sclerous cone penetrated by six strands of parenchyma
enclosing vascular bundles (fig. 494, D) which pass up from the
conducting tissue immediately external to the nucellus. The nucellus
is coalescent with the integument, as in _Physostoma_, as far as the
level of the domical free apex of the nucellus where the tapetal tissue
that lines the seed-cavity passes across the almost flat top of the
central region originally occupied by the megaspore. In some sections
prothallus-tissue was found with an apical ‘tent-pole’ protuberance.
A striking feature of _Conostoma_ is the mechanism for the reception
of the microspores. The free part of the nucellus consists of the
plinth, a broad tapering region originally filled with parenchyma but
in most cases represented only by its epidermis: the plinth, _p_, is
seen in fig. 494, B, to be two-storeyed, the upper and narrower storey
being a space formerly filled by a pad of tissue suspended from the
floor of the superposed lagenostome (pollen-chamber)[835]. The greater
development of the domical plinth is a feature in which _Conostoma_
differs from _Physostoma_. At the apex of the plinth and resting on
a slight depression is a small lagenostome, bowl-shaped in section,
and like the pollen-chamber of _Lagenostoma_, formed as the result of
enzyme-action on the apical papilla of the nucellus (fig. 494, B, B′,
_l′_). The mouth of the lagenostome engages with the micropylar tube by
a projecting flange (fig. 494, B′, _f_) of tissue lining the micropylar
canal and by a second flange (_f′_) at the base of the lagenostome
where the roof of the plinth (fig. 494, B′, _p_) bends downwards and
inwards. The walls of the lagenostome are formed by strong cells with
thickening bands giving them the appearance of tracheids (_l′_),
but the floor of the lagenostome is made of thinner cells which
become disorganised, allowing the microspores to fall into the large
plinth-cavity below (_p_, fig. 494, B), an arrangement comparable
with the two-storeyed pollen-chamber of _Bowenia_[836] and, to a less
extent, with the micropyle of the Conifer _Tsuga_. The microspores are
multicellular and ellipsoidal measuring 75μ × 65μ.
[Illustration: Fig. 494. A, _Stephanospermum akenioides_; _s_,
space between integument and nucellus; _n_, nucellus; _e_, inner
part of testa; _sc_, sclerotesta; _t_, _t_, tracheidal mantle; _pc_,
pollen-chamber; _a_, archegonia; _td_, tracheidal pad. B–D, _Conostoma
oblongum_; _a_, level of the transverse section shown in fig. C; _b_,
level of transverse section D; _m_, lobes of tissue surrounding the
micropyle; _sa_, sarcotesta; _v_, vascular bundles; _p_, plinth. B′,
section of lagenostome and part of the integument; _f_, _f′_, flanges;
_l′_, wall of micropyle-funnel. E, F, G, _Gnetopsis elliptica_;
_t_, tent-pole. F, section of the plumes. G, section through the
lagenostome. H, _Gnetopsis_ from Barnsley, Yorkshire, slightly
enlarged. I, _Physostoma elegans_; _h_, hairs. K, L, _Mitrospermum
compressum_; _cf_, inner flesh; _n_, nucellus and megaspore. L,
transverse section. M, microspores and (?) male gametes of _Physostoma
elegans_. N, microspore of _Physostoma_ showing part of exine and the
internal cells. (A–I, N, M, after Oliver; H, Kidston Coll. No. 1255; K,
L, after Mrs Arber.)]
The species _Conostoma anglo-germanicum_ agrees closely with _C.
oblongum_ in general form and organisation, but it has eight ribs,
four more prominent than the others, and differs also in other
minor characters from the rather shorter seeds of the type-species.
_Conostoma_ differs from _Lagenostoma_ in the absence of the tubular
prolongation of the lagenostome, the micropyle being like that in
recent Gymnosperms. In _Conostoma_ the tracheid-like elements of the
lateral wall of the lagenostome are a characteristic feature, and no
evidence has been found of the existence of a central core of tissue
such as occupies the centre of the seed-apex in _Lagenostoma_. The
long hairs of _Physostoma_ are represented in _Conostoma_ by the much
smaller mucilaginous cells of the epidermis and in _Lagenostoma_ by
the less closely united mucilage-cells of the superficial layer of the
testa.
=Sphaerostoma.= Benson.
As already pointed out in Chapter +xxix.+ where this genus is described
as probably the seed of _Heterangium_, there is a fairly close general
resemblance between _Sphaerostoma_ and _Lagenostoma_. In the presence
of free apical lobes the former genus resembles _Conostoma_, and
while agreeing with _Lagenostoma_ in its annular pollen-chamber it
is peculiar in the retention of an epidermis over the roof of the
pollen-chamber: as in _Lagenostoma_ the seed is enclosed by an outer
integument or cupule.
=Lagenostoma.= Williamson.
An account of this type of seed is included in the description of
_Lyginopteris_[837]. The more striking peculiarities are exhibited by
the pollen-chamber and the free region of the integument: the annular
pollen-chamber (fig. 493, D, _c_; fig. 409) surrounds a central
nucellar cone and is prolonged upwards as a tube engaging with the
micropyle in contrast to the form of the pollen-chamber and the absence
of a tubular prolongation in _Conostoma_. The tentacles of _Physostoma_
and the short apical lobes of _Conostoma_ are replaced by an apical
cone formed by the coalescence of the integument containing nine
cavities originally filled with parenchyma (figs. 409; 493, B). The
presence of a cupule is a characteristic feature of young seeds, but
from negative evidence in the case of most other seeds it is unsafe
to assume that the cupule of _Lagenostoma_ is an exceptional feature.
The nucellus and testa are united as far as the shoulders of the seed
as in the seeds of recent Cycads and in contrast to their lateral
independence in _Trigonocarpus_, _Stephanospermum_, and other genera.
=Gnetopsis.= Renault.
This generic name was given by Renault[838] to some small petrified
seeds from the Stephanian of Grand’ Croix and to impressions from the
Commentry coal-field which he believed to belong to some Gnetaceous
plant. Saporta and Marion[839] and other authors have accepted
these seeds as evidence of the existence of Palaeozoic Gnetales: it
has, however, been shown[840] that _Gnetopsis_ has no claim to such
relationship and is a type of seed closely allied to _Conostoma_.
Renault described three species, afterwards adding three from another
locality[841]; the genus is recorded also from Commentry[842] and
Gard[843]. More recently Depape and Carpentier[844] have described
examples from the Westphalian of Valenciennes which they place in
the Pteridosperms in accordance with the conclusion of Oliver and
Salisbury. _Gnetopsis_ has also been discovered by Mr Hemingway in the
Middle Coal Measures of England[845] (fig. 494, H).
_Gnetopsis elliptica_ Renault.
The seeds of this species, slightly oval in section, occur in groups of
2–4 in a cupular investment (fig. 506, E, p. 359) described by Renault
as an ovary but correlated by Oliver and Salisbury with the cupule
which surrounds the single ovule of _Lagenostoma_. The cupule is lined
with hairs similar to those on the wall of the cupule of _Lagenostoma_.
A characteristic feature of the French seeds is the presence of three
or four long plumes of hairs at the apex (fig. 494, E, F). As seen in
fig. 494, E, a small lagenostome (pollen-chamber) rests on the roof of
a broad plinth precisely as in _Conostoma_, and four vascular bundles,
corresponding to the six bundles in _Conostoma_, pass into the apical
cap of sclerous tissue enclosed by a sarcotesta, _sa_ (fig. 494, E,
G). A ‘tent-pole’ prolongation (fig. 494, E, _t_) occurs at the apex
of the prothallus. Renault described a portion of the integument as
consisting of lacunar tissue which Oliver and Salisbury homologise with
the superficial mucilaginous layer of _Conostoma_: this is seen above
the sclerotesta in the apical region of fig. 494, E, _sa_.
_Gnetopsis anglica_ Kidston +MS.+
This species (fig. 494, H) is represented by seeds from the Middle Coal
Measures near Barnsley, Yorkshire, 4 mm. long with apical appendages
at least 2·2 cm. in length and probably four in number. The appendages
do not show the hairs which form a prominent feature in the French
specimens, but this is probably the result of imperfect preservation:
there are indications of hairs on other specimens in Dr Kidston’s
Collection. The type-specimen, in Dr Kidston’s Collection, was
generously lent to me for examination.
_Gnetopsis_, while agreeing with _Conostoma_ in the more important
features, is distinguished by the apical plumes, the very slight
development of a tent-pole prolongation of the nucellar apex (fig. 494,
E, _t_), the smaller number of vascular bundles, and by the presence of
an enclosing cupule (fig. 506, E). In its slight departure from radial
symmetry _Gnetopsis_ forms a transition between the Radiospermeae and
the Platyspermeae. It is undoubtedly the seed of a Pteridosperm, but
nothing is known as to the nature of the vegetative organs of the
parent-plant.
II. +Trigonocarpales.+
In this group are included radially symmetrical seeds for the most
part belonging to members of the Medulloseae. The peripheral zone of
the nucellus is supplied with vascular tissue and the nucellus is
free within the integument except at the base; it is superior and not
semi-inferior[846] as in recent Cycads and in Lagenostomales. The ovule
of _Myrica Gale_, in which the nucellus stands free within the single
integument, affords an interesting parallel to seeds of this class in
contrast to the usual Angiospermous type with a laterally coalescent
nucellus. In _Myrica Gale_[847] the vascular supply is confined to
the integument. There is a comparatively broad pollen-chamber and in
some types the lateral tissue of the nucellus is prolonged as a tube
within the micropyle. The usually ribbed testa is differentiated into
an outer flesh (sarcotesta), a sclerotesta, and probably in most cases
an endotesta or inner flesh. The ribs of the sclerotesta are in the
majority of genera in multiples of three and in position correspond to
the outer ring of vascular bundles. The presence of lacunar tissue in
the sarcotesta of several genera may be correlated with dispersal by
water. The apical region of the integument is not lobed but extends as
a longer or shorter micropylar tube above the summit of the nucellus.
In the differentiation of the testa, the form of the pollen-chamber,
and in some other features, the seeds of this group present a general
agreement with those of recent Cycads.
The microspores are multicellular and larger than those of the
Lagenostomales: in _Stephanospermum akenioides_ they measure 160μ ×
100μ[848] while in _Aetheotesta_[849] they reach 360μ × 290μ.
Genera. _Trigonocarpus_; _Tripterospermum_; _Ptychotesta_;
_Hexapterospermum_; _Polypterospermum_; _Pachytesta_;
_Stephanospermum_; _Polylophospermum_; _Codonospermum_; _Aetheotesta_;
_Eriotesta_; _Gaudrya_.
=Trigonocarpus.= Brongniart.
A description of the morphological features of _Trigonocarpus
Parkinsoni_ and _T. shorensis_ is given in the chapter on _Medullosa_
(p. 117), as there is good evidence that they are the seeds of that
genus. There is considerable difference in size and to some extent
in the form of casts included in _Trigonocarpus_ and, in the absence
of anatomical data, it is hardly possible to determine the actual
systematic position of many of the specimens so named. Dr Arber[850]
has recently proposed a new generic name _Schizospermum_ for casts
very like those of _T. Parkinsoni_, but distinguished by the splitting
of the shell into three valves, a character which leads him to
conclude that it is the external surface which is preserved and not
a mere cast of the seed-cavity. It is, however, more probable that
the specimens are casts of a split sclerotesta. In _Trigonocarpus
pusillus_[851] the shell is divided into three valves, the dividing
lines being marked by greatly reduced ribs, and in _T. schizocarpoides_
Grand’Eury[852], a species that may not be a true _Trigonocarpus_,
there is also evidence of splitting. Arber points out that the species
_Rhabdocarpus Boschianus_ Berg. is founded on a _Trigonocarpus_
from which the outer flesh has disappeared leaving the shell as the
external covering. _Trigonocarpus_ seeds are widely distributed in
Carboniferous and Permian rocks in Europe and North America: from the
latter continent Newberry[853] has described several different forms
that afford good examples of the abundance and variety of the genus.
Some of the specimens included by Newberry[854] in _Trigonocarpus_
are probably distinct generic types: his species _T. multicarinatus_
may be identical with the ribbed cast shown in fig. 506, A. The
casts described by Lindley and Hutton and by other authors as _T.
Dawesi_[855] are nearly 5 cm. long, and if these are correctly included
in the genus they point to the occurrence of seeds much larger than
the type-species. The French species _Trigonocarpus pusillus_[856]
Brongn., one of the smallest Palaeozoic seeds, from 6·5 to 15 mm. long,
differs from _Trigonocarpus Parkinsoni_ and _T. shorensis_ in the
absence of prominent ribs and in the much feebler development of the
sarcotesta. Specimens of the German type _T. sporites_ Weiss, believed
by some authors to be megaspores, were described by Zeiller[857] from
Valenciennes as seeds: these are from 2·5 to 3·5 mm. long and have
three small ribs. Zeiller quotes the presence of cell-outlines on the
surface as evidence of their seed-nature, but it may be that this
feature represents a sculpturing of the exine of a spore. Typical
_Trigonocarpus_ seeds agree in several morphological characters with
those of recent Cycads. They differ in the lack of a lateral union
between nucellus and integument; the presence of nucellar tracheids,
though a feature shared with _Bowenia_, distinguishes them from the
majority of recent Cycadean seeds. In the comparatively long and fleshy
micropylar tube a seed of _Encephalartos Lehmanni_ presents a fairly
close resemblance to a _Trigonocarpus_. Salisbury has pointed out that
the three species _T. Parkinsoni_, _T. shorensis_, _T. pusillus_ form
a consecutive series illustrating the gradual disappearance of the
secondary ribs that form a prominent feature in _T. Parkinsoni_; but
for a comparison of these with other types of fossil and recent seeds
the reader is referred to Salisbury’s summary[858].
=Tripterospermum.= Brongniart.
The seed on which this genus was founded by Brongniart[859] is clearly
very closely related to _Trigonocarpus_ and, as Oliver[860] says,
the distinguishing character described by the author of the genus is
unimportant. Brongniart describes the type species, _T. rostratum_,
as characterised by the presence of three prominent wings composed of
a testa differentiated into an inner hard tissue and an outer lacunar
tissue. It is, however, hardly possible to say whether the outer soft
tissue was originally flattened in the form of ‘wings’ or pressed down
on to the harder shell. Renault[861] notes the association of seeds
that he refers to this genus with the leaves of _Dorycordaites_, but
apart from the improbability of any connexion between _Tripterospermum_
and _Cordaites_, Renault’s seeds are too imperfect to demonstrate
their identity with Brongniart’s genus. Kidston[862] has described an
impression of a three-winged seed from the coal-field of Staffordshire
as _Tripterospermum ellipticum_, a form described on page 357 as
_Polypterocarpus anglicus_ (fig. 496, B).
=Ptychotesta.= Brongniart.
The type-species of this genus[863], _Ptychotesta tenuis_[864], about
3 cm. long, is characterised by six very prominent flanges or wings
formed by the fissured or folded sclerotesta (fig. 495, B). There is no
information as to the vascular supply or other anatomical details. It
is not at all improbable that there is no real distinction between this
genus and Brongniart’s genus _Hexapterospermum_.
=Hexapterospermum.= Brongniart.
In this genus Brongniart[865] included two species, _Hexapterospermum
stenopterum_ and _H. pachypterum_: the shell is hexagonal in
transverse section, each angle being prolonged as a narrow flange.
In one of the sections figured by Brongniart (fig. 495, E) the
ribs are not fissured: this is said to be a feature distinguishing
_Hexapterospermum_ from _Ptychotesta_, but the occurrence of a
fissured rib in another section suggests that in the structure of
the ribs there is no essential difference between the two genera.
In _Ptychotesta pachypterum_ the testa is prolonged at the chalazal
end as in _Polylophospermum_, and it is possible that there is
no generic difference. Williamson described a cast from the Coal
Measures of Lancashire as _Hexapterospermum_ [= _Hexagonocarpus_]
_Noegerrathi_[866] (fig. 506, H), but in the absence of anatomical
characters it is preferable to avoid the use of Brongniart’s term and
to assign them to Renault’s genus _Hexagonocarpus_[867]. Similarly
the seeds referred by Dr P. Bertrand[868] to _Hexapterospermum_ may
appropriately be included in the genus _Hexagonocarpus_.
[Illustration: Fig. 495. A, _Polypterospermum Renaultii_. B,
_Ptychotesta tenuis_. C, D, _Polylophospermum_ _stephanense_; _sa_,
sarcotesta; _s_, spaces; _v_, vascular bundles. E, _Hexapterospermum
stenopterum_. F, _Diplotesta avellana_. G, _Taxospermum_ _Grüneri_. H,
_Diplotesta Grand’Euryana_. (After Brongniart; C, after Oliver.)]
=Polypterospermum.= Brongniart.
This generic name was proposed for an ovoid seed from St Étienne
described as _Polypterospermum Renaultii_[869]; hexagonal in section
with six deep and sharp flanges at the angles alternating with six
secondary ridges distinguished by their blunter edges and slightly
smaller depth (fig. 495, A). Without further anatomical details it
is hardly possible to say whether or not the species represents a
well-defined generic type, but it is not improbable that a fuller
knowledge would confirm Brongniart’s institution of a new generic
designation. The seeds described by Arber[870] and Kidston[871]
respectively as _Radiospermum ornatum_ and _Polypterospermum ornatum_
are transferred to _Polygonocarpus_ on the ground that they afford no
evidence of anatomical characters of the _Polypterospermum_ type.
=Pachytesta.= Brongniart.
Brongniart[872] established this genus for some unusually large seeds
(fig. 496, A) from the Coal Measures of France reaching a length of
11–12 cm. and resembling in form and size a hen’s egg. Petrified
examples have been described by Brongniart and Renault[873], and
Oliver[874] has given a lucid statement of the more interesting
features of this Permo-Carboniferous genus.
_Pachytesta gigantea_ (Grand’Eury).
In the separation of nucellus and integument _Pachytesta_ agrees with
_Stephanospermum_ and _Trigonocarpus_. There is a double series of
vascular bundles in the outer part of the testa or exotesta (fig. 497,
_ex_) consisting of a spongy tissue bounded externally and internally
by sclerotic layers: the exotesta is succeeded by a broader endotesta
of spongy consistency which Renault suggested may have served as
a floating mechanism, and this is intimately associated with the
nucellus by means of grooves engaging with short ridges (fig. 497,
_nr_) on the surface of the nucellar tissue. The exotesta is divided
into three valves by radial extensions of the sclerotic tissue (_b_,
_c_, fig. 497; at _a_ the exotesta is shown in an unsplit condition).
In connexion with each radial plate are two curved plates of fibrous
tissue which extend towards the grooves in the endotesta (fig. 497,
_t_): as Oliver suggests, these plates may have served a mechanical
purpose for the support of the bulky nucellus. The peripheral region
of the nucellus is supplied by several vascular bundles (fig. 497,
_n_) from the tracheal chalazal disc. The exotesta is regarded by
Oliver as corresponding to the sarcotesta and sclerotesta of other
seeds, and the grooves in the endotesta interlocking with the nucellar
ridges are compared with the interlocking of nucellus and canopy in
_Lagenostoma_, as also with the ruminated endosperm of _Torreya_.
[Illustration: Fig. 496. A, _Pachytesta incrassata_. B,
_Polypterocarpus anglicus_. (A, after Renault; B, after Kidston.)]
[Illustration: Fig. 497. _Pachytesta_. Transverse section of a seed
showing the exotesta, _ex_, with sclerous layers and vascular strands
(black), the endotesta (dotted), the nucellus with its ring of vascular
bundles, _n_, and the prothallus, _p_. The three radial plates in the
exotesta are seen at _a_, _b_, _c_, with the trabeculae, _t_, and
internal to these are the grooves in the endotesta engaging with ribs
on the nucellus, _nr_. (After Oliver.)]
We have no definite information as to the plants which bore
_Pachytesta_, but it is probable that they were members of the
Medulloseae. Grand’Eury[875] believes _Pachytesta_ to be the seed of
_Alethopteris Grandini_, though this view requires confirmation. This
author figures several _Pachytesta_ seeds attached in two oblique rows
to a comparatively slender axis which may be the rachis of a large
compound frond[876]. Renault and Zeiller[877] have figured specimens
of _P. gigantea_ and _P. incrassata_ from Commentry which afford a
good idea of the form of these large seeds; the genus is recorded also
from Gard, Blanzy[878], and other localities. Kidston[879] described a
large oval seed, blunt at each end, from the Middle Coal Measures of
Lancashire as _Carpolithus Wildii_ which he thinks may be allied to
_Pachytesta_, but adds, ‘I do not think we are justified in placing
mere impressions of plants in genera, whose distinctive characters are
derived from their internal organisation, unless there is conclusive
evidence to show their identity.’ On this specimen Arber[880] has
founded a new genus _Megalospermum_, but as the type-specimen shows
no distinctive features other than large dimensions it would seem
preferable to retain the more general designation _Carpolithus_[881].
Another example of a seed that may be generically identical with
_Pachytesta_ is that described by Lesquereux from North American Coal
Measures as _Rhabdocarpus Mansfieldi_ and more recently recorded by
White from Missouri as _Rhabdocarpus (Pachytesta) Mansfieldi_[882].
There is little doubt that this and other species of seeds preserved
as impressions are examples of _Pachytesta_ but, as Kidston says, it
is preferable to reserve the name for specimens showing anatomical
features.
=Stephanospermum.= Brongniart.
The genus _Stephanospermum_, founded by Brongniart[883] on petrified
specimens from French Stephanian beds, affords a good example of
a radiospermic seed without ribs differing in certain well-marked
characters from such seeds as _Lagenostoma_, _Physostoma_, and
_Conostoma_, notably in the complete separation of the nucellus from
the integument and in the possession of a nucellar vascular system. The
descriptions by Brongniart and Renault[884] have been extended by the
thorough investigations of Oliver[885].
_Stephanospermum akenioides_ Brongniart.
This species is represented by ellipsoidal seeds, 10 mm. long with
a maximum breadth of 4–4·5 mm., circular in transverse section.
The integument consists of a thick sclerotesta lined internally by
a thinner soft layer, the endotesta (fig. 494, A, _sc_, _e_), and
in all probability covered by an outer flesh or sarcotesta as in
_Trigonocarpus_, though this tissue is not preserved and is omitted
from the section shown in fig. 494, A. The sclerotesta is composed
of a broad outer band of thick-walled palisade cells—the contracted
contents of which are a striking feature in the silicified seeds (fig.
493, F); these are succeeded by an inner zone of longitudinal fibres.
In the structure of the palisade-layer _Stephanospermum_ bears a close
resemblance to the sporocarp wall of _Pilularia_[886]. In the apical
region the shell forms a circular ridge surrounding a peri-micropylar
trough, a character expressed by the term ‘crown-seed[887]’ employed
by Grand’Eury: the trough sometimes contains partially destroyed
tissue that may be a remnant of a sarcotesta. A _Stephanospermum_
seed, with its apical core and surrounding depression with remnants
of some partially destroyed tissue, may be compared with a fruit of
_Attolia speciosa_ (Palmae) in which a tuft of fibrous tissue picked
out by decay from the mesocarp surmounts the conically pointed apex
of the harder interior of the fruit-wall. The base of the sclerotesta
is perforated by a vascular strand which expands into a tracheal
disc, _td_, fig. 494, A, at the base of the megaspore from which a
continuous mantle 2–3 cells broad, of short spiral and scalariform
tracheids (fig. 493, G), spreads over the whole of the nucellus
immediately below the nucellar epidermis as far as the lower part of
the sides of the pollen-chamber: this mantle forms the floor of the
large domical pollen-chamber excavated out of the nucellar cone (fig.
494, A, _pc_). The fact that in sections of older seeds the tracheal
floor shows signs of splitting and disorganisation led Oliver to
conclude that in the living seed the tracheids underwent a gradual
disintegration prior to fertilisation, thus allowing the passage of
the antherozoids to the egg-cells (fig. 494, A, _a_). The presence of
a continuous tracheal sheath instead of separate vascular bundles is a
special feature in which _Stephanospermum_ differs from _Trigonocarpus_
and other Palaeozoic seeds as well as from those of recent Cycads;
as Oliver suggests, ‘the apparent perfection of the vascular
mantle in _Stephanospermum_ may have proved an obstacle to further
development[888]’ and was not retained by the more successful types. In
its tracheal sheath _Stephanospermum_ resembles the seeds of _Ginkgo_.
The nucellar cone is prolonged as a beak into the micropyle formed by
the tubular integument. The megaspore occupies the central portion
of the seed and in the course of its development it compressed the
megasporangium (nucellus) to such an extent that little more than the
epidermis remains: there is a definite megaspore-membrane surrounding
the prothallus-tissue in which there were probably two archegonia (fig.
494, A, _a_)[889]. The nucellus stands free within the integument from
which it is separated by a space (_s_, fig. 494, A). Microspores are
frequently met with in the pollen-chamber in which they matured after
their introduction through the micropyle, probably with the aid of a
drop of mucilage: they are large oval bodies with an average size of
160μ × 100μ, some reaching over 200μ, in length and, as Renault was
the first to point out, within a finely granulate exospore there are
several thin-walled cells: this statement was not accepted by some
authors but Prof. Oliver’s researches have amply confirmed it, and an
examination of the original preparations convinced me that Renault had
correctly described the structural features. Oliver shows that there
are about 20 cells within each microspore regularly arranged as rows
of five wedge-shaped elements with their pointed ends directed towards
the centre, and he thinks that these cells may have undergone further
division to produce sperm-mother-cells, though there is not such good
evidence of this as in the differently constituted microspores of
_Stephanospermum caryoides_. No trace of pollen-tubes was found and it
is probable that the antherozoids were liberated by the rupture of the
delicate prothallus-tissue. The tracheal sheath afforded an adequate
means of water-supply to the pollen-chamber and this enabled the motile
antherozoids to reach the archegonia.
_Stephanospermum caryoides_ Oliver[890].
This species, founded on a specimen from ., is a larger seed (15·5 ×
12·5 mm.) and more globular than _S. akenioides_; there is a shorter
micropylar beak and a less developed peri-micropylar trough. As in _S.
akenioides_ the nucellus is free from the base and the two species
conform to the same general type of construction. The microspores are
distinguished by the presence of only two internal cells which do not
occupy the whole of the spore-cavity but are surrounded by a large
peripheral cell comparable with the tube-cell in recent microspores,
though there is no proof that a tube was formed: in the case of _S.
akenioides_ Oliver considers that fertilisation was not assisted by
the production of a pollen-tube. The two cells by further division
gave rise to a secondary cell-complex consisting of at least eight
antherozoid-mother-cells. No antherozoids have been found in the
microspores but it is possible that two small bodies, 17μ × 15μ, met
with in a pollen-chamber may represent the nuclei of sperms. Their
small size differentiates them from the much larger male gametes of
Cycads and from the supposed sperms of _Physostoma_ and _Lagenostoma_.
We have no proof as to the nature of the plant which bore seeds of the
_Stephanospermum_ type, but it is significant that the specimens occur
in close association with fragments of _Alethopteris_ and _Myeloxylon_,
a circumstance that favours the view, based on the resemblance of these
seeds to _Trigonocarpus_, that _Stephanospermum_ is the seed of a
member of the Medulloseae.
=Polylophospermum[891].= Brongniart.
The type-species _Polylophospermum stephanense_ Brongn.[892],
founded on partially preserved material from Grand’ Croix, is a
narrow hexagonal seed 15 mm. long. Additional facts as to the
structural features have been contributed by Oliver[893]. The testa
is differentiated into an inner shell (sclerotesta) and an outer
flesh (_sa_, fig. 495, C, D): the sclerotesta has six prominent,
fissured, ribs, one at each angle, and between these are six solid and
less prominent secondary ribs. Oliver states that there is an outer
series of vascular bundles in the sarcotesta, one bundle immediately
external to each secondary rib (fig. 495, C, _v_). In the presence
of two kinds of ribs and in the relation of ribs to tracheal strands
_Polylophospermum_ agrees with _Trigonocarpus Parkinsoni_. Strands of
short tracheids supply the peripheral region of the nucellus and, as in
_Stephanospermum_, reticulate elements extend as far as the floor of
the large pollen-chamber. There was probably no lateral union between
nucellus and integument. A striking feature is the prolongation of the
testa at each end of the seed to form an open chamber surrounding the
micropylar beak and the seed-base (fig. 495, D, _s_, _s_): the apical
chamber, though relatively more prominent, is comparable with that in
_Stephanospermum_, while the basal chamber recalls that in the seed
named by Scott and Maslen _Trigonocarpus Oliveri_[894] but subsequently
removed by Salisbury[895] from that genus. There is no evidence as to
the parent-plant of _Polylophospermum_, but it may be assumed to have
been a Pteridosperm, probably one of the Medulloseae.
[Illustration: Fig. 498. A, B, _Codonospermum_. A, _C. olivaeforme_,
longitudinal section showing the seed-proper and the air-chamber, _s_;
_pc_, pollen-chamber. B, Diagrammatic sketch of a _Codonospermum_
showing the vascular supply, _v_, and the fibrous arcs, _f_, in the
wall of the lower part of the seed. C, _Aetheotesta elliptica_, _s_,
sarcotesta with lacunae; _v_, vascular supply. (After Renault.)]
=Codonospermum[896].= Brongniart.
Our knowledge of this peculiar genus is derived from Brongniart’s
description of the type-species _Codonospermum anomalum_[897]
(fig. 506, B, C) from St Étienne and from Renault’s account of _C.
olivaeforme_[898]. The seeds are globular or ellipsoidal and reach a
length of 2 cm.: the testa is differentiated into an outer flesh and a
sclerotesta and has usually eight ribs. The most striking feature is
the division of the seed into two regions, an upper portion containing
the nucellus and megaspore and a lower portion in the form of an
empty chamber that probably served as a float (fig. 498, A, _s_).
Externally the upper half or seed-proper forms a depressed cupola with
eight slightly developed ribs, separated by a circular transverse
constriction from the basal chamber (fig. 498, B). There is a fairly
large pollen-chamber, _pc_, in which Renault found multicellular
microspores in _C. olivaeforme_. In _C. anomalum_ 16 vascular bundles
(fig. 498, B, _v_) surround the central region of the seed probably
in the peripheral tissue of the nucellus; these bundles unite in the
chalazal region to form a strand that passes up the centre of the empty
chamber. In _C. anomalum_ the testa of the lower half has eight ribs
and corresponding with each rib is a strand of fibrous tissue (fig.
498, B, _f_). In _C. olivaeforme_ the testa is thicker than in _C.
anomalum_ and the lower region of the seed is smooth and circular in
section.
Impressions of _Codonospermum_ are described[899] from Commentry, from
the Loire—the Gard district[900], and by Zeiller[901] from Blanzy.
There is no decisive evidence as to the parent-plant, but some French
authors[902] regard the frequent association of _Codonospermum_ with
leaves of _Dolerophyllum_ as significant.
=Aetheotesta.= Brongniart.
_Aetheotesta elliptica_ Renault.
Brongniart instituted the genus _Aetheotesta_[903] for an incomplete
seed (_A. subglobosa_) from Grand’ Croix, and Renault[904] subsequently
founded the species _A. elliptica_ on much better material. The testa
of the elliptical seed, 3 cm. long, consists of an outer region
very thin on the flanks but highly developed at the apex and base
which forms a sarcotesta characterised in the apical region by large
radially disposed spaces, fig. 498, C, _s_; the sclerotesta, composed
of harder tissue, is prolonged as an apical beak. There is a large
pollen-chamber, _pc_, in which Renault found multicellular microspores
(320μ–400μ). In the chalazal region the main vascular supply forms
a cup-like investment, _v_, in the basal portion of the nucellus,
and at a higher level this breaks up into several nucellar strands.
Renault thinks that _Aetheotesta_ is the seed of _Dolerophyllum_, but
there is no proof of any connexion. The presence of large spaces in
the sarcotesta may be interpreted, as Renault suggests, as evidence of
adaptation to dispersal by water.
=Eriotesta.= Brongniart.
Brongniart[905] instituted this genus[906] on an incomplete transverse
section of a seed from Grand’ Croix, 8 mm. in diameter, which he called
_Eriotesta velutina_, characterised by a ribbed and probably octagonal
testa bearing numerous elongated hairs over the whole surface. The
material is, however, too meagre to throw any light on the important
features of the seed.
=Gaudrya.= Grand’Eury.
_Gaudrya trivalvis_ Grand’Eury.
The generic name _Gaudrya_[907] was proposed for two petrified seeds
from the Gard coal-field briefly described as _G. trivalvis_, the
type-species, and _G. lagenaria_. The testa of the former species
shows signs of splitting along three equidistant lines; it consists
of a sarcotesta enclosing an endotesta with six longitudinal ribs:
Grand’Eury speaks of lacunae between the endotesta and nucellus which
he regards as spaces in a tissue which made the seed buoyant and
facilitated dispersal by water. It is not clear whether the nucellus
and integument were originally connected or laterally free: the lacunae
may be the remains of an inner flesh and not spaces in a spongy tissue.
The genus is radiospermic and characterised by a long micropyle and
a broad pollen-chamber. On the available evidence it is difficult to
assign the specimens to their systematic position, but the genus is
probably a member of the Trigonocarpales.
III. +Cardiocarpales.+