LYCOPODIALES.
The recent members of the Lycopodiales are considered apart from
the extinct genera in order that our examination of the latter may
be facilitated by a knowledge of the salient characteristics of the
surviving types of this important section of the Pteridophyta. A
general acquaintance with the extinct as well as with the recent genera
will enable us to appreciate the contrasts between the living and the
fossil forms and to realise the prominent position occupied by this
group in the Palaeozoic period, a position in striking contrast to
the part played by the diminutive survivors in the vegetation of the
present day. In the account of the recent genera special attention is
drawn to such features as afford a clue to the interpretation of the
fossils, and the point of view adopted, which at times may appear to
lead to an excessive attention to details, is necessarily somewhat
different from that represented in botanical text-books[83].
A. =HOMOSPOREAE.=
=Lycopodiaceae=: genera _Phylloglossum_, _Lycopodium_.
B. =HETEROSPOREAE.=
=Selaginellaceae=: genus _Selaginella_.
=Isoetaceae=: genus _Isoetes_.
The existing plants included in the Lycopodiales are in nearly all
cases perennial herbaceous pteridophytes, exhibiting in their
life-histories a well marked alternation of generations. The sporophyte
(asexual generation) is characterised by the relatively small size
of the leaves except in the genus _Isoetes_ (fig. 132) and in the
Australian and New Zealand genus _Phylloglossum_. The stems are usually
erect or trailing, pendulous in epiphytic species or small and tuberous
in _Isoetes_ and _Phylloglossum_. The repeated forking of the shoots
(monopodial and dichotomous branching) is a prominent feature of the
group. The vascular tissue of the stem usually assumes the form of a
single axial strand (stele) (fig. 125), but the shoots of some species
of _Selaginella_ often contain two or more distinct steles (fig. 131).
The group as a whole is characterised by the centripetal development of
the xylem composed almost entirely of scalariform tracheids: secondary
xylem and phloem of a peculiar type occur in _Isoetes_, and the
production of secondary xylem elements in a very slight degree has been
noticed in one species of _Selaginella_ (_S. spinosa_)[84]. The roots
are constructed on a simple plan, having in most cases only one strand
of spiral protoxylem elements (monarch structure). In _Lycopodium_,
in which stem and root anatomy are more nearly of the same type than
in the majority of plants, several protoxylem strands may be present.
The sporangia are axillary or, more frequently, borne on the upper
surface of sporophylls, which are either identical with or more or less
distinct from the foliage leaves; in the latter case the sporophylls
often occur in the form of a well defined strobilus (cone) at the tips
of branches.
The gametophyte (sexual generation) is represented by prothalli which,
in the homosporous genera, may live underground as saprophytes, or the
upper portion may develop chlorophyll and project above the surface of
the ground as an irregularly lobed green structure (_e.g._ _Lycopodium
cernuum_)[85]. In the heterosporous forms the prothalli are much
reduced and do not lead an independent existence outside the spore by
the membrane of which they are always more or less enclosed. The sexual
organs are represented by antheridia and archegonia; the male cells
are provided with two cilia except in _Isoetes_ which has multiciliate
antherozoids like those of the ferns.
The existing Lycopods, though widely distributed, never grow in
sufficiently dense masses to the exclusion of other plants to form a
conspicuous feature in the vegetation of a country. The inconspicuous
rôle which they play among the plant-associations of the present era
affords a striking contrast to the abundance of the arborescent species
in the Palaeozoic forests of the northern hemisphere.
• • • • •
=Lycopodiaceae.= _Lycopodium_, represented by nearly 100 species,
forms a constituent of most floras: epiphytic species predominate in
tropical regions, while others flourish on the mountains and moorlands
of Britain and in other extra-tropical countries. For the most part
_Lycopodium_ exhibits a preference for a moist climate and appears to
be well adapted to habitats where the amount of sunlight is relatively
small and the conditions of life unfavourable for dense vegetation.
Mountains and islands constantly recur as situations from which species
have been recorded. Some species are essentially swamp-plants, _e.g._
_Lycopodium inundatum_, a British species, and _L. cruentum_ from
the marshes of Sierra Nevada. A variety of the American species, _L.
alopecuroides_ (var. _aquaticum_) affords an instance of a submerged
form, which has been collected from an altitude of 12–14,000 ft. on the
Andes and Himalayas. It is noteworthy that a considerable variety of
habitats is represented within the limits of the genus and that many
species are sufficiently hardy to exist in circumstances which would be
intolerable to the majority of flowering plants[86].
The British species frequently spoken of as Club Mosses, include
_Lycopodium Selago_, _L. annotinum_, _L. clavatum_, _L. alpinum_, and
_L. inundatum_.
• • • • •
=Selaginellaceae.= The species of _Selaginella_, over 300 in number,
are widely spread in tropical and subtropical forests, growing on the
ground with trailing, suberect or erect stems climbing over taller and
stouter plants or as pendulous epiphytes on forest trees.
_Selaginella lepidophylla_, a tropical American type, popularly known
as the Resurrection plant, and often erroneously spoken of as the Rose
of Jericho[87], possesses the power of rolling up its shoots during
periods of drought and furnishes an example of a species adapted to
conditions in marked contrast to those which are most favourable to the
majority of species.
The only British species is _Selaginella spinosa_ named by Linnaeus
_Lycopodium selaginoides_ and occasionally referred to as _Selaginella
spinulosa_ A. Br. (not to be confounded with a Javan species _S.
spinulosa_ Spring[88]).
• • • • •
=Isoetaceae.= _Isoetes_ (fig. 132), of which Mr Baker in his _Handbook
of the Fern-Allies_ enumerates 49 species, is a type apart, differing
in habit as in certain other characters from the other members of the
Lycopodiales. Some botanists[89] prefer to include the genus among the
Filicales, but the balance of evidence, including resemblances between
_Isoetes_ and extinct Lycopodiaceous plants, would seem to favour its
retention as an aberrant genus of the group Lycopodiales. Some species
are permanently submerged, others occur in situations intermittently
covered with water, and a few grow in damp soil. _Isoetes lacustris_
is found in mountain tarns and lakes of Britain and elsewhere in
Central and Northern Europe and North America. _Isoetes hystrix_[90], a
land-form occurs in Guernsey, North-East France, Spain and Asia Minor.
Lycopodiaceae.
The monotypic genus _Phylloglossum_, represented by _P. Drummondii_ of
Australia and New Zealand, though interesting from the point of view of
its probable claim to be considered the most primitive type of existing
Lycopodiaceous plants, need not be dealt with in detail. A complete
individual, which does not exceed 4 or 5 cm. in length, consists of a
very small tubercle or protocorm bearing a rosette of slender subulate
leaves and prolonged distally as a simple naked axis which overtops the
foliage leaves and terminates in a compact cluster of small scale-like
sporophylls, each subtending a single sporangium[91].
_Lycopodium._ It would be out of place in a volume devoted mainly
to fossil plants to attempt a comprehensive account of the general
morphology of recent species, and indeed our knowledge of the
anatomical characters of the genus is still somewhat meagre. For
purposes of comparison with extinct types, it is essential that some of
the more important morphological features of existing species should
be briefly considered. The additions made to our knowledge of the
gameophyte[92] of European and tropical species during the last two
decades have revealed a striking diversity in habit.
In several species, grouped round the widely distributed type
_Lycopodium Selago_ Linn., the comparatively short, erect or suberect,
shoots form fairly compact tufts; the ordinary foliage-leaves function
as sporophylls, and the sporangia are not localised on special portions
of shoots. From this type, we pass to others in which the fertile
leaves tend to be confined to the tips of branches, but hardly differ
in form from the sterile. A further degree of specialisation is
exhibited by species with well-defined cones composed of leaves (or
bracts), the primary function of which is to bear sporangia and to
afford a protective covering to the strobilus[93].
_Lycopodium rufescens_ Hook. An Andian species with stout dichotomously
branched erect stems bears on the younger shoots crowded leaves with
their thick and broadly triangular laminae pointing upwards, but on the
older and thick shoots the laminae are strongly reflexed (fig. 121,
A). The lower part of the specimen represented in fig. 121, A, shows
tangentially elongated scars and persistent leaf-bases or cushions left
on the stem after the removal of the free portions of the leathery
leaves, a surface-feature which also characterises the Palaeozoic genus
_Lepidodendron_. The reflexed leaves and persistent leaf-cushions are
clearly seen in the piece of old stem of _Lycopodium dichotomum_ Jacq.,
a tropical American species reproduced in fig. 121, B. Such species
as _L. erythraeum_ Spring, and others with stiff lanceolate leaves
exhibit a striking resemblance to the more slender shoots of some
recent conifers, more especially _Araucaria excelsa_, _A. Balansae_,
_Cryptomeria_, _Dacrydium_ and other genera.
[Illustration: FIG. 121. _Lycopodium._
A. _Lycopodium rufescens._
B. _L. dichotomum._
C. _L. tetragonum._
D. _L. nummularifolium._
E. _L. Dalhousianum._
F. _L. casuarinoides._
G. _L. volubile._
(From specimens in the Cambridge Herbarium and Botanic Garden. M.S.)]
[Illustration: FIG. 122. _Lycopodium squarrosum._ The branches of the
larger shoot terminate in cones. (From a plant in the Cambridge
Botanic Garden. Reduced.)]
In _Lycopodium tetragonum_ Hook., (fig. 121, C), a species from the
Alpine region of the Andes, the long, pendulous and repeatedly forked
branches bear four rows of fleshy ovate leaves and simulate the
vegetative characters of certain conifers.
[Sidenote: LYCOPODIUM]
[Illustration: FIG. 123. _Lycopodium cernuum._
(From a specimen in the Cambridge Herbarium. ½ nat. size.)]
_L. squarrosum_ Forst. (fig. 122) a tropical species from India,
Polynesia, and other regions, is characterised by its stout stems
reaching a diameter of 2·5 cm., bearing long pendulous branches with
large terminal cones composed of sporophylls differing but slightly
from the foliage leaves. The plant represented in the photograph serves
as a good illustration of the practical identity in habit between
Palaeozoic and recent genera.
[Illustration: FIG. 124. _Lycopodium obscurum._]
_L. Dalhousianum_ Spring, from the mountains of the Malay Peninsula
and Borneo, has larger leaves of finer texture with a distinct midrib
reaching a length of 2–3 cm. (fig. 121, E). Another type is illustrated
by _L. nummularifolium_ Blume, also a Malayan species, in which the
leaves are shorter, broadly oblong or suborbicular, and the branches
terminate in narrow and often very long strobili (sometimes reaching
a length of 30 cm.) with small bracts in striking contrast to the
foliage leaves (fig. 121, D). A similar form of long and slender
strobilus occurs in _L. Phlegmaria_ Linn., a common tropical Lycopod:
the frequent forking of the strobili noticed in this and other species
is a character not unknown among fossil cones (_Lepidostrobi_).
_L. cernuum_ Linn. (fig. 123), another widely spread tropical type,
offers an even closer resemblance than _L. squarrosum_ to the fossil
Lepidodendra. The stiff erect stem, reaching in some cases a length
of several feet, bears numerous repeatedly forked branches, with
crowded linear leaves, terminating in short cylindrical cones with
broadly ovate sporophylls. A similar habit characterises the North
American species _L. obscurum_ Linn. (fig. 124) bearing cones several
centimetres in length.
_L. casuarinoides_ Spring (fig. 121, F) an eastern tropical species,
is worthy of notice as exhibiting a peculiar form of leaf consisting
of a very small lamina, 3 mm. in length, borne on the top of a long
decurrent base, which forms a narrow type of leaf-cushion, bearing
some resemblance to the long and rib-like cushions of certain species
of _Sigillaria_, and recalling the habit of slender fossil twigs
referred to the Coniferae under such names as _Widdringtonites_,
_Cyparissidium_, _Sphenolepidium_.
_L. volubile_ Forst. (fig. 121, G) a New Zealand species, in habit
and leaf-form bears a close resemblance to the Jurassic _Lycopodites
falcatus_ Lind. and Hutt. (fig. 137): it is also a representative of a
few species of _Lycopodium_ which agree with the majority of species of
_Selaginella_ in having two kinds of sterile leaves, comparatively long
falcate leaves forming two lateral rows and smaller appressed leaves on
the upper surface of the branches.
These examples suffice to illustrate the general appearance presented
by the vegetative shoots of recent species of which the foliage
leaves vary considerably—from the small scale-leaves of _Lycopodium
tetragonum_, to the very slender linear subulate leaves of such a
species as _L. verticillatum_ Linn. or the long and broader lamina of
_L. Dalhousianum_ (fig. 121, E). It is obvious that fragments of the
various types preserved as fossils might well be mistaken either for
some of the larger mosses or for twigs of conifers. As Dr Bommer[94]
has pointed out in his interesting paper on “Les causes d’erreur dans
l’étude des empreintes végétales” some dicotyledonous plants may also
simulate the habit of Lycopods: he cites _Phyllachne clavigera_ Hook
(Candolleaceae), _Tafalla graveolens_ Wedd (Compositae) and _Lavoisiera
lycopodioides_ Gard. (Melastomataceae). Another point illustrated
by fig. 121 is the close agreement in habit and in the form of the
leaves and leaf-cushions between the recent plants and the Palaeozoic
Lepidodendreae.
In his masterly essay “On the vegetation of the Carboniferous Period,
as compared with that of the present day” Sir Joseph Hooker called
attention to the variation in the shape and arrangement of the
leaves in the same species of _Lycopodium_. The three woodcuts which
he publishes of _Lycopodium densum_, a New Zealand species, afford
striking examples of the diversity in habit and leaf-form and justify
his warning “that if the species of _Lepidodendron_ were as prone to
vary in the foliage as are those of _Lycopodium_, our available means
for distinguishing them are wholly insufficient[95].”
As we have already noticed, there is a considerable diversity among
recent species, both as regards habitat and habit; in the anatomy
of the stem also corresponding variations occur within the limits
of a well-defined generic type of stele. In species with creeping
stems, such as _L. clavatum_[96], the stele exhibits an arrangement
of vascular tissue characteristic of the plagiotropic forms. The
xylem consists of more or less horizontal plates of scalariform
tracheae, each surrounded by small-celled parenchyma, alternating with
bands or groups of somewhat ill-defined phloem. The protoxylem and
protophloem elements occupy an external position (exarch), pointing
to a centripetal development of the metaxylem. This centripetal or
root-like character of the primary xylem is an important feature in
recent as in fossil Lycopods. The close agreement between the roots and
stems of recent species in the disposition of the vascular elements
also denotes a simpler type of anatomy than occurs in the majority of
vascular plants in which stem and root have more pronounced structural
peculiarities. A pericycle, 2–6 cells in breadth, encloses the xylem
and phloem bands and this is succeeded by an endodermis, 2–3 cells
broad, with vaguely defined limits. In _L. clavatum_, as in _L.
alpinum_, another British species, the broad cortex is differentiated
into three fairly distinct regions; abutting on the endodermis is a
zone several layers broad of thick-walled cells constituting an inner
cortex modified for protection and support; the central region consists
of larger and thinner-walled cells adapted for water-storage and
aeration; beyond this is an outer cortical zone of firmer and thicker
elements. The prominent leaf-bases or leaf-cushions (fig. 125, A, _lc_)
give to the surface of a transverse section a characteristic appearance
which presents the closest agreement with that of the younger shoots of
_Lepidodendron_. From the peripheral protoxylem groups small strands of
xylem are given off, which follow a steeply ascending course through
the cortex to the single-veined leaves. The leaf-traces, in several
species at least, are characterised by a mesarch structure (fig. 125,
F, G), the spiral protoxylem elements occupying an approximately
central position. The mesophyll of the leaves varies in regard to the
extent of differentiation into a palisade and spongy parenchyma; in all
cases there is a single vascular bundle occasionally accompanied by a
secretory duct.
[Illustration: FIG. 125.
A. _Lycopodium dichotomum._ Transverse section of stem: _lc_,
leaf-cushion; _lt_, leaf-trace; R, roots.
B. _L. cernuum_, portion of cortex of fig. H, enlarged.
C. _L. saururus._ Cortex: _lt_, leaf-trace; _a_, thin-walled
tissue; _b_, thick-walled tissue; _lc_, lacuna.
D. _L. saururus._ Stele: _x_, xylem; _p_, phloem.
E. Portion of fig. D, enlarged: _px_, protoxylem; _p_, phloem.
F. Transverse section of leaf of _Lycopodium_.
G. Vascular bundle of leaf: _px_, protoxylem.
H. _L. cernuum_: _b_, branch of stele; _c_–_c″_, cortex; _s_, space
in cortex; _lt_, leaf-trace.
I. Stele of fig. H, enlarged (phloem omitted).]
In erect stems of _Lycopodium_, as represented by _L. cernuum_ (figs.
123, 125, H, I), _L. Dalhousianum_, _L. squarrosum_ (fig. 122) and
many others, the stele presents a characteristic appearance due to the
xylem plates being broken up into detached groups or short uniseriate
bands with the interspaces occupied by phloem islands. This type of
structure bears a superficial resemblance to that in the single stele
of certain species of the fern _Lygodium_[97], but it is distinguished
by the islands of phloem scattered through the stele. In other species
the xylem tends to assume the form of a Maltese cross (_e.g._ _L.
serratum_ Thbg.) or it may be disposed as =V=-shaped and sinuous bands
terminating in broad truncate ends composed of protoxylem elements.
This form of the xylem and the distribution of the phloem groups are
shown in fig. 125, D, E, drawn from a section of a plant of _Lycopodium
saururus_ Lam.[98] collected by Mr A. W. Hill at an altitude of 15,000
feet on the Andes of Peru. The position of the protoxylem is shown fig.
125, E, _px_.
While several species possess a cortex of three distinct zones (fig.
125, H, _c_, _c_′, _c_″), in others the extra-stelar tissue is much
more homogeneous, consisting of thin-walled parenchyma or in some
cases of thick-walled elements; as a general rule, however, there
is a tendency towards a more compact arrangement in the inner and
outer portions of the cortex as contrasted with the larger and more
loosely connected cells of the middle region. In certain types the
middle cortex contains fairly large spaces, as in the swamp-species
_L. inundatum_, which with _L. alopecuroides_ exhibits another feature
of some interest first described by Hegelmaier[99]. If a transverse
section of the stem of _L. inundatum_ be examined the leaf-traces are
seen to be accompanied by a circular canal containing mucilage which
extends into the lamina of the leaf. In a specimen of _L. cernuum_[100]
obtained at a height of 2500 ft. by Professor Stanley Gardiner in
the Fiji Islands, the leaf-traces (fig. 125, B _lt_) were found to
be accompanied for part of their course by a well-marked secretory
space (fig. 125, B, _s_). There is little doubt that the presence of
these mucilage canals is directly connected with a certain type of
habitat[101] and attention is called to them in view of a resemblance
which they offer to a characteristic strand of tissue, known as the
parichnos, which is associated with the leaf-traces of _Lepidodendreae_
and _Sigillarieae_. In the section shown in fig. 125, H, the xylem of
the stele forms more continuous bands than is often the case in _L.
cernuum_ which has already been described as having its xylem in small
detached groups. The presence of the smaller branch-stele (fig. 125, H,
_b_) affords an example of monopodial branching. The outer cortex of
_L. saururus_ (fig. 125, C) exhibits a somewhat unusual feature in the
distribution of the thicker-walled tissue (_b_) which encloses a patch
of more delicate parenchyma (_a_) with large lacunae (_lc_) in the
region of the leaf-bases, and presents the appearance of an irregular
reticulum. This arrangement of the mechanical tissue in the outer
cortex is comparable with that in stems of some species of _Sigillaria_.
In certain species of _Lycopodium_ the roots[102], which arise
endogenously from the axial vascular cylinder, instead of passing
through the cortex of the stem by the shortest route, bend downwards
and bore their way in a more or less vertical direction before emerging
at or near the base of the aerial shoot. The transverse section of _L.
dichotomum_ represented in fig. 125, A, shows several roots (R) in the
cortex; they consist of a xylem strand of circular or crescentric form
accompanied by phloem and enclosed by several layers of root-cortex.
The roots of _Lycopodium_ do not always present so simple a structure
as those of _L. dichotomum_; the xylem may have an irregularly stellate
form with as many as ten protoxylem groups.
_Reproductive Shoots[103]._ In _Lycopodium Selago_ the foliage
leaves serve also as sporophylls and, as Professor Bower[104] has
pointed out, the branches exhibit to some extent a zonal alternation
of sterile and fertile leaves; in other species, in which foliage
leaves and sporophylls are practically identical, the sporangia occur
sporadically on the ordinary leaves. In species with well-defined
terminal cones the lower sporophylls may bear arrested sporangia and
thus form transitional stages between sterile and fertile leaves, a
feature which occurs also in the male and female flowers of many recent
Araucarieae[105]. The sporangia[106] (fig. 126, D, F) are usually
reniform and compressed in a direction parallel to the surface of the
cone-scales; they are developed from the upper surface and close to
the base of the fertile leaf to which they are attached by a short and
thick stalk (_e.g._ _L. inundatum_) or by a longer and more slender
pedicel (_L. Phlegmaria_, fig. 126, E). On maturity the sporangia open
as two valves in the plane of compression and the line of dehiscence
is determined in some species at least by the occurrence of smaller
cells in the wall. In transverse sections of cones in which the
sporangia are strongly saddle-shaped, the sporophylls may appear to
bear two sporangia. This is well shown in the section of a cone of
_L. clavatum_ shown in fig. 126, F. The sporangia _a_ and _b_ are cut
through in an approximately median plane showing the irregular outline
of the sterile pad (_p_) of tissue in the sporogenous cavity. Those
at _c_ and _d_ have been traversed at a lower level and the two lobes
of the saddle-shaped sporangia are cut below the attachment to the
sporophyll. The distal laminae of the sporophylls, cut at different
levels, are seen at the periphery of the cone.
[Illustration: FIG. 126.
A. _Lycopodium cernuum_, longitudinal section of strobilus; _a_,
band of lignified cells.
B. _L. cernuum._ Cell from sporangium wall.
C. _L. cernuum._ Sporophyll and sporangium; _lt_, vascular bundle.
D. _L. clavatum._ Part of radial longitudinal section of strobilus;
_p_, sterile tissue.
E. _L. Phlegmaria._ Sporophyll and stalked sporangium.
F. _L. clavatum._ Transverse section of strobilus; _p_, sterile pad.]
In longitudinal radial section of some cones the sporangia appear to
occupy an axillary position, but in others (_e.g._ _L. clavatum_)
they are attached to the horizontal portion of the sporophyll almost
midway between the axis of the cone and the upturned distal end of the
sporophyll (fig. 126, D). The wall of a sporangium frequently consists
of 2–3 cell-layers and in some cases (_e.g._ _L. dichotomum_), it may
reach a thickness of seven layers, resembling in this respect the
more bulky sporangia of a certain type of Lepidodendroid cone. The
sporogenous tissue is separated from the stalk of the sporangium by
a mass of parenchymatous tissue which may project as a prominent pad
(fig. 126, D, F, _p_) into the interior of the sporogenous cavity. This
basal tissue (the subarchesporial pad of Bower[107]) has been observed
in _L. clavatum_ to send up irregular processes of sterile cells among
the developing spores, suggesting a comparison with the trabeculae
which form a characteristic feature of the sporangia of _Isoetes_ and
with similar sterile strands noticed by Bower[108] in _Lepidostrobus_
(cone of _Lepidodendron_).
Each sporophyll is supplied by a single vascular bundle which according
to published statements never sends a branch to the sporangium base.
The fertile tips of the foliage shoots of _L. cernuum_ (figs. 126, A–C)
afford good examples of specialised cones. The surface of the cone is
covered by the broadly triangular laminae of sporophylls (fig. 126, C)
which in their fimbriate margins resemble the Palaeozoic cone-scales
described by Dr Kidston[109] as _Lepidostrobus fimbriatus_. The distal
portions of the sporophylls are prolonged downwards (fig. 126, A)
to afford protection to the lower sporangia, their efficiency being
increased by the lignified and thicker walls (A, _a_) of the cells in
the lower portion of the laminar expansion. The cells of the sporangial
wall are provided with strengthening bands which in surface-view
(fig. 126, B) present the appearance of prominent pegs. Since the
appearance of Miss Sykes’s paper on the sporangium-bearing organs of
the Lycopodiaceae, Dr Lang[110] has published a more complete account
of the structure of the strobilus of _Lycopodium cernuum_ in which he
records certain features of special interest. The importance of these
morphological characters is increased by their agreement, as shown
by Lang, with those of the Palaeozoic cone _Spencerites_[111]. The
sporophylls of a cone (12 mm. long by 3 mm. in diameter) of _Lycopodium
cernuum_ show an abrupt transition from the foliage leaves, but like
these they occur in alternate whorls of five. A large sporangium is
attached to the upper face of each sporophyll close to the base of
the obliquely vertical distal lamina (fig. 127); each sporophyll,
which is supplied with a single vascular bundle, has a large
mucilage-cavity (_m_) in its lower region. “The mucilaginous change”
in the sub-sporangial portion of a sporophyll “extends to the surface
involving the epidermis, so that this portion of the sporophyll-base
may be described as consisting of a mass of mucilage bounded below by
a structureless membrane[112].” Dehiscence of the sporangia occurs at
the middle of the distal face (fig. 127, _x_). As seen in the radial
section (fig. 127, _ma_) the outer margin of the base of the sporophyll
bears a short outgrowth. The leaf-bases of each whorl hang down between
the sporangia of the alternating whorl below, and the base of each
sporophyll is coherent with the margins of the two sporophylls of
the next lower whorl between which it lies, the sporangia being thus
closely packed and lying in a pocket “open only on the outer surface
of the cone.” Fig. 128 represents a transverse section through a cone
in the plane AA of fig. 127; this traverses the sporangia and their
subtending bracts (_b_) of one whorl and the dependent bases of the
sporophylls of the next higher whorl in the region of the mucilage-sacs
(_m_), which are bounded at the periphery by the outer tissue of the
sporophylls (_a_). A transverse section in the plane BB of fig. 127
is shown in fig. 129: the pedicels and a part of each vascular strand
are seen at _b_ radiating from the axis of the cone; one sporophyll
(_sp, a_) is cut through in the region of the pad of tracheal tissue
that characterises the short sporangial stalks. The upper portions of
the sporangia of the next lower whorl, which project upwards against
the mucilaginous bases of the sporophylls above (cf. fig. 127, BB) are
shown at _c_ and external to them, at _a_, the section has cut through
the outer persistent portions of these sporophyll bases.
[Illustration: FIG. 127. Radial longitudinal section of the cone of
_Lycopodium cernuum_. (After Lang.)]
[Illustration: FIG. 128. Transverse section of the cone of _Lycopodium
cernuum_, in its plane AA of fig. 127. (After Lang.)]
[Illustration: FIG. 129. Transverse section of the cone of _Lycopodium
cernuum_ in the plane BB of fig. 127. (After Lang.)]
As Lang points out, this highly complex structure is an expression of
the complete protection afforded to the sporangia of a plant met with
in exposed situations in the tropics; it is also of importance from a
morphological standpoint as exhibiting an agreement with the extinct
type of Lycopod cone represented by _Spencerites_.
=Selaginellaceae.=
_Selaginella_ differs from _Lycopodium_ in the production of two kinds
of spores, megaspores and microspores, and, in the great majority
of species, in the dimorphic character of the foliage leaves, which
are usually arranged in four rows, the laminae of the upper rows
being very much smaller than those of the lower (fig. 130, 1–3). The
smaller leaves are shown more clearly in fig. 130, 1_a_. It is obvious
from an examination of a Selaginella shoot, such as is shown in fig.
130, that in fossil specimens it would often be almost impossible to
recognise the existence of two kinds of leaves. Some species, _e.g._
_Selaginella spinosa_[113], the sole British representative of the
genus, are homophyllous and agree in this respect with most species
of _Lycopodium_. Another feature characteristic of _Selaginella_, as
contrasted with _Lycopodium_, is the presence of a ligule in both
foliage leaves and sporophylls. This is a colourless thin lamina
attached by a comparatively stout foot to the base of a pit on the
upper surface and close to the lower edge of the leaf (fig. 130, 4,
_l_; fig. 131, E, F, _l_).
[Illustration: FIG. 130. _Selaginella grandis._ (1–3, nat. size.)]
In an erect species, such as _S. grandis_ Moore[114] (fig. 130 and
fig. 131, G) from Borneo, the main shoots, which may attain a height
of 2–3 feet, bear small and inconspicuous leaves of one kind, but the
lateral and repeatedly forked shoots are heterophyllous. The passage
from the homophyllous to the heterophyllous arrangement is shown in
the transition from the erect to the dorsiventral habit of the lateral
shoots (fig. 130, 2). The monopodially or dichotomously branched shoots
produce long naked axes at the forks; these grow downwards to the
ground where they develop numerous dichotomously forked branches. For
certain reasons these naked aerial axes were named rhizophores and have
always been styled shoots, the term root being restricted to repeatedly
forked branches which the rhizophores produce in the soil. It has,
however, been shown by Professor Harvey-Gibson[115] that there is no
sufficient reason for drawing any morphological distinction between
rhizophores and roots, the term root being applicable to both.
Our knowledge of the anatomy of _Selaginella_, thanks chiefly to the
researches of Harvey-Gibson[116], is much more complete than in the
case of _Lycopodium_. The stems, which may be either trailing or erect,
are usually dorsiventral, and it is noteworthy that different shoots of
the same plant or even the same axis in different regions may exhibit
considerable variation in the structure and arrangement of the vascular
tissue. In the well-known species, _Selaginella Martensii_, the
stem, which is partly trailing, partly ascending, possesses a single
ribbon-shaped stele composed of scalariform tracheids with two marginal
protoxylems formed by the fusion of the leaf-traces of the dorsal
and ventral leaves respectively. As in _Lycopodium_ the metaxylem
tracheae are as a rule scalariform, but reticulate xylem elements are
by no means unknown. The tracheal band, surrounded by parenchymatous
elements, is enclosed by phloem with external protophloem elements.
The characteristic features of the stele are shown in the diagrammatic
drawing of a section of another species—_S. Willdenowii_—represented in
fig. 131, A.
[Illustration: FIG. 131.
A. _Selaginella Willdenowii._ Transverse section of stem: _a_,
outer cortex; _p_, phloem; _t_, trabeculae.
B. _S. spinosa_, stem: _px_, protoxylem.
C. _S. laevigata_ var. _Lyallii_, section of stele: _t_, ridge of
xylem cylinder; _e_, endodermis.
D. _S. rupestris_, seedlings with cotyledons (_c_) protruding
beyond the sporophylls (_b_).
E. Transverse section of _Selaginella_ leaf-base: _l_, ligule;
_lt_, leaf-trace.
F. Portion of G. enlarged.
G. _S. grandis._ Longitudinal section of strobilus: _bb_,
sporophyll-trace; _l_, ligule.
(A, B, C, E, F, after Harvey-Gibson; D, after Miss Lyon.)]
A pericycle composed of one or two layers of chlorophyll-containing
cells encircles the whole stele which is suspended in a lacuna by
trabeculae (fig. 131, A, B, _t_) connecting the pericycle with the
inner edge of the broad cortex. The trabeculae consist in part of
endodermal cells characterised by cuticular bands. The cortex is
usually differentiated into three fairly distinct regions. Mechanical
tissue of thick-walled fibres constitutes the outer region (_a_); the
middle cortex consists of thinner-walled parenchyma, the elements of
which become smaller and rather more compactly arranged in the inner
zone. The middle cortex is frequently characterised by the presence
of spaces and by the hyphal or trabecular structure of the tissue, a
feature which, as Bower[117] pointed out, is common to many recent
and fossil members of the Lycopodiales. In some cases, _e.g._ _S.
erythropus_, from tropical America, the cortex of the creeping stem
consists entirely of thick-walled cells. _Selaginella grandis_ (fig.
130) has “a short decumbent stem rooted at close intervals[118],”
from which thick erect aerial shoots rise to a height of one foot or
more. In the apical region these erect axes give off repeatedly forked
foliage shoots on which the spiral phyllotaxis of the homophyllous axis
is gradually replaced by four rows of two kinds of leaves (fig. 130,
2). The anatomy of this species agrees with that of _S. Martensii_.
The trailing or semi-erect and homophyllous shoots of _Selaginella
spinosa_[119] present a distinct type of vascular anatomy. The upper
part of the ascending stem has an axial strand of xylem with seven
peripheral groups of spiral protoxylem tracheae (fig. 131, B); in
the trailing portion of the shoot the protoxylem elements occur as
one central group in the solid rod of metaxylem through which the
leaf-traces pass on their way to the axial protoxylem. This type is
important as affording an exception, in the endarch structure of the
xylem, to the usual exarch plan of the stelar tissues. This species is
the only one in which any indication of the production of secondary
xylem elements has so far been recorded. Bruchmann[120] has shown
that, in the small tuberous swelling which occurs at the base of the
young shoot (hypocotyl), a meristematic zone is formed round the axial
vascular strand and by its activity a few secondary tracheids are added
to the primary xylem. With this exception _Selaginella_ appears to
have lost the power of secondary thickening, the possession of which
constitutes so striking a feature of the Palaeozoic Lycopods. Another
type is represented by _S. inaequalifolia_, an Indian species, the
shoots of which may have either a single stele or as many as five,
each in its separate lacuna. The homophyllous _S. laevigata_ var.
_Lyallii_ Spr., a Madagascan species, affords a further illustration
of the variation in plan of the vascular tissues within the genus.
There is a considerable difference in structure between the erect
and creeping shoots; in the former there may be as many as 12–13
steles, which gradually coalesce before the vertical axis joins the
creeping rhizome to form one central and four peripheral steles. In the
rhizome there is usually a distinct axial stele without protoxylem,
surrounded by an ill-defined lacuna and enclosed by a cylindrical
stele (solenostele)[121] usually two tracheae in width with four
protoxylem strands on its outer edge. The continuity of the tubular
stele is broken and, in transverse section, it assumes the form of a
horse-shoe close to the base of an erect shoot to which a crescentic
vascular strand is given off. Harvey-Gibson[122] has figured a section
of the rhizome of this type in which the axial vascular strand is
represented by a slight ridge of tracheae (fig. 131, C, _t_) projecting
towards the centre of the axis of the tubular stele. The cylindrical
stele consists of xylem with external and internal phloem (_p_):
cuticularised endodermal cells occur at _e_ and _e_.
Reference has already been made to the descending naked branches
given off from the points of ramification of the foliage shoots of
_Selaginella_. It has been shown by Harvey-Gibson[123] that these
branches, originally designated rhizophores by Nägeli and Leitgeb,
as well as the dichotomously branched roots which they produce below
the level of the ground, possess a single vascular strand of monarch
type. It is interesting to find that in some species the aerial portion
of the rhizophore has a xylem strand with a central protoxylem, an
instance of endarch structure like that in certain portions of the
shoot-system of _S. spinosa_. The root-anatomy of _Selaginella_ and
the dichotomous habit of branching afford points of agreement with the
subterranean organs of _Lepidodendron_ and _Sigillaria_.
_Leaves._ The leaves of _Selaginella_[124] usually consist of a
reticulum of loosely arranged cells, but in some cases part of the
mesophyll assumes the palisade form. The single vascular bundle
consists of a few small annular or spiral tracheae and at the apex of
the lamina the protoxylem elements are accompanied by several short
reticulated pitted elements. Both foliage leaves and sporophylls are
characterised by the possession of a ligule, a structure which may
present the appearance of a somewhat rectangular plate (fig. 130,
4, _l_, and fig. 131, E–G, _l_) or assume a fan-shaped form with a
lobed or papillate margin. The base, composed of large cells, is sunk
in the tissue of the leaf close to its insertion on the stem (fig.
131, E, _l_) and enclosed by a well-marked parenchymatous sheath. The
sheath is separated from the vascular bundle of the leaf by one or
more layers of cells, and in some species these become transformed
into short tracheids. The ligule is regarded by Harvey-Gibson[125] as
a specialised ramentum which serves the temporary function of keeping
moist the growing-point and young leaves.
_Cones._ The terminal portions of the branches of _Selaginella_ usually
bear smaller leaves of uniform size which function as sporophylls, but
in this genus the fertile shoots do not generally form such distinct
cones as in many species of _Lycopodium_. In _S. grandis_ (figs. 130,
3; 131, G) the long and narrow strobili consist of a slender axis
bearing imbricate sporophylls in four rows: each sporophyll subtends
a sporangium situated between the ligule and the axis of the shoot.
The sporangium may be developed from the axis of the cone or, as in
_Lycopodium_, from the cells of the sporophyll[126]. In some species
the lower sporophylls bear only megasporangia, each normally containing
four megaspores, the microsporangia being confined to the upper part of
the cone. This distribution of the two kinds of sporangia is, however,
by no means constant[127]: in some cases, _e.g._ _S. rupestris_, cones
may bear megasporangia only, and in the cone of _S. grandis_, of which
a small piece is represented in fig. 131, G, all the sporangia were
found to contain microspores.
The occurrence of two kinds of spores in _Selaginella_ constitutes
a feature of special importance from the point of view of the
relationship between the Phanerogams, in which heterospory is a
constant character, and the heterosporous Pteridophytes. One of the
most striking distinctions between the Phanerogams and the rest of the
vegetable kingdom lies in the production of seeds. Recent work has,
however, shown that seed-production can no longer be regarded as a
distinguishing feature of the Gymnosperms and Angiosperms. Palaeozoic
plants which combined filicinean and cycadean features resembled the
existing Phanerogams in the possession of highly specialised seeds.
This discovery adds point to the comparison of the true seed with
structures concerned with reproduction in seedless plants, which in
the course of evolution gave rise to the more efficient arrangement
for the nursing, protection, and ultimate dispersal of the embryo. In
the megaspore of _Selaginella_ we have, as Hofmeister was the first
to recognise in 1851, a structure homologous with the embryo-sac of
the Phanerogam. The embryo-sac consists of a large cell produced in a
mass of parenchymatous tissue known as the nucellus which is almost
completely enclosed by one or more integuments. Fertilisation of the
egg-cell within the embryo-sac takes place as a rule while the female
reproductive organ is still attached to the parent-plant and separation
does not occur until the ovule has become the seed.
In a few cases, notably in certain plants characteristic of Mangrove
swamps, continuity between the seed and its parent is retained until
after germination. The megasporangium of _Selaginella_ dehisces[128]
along a line marked out by the occurrence of smaller cells over the
crest of the wall. It has been customary to describe the megaspores
as being fertilised after ejection from the sporangia. This earlier
separation from the parent and the absence of any protective covering
external to the spore-wall constitute two distinguishing features
between seeds and megaspores. In _Selaginella apus_, a Californian
species, Miss Lyon has shown that fertilisation of the egg-cell usually
takes place while the megaspore is still in the strobilus. On examining
withered decayed strobili of this species which had been partially
covered with the soil for some months after fertilisation of the
megaspores, several young plants were found with cotyledons and roots
projecting through the crevices of the megasporangia[129]. From this,
adds Miss Lyon, “it seems safe to assume that an embryo may have two
periods of growth separated by one of quiescence quite comparable to
those of seed plants with marked xerophilous features.”
In another Western American species _S. rupestris_ described by the
same writer the cotyledons of young plants were found protruding
from the imbricate sporophylls of a withered cone (fig. 131, D).
This species is interesting also from the occasional occurrence
of one instead of four megasporangia in a sporangium; a condition
which affords another connecting link between the heterosporous
Pteridophytes, on the one hand, and the seed-bearing Phanerogams in
which the occurrence of a single embryo-sac (megaspore) in each ovule
is the rule. The cones of _Selaginella rupestris_ retain connexion
with the plant through the winter and fertilisation occurs in the
following spring. After the embryo has been formed the megasporangium
“becomes sunken in a shallow pit formed by the cushion-like outgrowth
of the sporophyll around the pedicel.” It is suggested that this
outgrowth may be comparable with the integument which grows up from
the sporophyll in the fossil genus _Lepidocarpon_[130] and almost
completely encloses the sporangium. In the drawings given by Miss Lyon
no features are recognisable which afford a parallel to the integument
of _Lepidocarpon_. I have, however, endeavoured to show, by a brief
reference to this author’s interesting account of the two Californian
species, that the physiological and morphological resemblances between
the megasporangia of _Selaginella_ and the integumented ovules of the
seed-bearing plants are sufficiently close to enable us to recognise
possible lines of advance towards the development of the true seed.
Professor Campbell[131] records an additional example of a
_Selaginella_—probably _S. Bigelovii_—from the dry region of Southern
California in which the spores become completely dried up after the
embryo has attained some size, remaining in that state until the more
favourable conditions succeeding the dry season induce renewed activity.
=Isoetaceae.=
The genus _Isoetes_ is peculiar among Pteridophytes both in habit and
in anatomical features. In its short and relatively thick tuberous
stem, terminating in a crowded rosette of subulate leaves like those of
_Juncus_ and bearing numerous adventitious roots, _Isoetes_ presents an
appearance similar to that of many monocotyledonous plants. The habit
of the genus is well represented by such species as _Isoetes lacustris_
and _I. echinospora_[132] (fig. 132) both of which grow in freshwater
lakes in Britain and in other north European countries. The latter
species bears leaves reaching a length of 18 cm. The resemblance in
habit between this isolated member of the Pteridophytes and certain
Flowering plants, although in itself of no morphological significance,
is consistent with the view expressed by Campbell that _Isoetes_ may be
directly related to the Monocotyledons[133].
[Illustration: FIG. 132. _Isoetes echinospora_ (After Motelay and
Vendryès).
A. Stem of _I. lacustris._
B. Base of sporophyll: _l_, ligule; _spg_, sporangium partially
covered by velum.]
There is as a rule little or no difference between the foliage leaves
and sporophylls; in _I. lacustris_ the latter are rather larger and
in the terrestrial species _I. hystrix_[134] the sterile leaves are
represented by the expanded basal portions only, which persist like the
leaf-bases of _Lepidodendron_ as dark brown scales to form a protective
investment to the older part of the stem. The innermost leaves are
usually sterile; next to these are sporophylls bearing megasporangia,
and on the outside are the older sporophylls with microsporangia. The
long and slender portion of the leaf becomes suddenly expanded close
to its attachment to the stem into a broad base of crescentic section
which bears a fairly conspicuous ligule (figs. 132, B, _l_, 133, E,
_l_) inserted by a foot or glossopodium in a pit near the upper part
of the concave inner face. The ligule is usually larger than that
of _Selaginella_, though of the same type. The free awl-like lamina
contains four large canals bridged across at intervals by transverse
diaphragms, and in the axial region a single vascular bundle of
collateral structure. Other vascular elements, in the form of numerous
short tracheids occur below the base of the transversely elongated
ligule.
Stomata are found on the leaves of _I. hystrix_, _I. Boryana_[135],
and in other species which are not permanently submerged. Both
microsporangia and megasporangia are characterised by their large
size and by the presence of trabeculae or strands of sterile tissue
(fig. 133, E, H, _t_) completely bridging across the sporangial
cavity or extending as irregular ingrowths among the spore-producing
tissue. Similar sterile bands, though less abundant and smaller, are
occasionally met with in the still larger sporangia of _Lepidostrobus_;
these may be regarded as a further development of the prominent pad
of cells which projects into the sporangial cavity in recent species
of _Lycopodium_ (fig. 126, D, _p_). The sporangia are attached by a
very short stalk to the base of a large depression in the leaf-base
below the ligule, from the pit of which they are separated by a ridge
of tissue known as the saddle, and from this ridge a veil of tissue
(the velum) extends as a roof over the sporangial chamber (fig. 133,
E, _v_). In most species there is a large gap between the lower edge
of the velum and that of the sporangial pit, but in _I. hystrix_
this protective membrane is separated from the base of the leaf by a
narrow opening, the resemblance of which to the micropyle of an ovule
suggested to one of the older botanists the employment of the same
term[136]. Mr T. G. Hill[137] has called attention to the presence
of mucilage canals in the base of the sporophylls of _I. hystrix_,
which he compares with the strands of tissue known as the parichnos
accompanying the leaf-traces of _Lepidodendron_ and _Sigillaria_ in
the outer cortex of the stem. The transverse section shown in fig.
133, H and I, shows two of these mucilage canals in an early stage
of development; a strand of parenchymatous elements distinguished
by their partially disorganised condition and more deeply stained
membranes (fig. 133, I) runs through the spandrels of the sporophyll
tissue close to the upper surface. There is a close resemblance
between the structure of these partially formed mucilage-canals and
the tissue which has been called the secretory zone in Lepidodendron
stems. Fig. 133, H, also shows a large microsporangium with prominent
trabeculae (_t_) lying below the velum. A longitudinal section (fig.
133, E) through a sporophyll-base presents an appearance comparable
with that of an Araucarian cone-scale with its integumented ovule
and micropyle. The megaspores are characterised by ridges, spines,
and other surface-ornamentation[138]. Though usually unbranched, the
perennial stem of _Isoetes_ (fig. 132) has in rare cases been found to
exhibit dichotomous branching, a feature, as Solms-Laubach[139] points
out, consistent with a Lycopodiaceous affinity. The apex is situated
at the base of a funnel-shaped depression. The stem is always grooved;
in some species two and in others three deep furrows extend from the
base up the sides of the short and thick axis towards the leaves: from
the sides of these furrows numerous slender roots are given off in
acropetal succession. A stele of peculiar structure occupies the centre
of the stem; cylindrical in the upper part (fig. 133, A), it assumes a
narrow elliptical or, in species in which there are three furrows, a
triangular form in the lower portion of the tuberous stem.
[Sidenote: ISOETES]
The stem of _I. lacustris_ represented in fig. 132, A, from which the
laminae of the leaves have been removed from the summit affords an
example of a species with two furrows. The drawing shows the widely
gaping sides of the broad furrow with circular root-scars and a few
simple and dichotomously branched roots. A short thick column of
parenchymatous tissue projects from a slightly eccentric position on
the base of the stem.
[Illustration: FIG. 133. _Isoetes lacustris_.
A. Transverse section of stem: _cr_, cortex; _x_, _x²_ xylem; _c_,
cambium; _a_, thin-walled tissue; _lt_, leaf-traces; _b_, dead
tissue.
B, C, D. Portions of A enlarged.
E. Longitudinal radial section of sporophyll-base: _v_, velum; _l_,
ligule; _bb_; vascular bundle; _m_, megaspores; _t_, sterile
tissue.
F. Longitudinal section through the base of a root.
G. Transverse section of root.
H. Transverse section of sporophyll, showing sporangium with
trabeculae, _t_; leaf-trace, (_lt_), and two groups of secretory
cells.
I. A group of secretory cells enlarged.]
The primary vascular cylinder[140] consists of numerous spiral,
annular or reticulate tracheids (fig. 133, A, _x_) which are either
isodiametric or longer in a horizontal than in a vertical direction,
associated with parenchyma. Lower in the stem crushed and disorganised
xylem elements are scattered through a still living trabecular network
of parenchymatous tissue. From the axial cylinder numerous leaf-traces
(fig. 133, A, _lt_) radiate outwards, at first in a horizontal
direction and then gradually ascending towards the leaves. The vascular
cylinder is of the type known as cauline; that is, some of the xylem is
distinct in origin from that which consists solely of the lower ends
of leaf-traces. As in _Lycopodium_ the development of the metaxylem is
centripetal.
Von Mohl[141], and a few years later Hofmeister[142], were the first
botanists to give a satisfactory account of the anatomy of _Isoetes_
but it is only recently[143] that fresh light has been thrown upon the
structural features of the genus the interest of which is enhanced
by the many points of resemblance between the recent type and the
Palaeozoic Lepidodendreae. A striking anatomical feature is the power
of the stem to produce secondary vascular and non-vascular tissue;
the genus is also characterised by the early appearance of secondary
meristematic activity which renders it practically impossible to draw
any distinct line between primary and secondary growth. A cylinder of
thin-walled tissue (fig. 133, A, _a_) surrounds the primary central
cylinder and in this a cambial zone, _c_, is recognised even close
to the stem-apex; this zone of dividing cells is separated from the
xylem by a few layers of rectangular cells to which the term prismatic
zone has been applied. The early appearance of the cambial activity
on the edge of the vascular cylinder is shown in fig. 133, C, which
represents part of a transverse section of a young stem. A leaf-trace,
_lt_, is in connexion with the primary xylem, _x′_, which consists of
short tracheids, often represented only by their spiral or reticulately
thickened bands of lignified wall, and scattered parenchyma. Some of
the radially elongated cells on the sides of the leaf-trace are seen
to be in continuity on the outer edge of the stele, at _st_, with
flattened elements, some of which are sieve-tubes. The position of a
second leaf-trace is shown at _lt′_. External to the sieve-tubes the
tissue consists of radially arranged series of rectangular cells, some
of which have already assumed the function of a cambium (_c_). The
tissue produced by the cambium on its inner edge consists of a varying
amount of secondary xylem composed of very short spiral tracheids; a
few of these may be lignified (fig. 133, A, _x²_) while others remain
thin.
Phloem elements, recognisable by the presence of a thickened reticulum
enclosing small sieve-areas (fig 133, B, _s_) are fairly abundant,
and for the rest this intracambial region is composed of thin-walled
parenchyma. In longitudinal section these tissues present an appearance
almost identical with that observed in a transverse section. Fig.
133, B represents a longitudinal section, through the intracambial
zone and the edge of the stele, of a younger stem than that shown
in fig. 133, A. Most of the radially disposed cells internal to
the meristematic region are parenchymatous without any distinctive
features; a few scattered sieve-tubes (_s_) are recognised by their
elliptical sieve-areas and an occasional tracheid can be detected. The
cambium cuts off externally a succession of segments which constitute
additional cortical tissue (fig. 133, A, _cr_) of homogeneous
structure, composed of parenchymatous cells containing starch and
rich in intercellular spaces. As the stem grows in thickness the
secondary cortex reaches a considerable breadth and the superficial
layers are from time to time exfoliated as strips of dead and crushed
tissue (fig. 133, A, _b_). The diagrammatic sketch reproduced in fig.
133, A, serves to illustrate the arrangement and relative size of the
tissue-regions in an _Isoetes_ stem. In the centre occur numerous
spirally or reticulate tracheae scattered in parenchymatous tissue
which has been considerably stretched and torn in the peripheral
region of the stele; the radiating lines mark the position of the
leaf-traces (_lt_) in the more horizontal part of their course. The
zone between the cambium (_c_) and the edge of the central cylinder
consists of radially disposed secondary tissue of short, and for the
most part unlignified, elements including sieve-tubes and parenchyma;
the secondary xylem elements consist largely of thin-walled rectangular
cells with delicate spiral bands, but discontinuous rows of lignified
tracheae (_x²_) occur in certain regions of the intracambial zone. The
rest of the stem consists of secondary cortex (_cr_) with patches of
dead tissue (_b_) still adhering to the irregularly furrowed surface.
The structure of the cambium and its products is shown in the detailed
drawing reproduced in fig. 133, D. Many of the elements cut off on the
inner side of the cambium exhibit the characters of tracheids: most
of these are unlignified, but others have thicker and lignified walls
(_tr_).
_I. hystrix_ appears to be exceptional in retaining its leaf-bases,
which form a complete protective investment and prevent the exfoliation
of dead cortex. Each leaf-trace consists of a few spiral tracheids
accompanied by narrow phloem elements directly continuous with the
secondary phloem of the intracambial zone. Dr Scott and Mr Hill have
pointed out that a normal cambium is occasionally present in the stem
of _I. hystrix_ during the early stages of growth; this gives rise
to xylem internally. The few phloem elements observed external to
the cambium may be regarded as primary phloem, a tissue not usually
represented in an Isoetes stem[144]. The occasional occurrence of this
normal cambium, may, as Scott and Hill suggest, be a survival from a
former condition in which the secondary thickening followed a less
peculiar course. The lower leaf-traces become more or less obliterated
as the result of the constant increase in thickness of the broad zone
of secondary tissues through which they pass.
The adventitious roots are developed acropetally and arranged in
parallel series on each side of the median line of the two or three
furrows. The three arms of the triangular stele of _I. hystrix_ and
the two narrow ends of the long axis of the stele of _I. lacustris_,
which in transverse section has the form of a flattened ellipse, are
built up of successive root-bases. A root of _Isoetes_ (fig. 133, G)
possesses one vascular bundle, _x_, with a single strand of protoxylem,
_px_, thus agreeing in its monarch structure with the root-bundle in
_Selaginella_ and many species of _Lycopodium_. The cortical region
of the root consists of a few layers of outer cortex succeeded by
a large space, formed by the breaking down of the inner cortical
tissue, into which the vascular bundle projects (fig. 133, F). The
peculiarity of the roots in having a hollow cortex and an eccentric
vascular bundle was noticed by Von Mohl[145]. In the monarch bundles,
as in the fistular cortex and dichotomous branching, the roots of
_Isoetes_ present a striking resemblance to the slender rootlets of the
Palaeozoic _Stigmaria_ (see Page 246). The longitudinal section through
the base of a root of _Isoetes lacustris_ shown in fig. 133, F, affords
a further illustration of certain features common to the fossil and
recent types.
FOSSIL LYCOPODIALES.
Isoetaceae
The geological history of this division of the Pteridophyta is
exceedingly meagre, a fact all the more regrettable as it is by no
means improbable that in the surviving genus _Isoetes_ we have an
isolated type possibly of considerable antiquity and closely akin to
such extinct genera as _Pleuromeia_ and _Sigillaria_. If Saporta’s
Lower Cretaceous species _Isoetes Choffati[146]_, or more appropriately
_Isoetites Choffati_, is correctly determined, it is the oldest fossil
member of the family and indeed the most satisfactory among the more
than doubtful species described as extinct forms of _Isoetes_.
Isoetites.
The generic name _Isoetites_ was first used by Münster[147] in the
description of a specimen, from the Jurassic lithographic slates of
Solenhofen in Bavaria, which he named _Isoetites crociformis_. The
specific name was chosen to express a resemblance of the tuberous
appearance of the lower part of the imperfectly preserved and
indeterminable fossil to a Crocus corm.
Impressions of Isoetes-like leaves from the Inferior Oolite of
Yorkshire figured by Phillips[148] and afterwards by Lindley[149] as
_Solenites Murrayana_ were compared by the latter author with _Isoetes_
and _Pilularia_, but these leaves are now generally assigned to Heer’s
gymnospermous genus _Czekanowskia_. An examination of the structure of
the epidermal cells of these Jurassic impressions convinced me that
they resemble recent coniferous needles more closely than the leaves
of any Pteridophyte. The genus _Czekanowskia[150]_ is recognised by
several authors as a probable member of the Ginkgoales.
_Isoetites Choffati._ Saporta.
The late Marquis of Saporta founded this species on two sets of
impressions from the Urgonian (Lower Cretaceous) of Portugal which,
though not found in actual organic connexion, may possibly be portions
of the same plant. Small relatively broad tuberous bodies reaching
a breadth of 1 cm. are compared with the short and broad stem of
_Isoetes_, which they resemble in bearing numerous appendages radiating
from the surface like the roots of the recent species; on the exposed
face of the stem occur scattered circular scars representing the
position of roots which were detached before fossilisation. Other
impressions are identified as the basal portions of sporophylls bearing
sporangia: these suggest the expanded base of the fertile leaves of
_Isoetes_ with vertically elongated sporangia, some of which have
a smooth surface while in others traces of internal structure are
exposed; the interior consists of an irregular network with depressions
containing carbonised remains of spores.
While recognising a general resemblance to the sporophylls of
_Isoetes_, certain differences are obvious: there is no ligule in
the fossil leaves nor are there any distinct traces of vascular
strands such as occur in the leaves of recent species. The form of
the sporangium, more elongated than in the majority of recent forms,
is compared by Saporta with that in a south European species _Isoetes
setacea_ Spr.
Such evidence as we have lends support to the inclusion of these
Portuguese fossils in the genus _Isoetites_, but apart from the fact
that we have no proof of any connexion between the stems and supposed
sporophylls, the resemblance of the latter to those of _Isoetes_ is,
perhaps, hardly sufficient to satisfy all reasonable scepticism.
The generic name _Isoetopsis_ was used by Saporta as more appropriate
than _Isoetes_ for some Eocene fossils from Aix-en-Provence which are
too doubtful to rank as trustworthy evidence of the existence of the
recent genus. The species, _Isoetopsis subaphylla_[151] is founded on
impressions of small scales, 4 mm. long, bearing circular bodies which
are compared with sporangia or spores.
Other records of fossils referred to _Isoetes_ need not be described
as they have no claim to be regarded as contributions towards the past
history of the genus. Heer’s Miocene species _Isoetites Scheuzeri_ and
_I. Braunii_ Unger[152] from Switzerland are based on unsatisfactory
material and are of no importance.
Pleuromeia.
The generic name _Pleuromeia_, was suggested by Corda[153] for a fossil
from the Bunter Sandstone, the original description of which was based
by Münster[154] on a specimen discovered in a split stone from the
tower of Magdeburg Cathedral.
The majority of the specimens have been obtained from the neighbourhood
of Bernburg, but a few examples are recorded from Commern and other
German localities: all are now included under the name _Pleuromeia
Sternbergi_. Germar, who published one of the earlier accounts of the
species, states that Corda dissented from Münster’s choice of the name
_Sigillaria_ and proposed the new generic title _Pleuromeia_. One of
the best descriptions of the genus we owe to Solms-Laubach[155] whose
paper contains references to earlier writers. Illustrations have been
published by Münster, Germar[156], Bischof[157], Solms-Laubach and
Potonié[158].
_Pleuromeia Sternbergi._ (Münster.)
Fig. 134.
1842. _Sigillaria Sternbergii_, Münster.
1854. _Sagenaria Bischofii_, Goeppert[159].
1885. _Sigillaria oculina_, Blanckenhorn.
1904. _Pleuromeia oculina_, Potonié.
_Pleuromeia Sternbergi_ is represented by casts of vegetative and
fertile axes, but the preservation of the latter is not sufficiently
good to enable us to draw any very definite conclusions as to the
nature of the reproductive organs. Casts of the stems reach a length
of about 1 metre and a diameter of 5–6 cm., or in some cases 10 cm.;
all of them are in a more or less decorticated state, the degree
of decortication being responsible for differences in the external
features which led Spieker[160] to adopt more than one specific name.
Fig. 134, A, represents a sketch, made some years ago, of a specimen
in the Breslau Museum which contains several examples of this species,
among others those described by Germar in 1852. The cylindrical cast
(38 cm. long by 12 cm. in circumference), which has been slightly
squeezed towards the upper end, bears spirally arranged imperfectly
preserved leaf-scars and the lower end shows the truncated base of one
of the short Stigmaria-like arms characteristic of the plant. As shown
clearly in a specimen originally figured by Bischof and more recently
by Potonié[161], the stem-base is divided by a double dichotomy into
four short and broad lobes with blunt apices and bent upwards like
the arms of a grappling iron (fig. 134, D). The surface of this
basal region is characterised by numerous circular scars (fig. 134,
D; 4 scars enlarged) in the form of slightly projecting areas with a
depression in the centre of each. These are undoubtedly the scars of
rootlets, remains of which are occasionally seen radiating through the
surrounding rock. As seen in fig. 134, D, _a_, the fractured surface
of a basal area may reveal the existence of an axial vascular cylinder
giving off slender branches to the rootlets.
[Illustration: FIG. 134. _Pleuromeia Sternbergi._
A. Cast of stem in the Breslau Museum (⅓ nat. size). (A.C.S.)
B. “_Sigillaria oculina_” Blanckenhorn. (After Weiss).
C, D. Leaf-scars and base of stem: _a_, vascular tissue. (After
Solms-Laubach.)]
The bulbous enlargement at the base of the Brown seaweed _Laminaria
bulbosa_ Lam.[162] simulates the swollen base of _Pleuromeia_; but a
confusion between these two plants is hardly likely to occur. Above
the Stigmaria-like base the gradually tapered axis, in the less
decorticated specimens, bears spirally disposed transversely elongated
areas consisting of two triangular scars between which is the point
of exit of a leaf-trace. The form of the leaf-scars is best seen on
the face of a mould figured by Solms-Laubach (fig. 134, C): in this
case the two triangular areas appear as slight projections separated
by a narrow groove marking the position of the vascular bundle of the
leaf. The curved lines above and below the leaf-scar probably mark the
boundary of the leaf-base. The two triangular scars are compared by
Solms-Laubach and by Potonié with the parichnos-scars of _Sigillaria_
and _Lepidodendron_ (cf. fig. 146, C), but the large size of the
Pleuromeia scars constitutes an obvious difference though possibly not
a distinction of importance.
The occurrence of a vertical canal filled with carbonaceous material
in some of the stems throws light on the internal structure: the
canal, which is described by Solms-Laubach as having a stellate
outline in transverse section recalls the narrow central cylinder
of a Lepidodendron stem, and this comparison is strengthened by the
presence of obliquely ascending grooves which represent leaf-traces
passing through the cortex. In specimens which have lost more of the
cortical tissues the surface is characterised by spirally disposed,
discontinuous vertical grooves representing portions of leaf-traces
precisely as they appear in similar casts of _Lepidodendron_. There
is no direct evidence of the existence of secondary wood in the stem,
but, as Potonié has pointed out, the greater transverse elongation of
the leaf-scars in the lower part of a cast (fig. 134, A) points to the
production of some secondary tissue either in the vascular cylinder or
cortex, or possibly in both regions.
In some specimens of _Pleuromeia_ the upper portion is clothed with
crowded and imbricate sporophylls which reach a length of 2·5 cm., a
maximum breadth of 2·7 cm., and a thickness of 1 mm. Each sporophyll
has a thin wing-like border, and on the lower face are several parallel
lines. Solms-Laubach describes the sporangium or ovule as attached to
the lower surface of the sporophyll and this opinion has been confirmed
by Fitting[163] who has also brought forward satisfactory evidence in
favour of the sporangial nature of the reproductive organs. Fitting
found numerous spores in the Bunter Sandstone near Halle; these are
flattened circular bodies 0·5–0·7 mm. in diameter with a granulated
surface and the three converging lines characteristic of spores
produced in tetrads. The comparison made by this author between the
sporophylls of _Pleuromeia_, which bore the sporangia on the lower
surface instead of on the upper as in other lycopodiaceous plants, and
the pollen-sacs of Conifers, is worthy of note in reference to the
possible relationship between Conifers and Lycopods.
A comparison of the _Isoetes_ stem represented in fig. 132, A, with the
base of a _Pleuromeia_ shows a striking similarity, but, as Fitting
points out, the Stigmaria-like arms of the fossil contained a vascular
cylinder whereas the blunt lobes of _Isoetes_ consist exclusively of
cortical tissue, the roots being given off from the grooves between the
lobes of the tuberous stem.
The position of _Pleuromeia_ must for the present be left an open
question; it is, however, clear that the plant bears a close
resemblance in the form of its base to the Stigmarian branches of
_Lepidodendron_ and _Sigillaria_. The vegetative shoot appears to be
constructed on a plan similar to that of these two Palaeozoic genera,
but the strobilus is of a different type. It would seem probable that
_Pleuromeia_ may be closely allied to _Isoetes_ and to the arborescent
Lycopods of Palaeozoic floras. It is not improbably a link in a chain
of types which includes _Sigillaria_ on the one hand and _Isoetes_ on
the other.
It is not improbable that a specimen from the Lower Bunter of Commern
which Blanckenhorn made the type of a new species, _Sigillaria
oculina_ (fig. 134, B) is specifically identical with _Pleuromeia
Sternbergi_. An examination of a cast of the type-specimen in the
Berlin Bergakademie led me to regard the fossil with some hesitation
as a true _Sigillaria_, but a more extended knowledge of _Pleuromeia_
lends support to the view adopted by Potonié[164] that Blanckenhorn’s
plant is not genetically distinct from _Pleuromeia Sternbergi_. The
resemblance between _Sigillaria oculina_ and some of the Palaeozoic
species of _Sigillaria_ emphasised by Weiss[165] has given rise to the
belief that the genus _Sigillaria_ persisted into the Triassic era; it
is, however, highly probable that the Bunter specimen has no claim to
the generic name under which it has hither to been known.
The Bunter Sandstone in which _Pleuromeia_ is the sole representative
of plant-life, at least in certain localities, is usually considered to
be a desert formation. We may not be far wrong in accepting Fitting’s
suggestion that in this isolated species we have a relic of the sparse
vegetation which was able to exist where the presence of lakes added a
touch of life to the deadness of the Triassic desert.
_Pleuromeia_ is recorded by Fliche as a rare fossil in the Middle Trias
of France in the neighbourhood of Lunéville[166].
Herbaceous fossil species of Lycopodiales.
The history of our knowledge of fossil representatives of the
Lycopodiales, as also of the Equisetales, affords a striking
illustration of the danger of attempting to found a classification
on such differences as are expressed by the terms herbaceous and
arborescent in the sense in which they are usually employed. As we have
seen[167], the presence of secondary wood in stems of the Palaeozoic
plant now known as _Calamites_ led so competent a botanist as Adolphe
Brongniart to recognise a distinct generic type _Calamodendron_, which
he placed in the Gymnosperms, reserving the designation _Calamities_
for species in which no indication of secondary thickening had been
found.
Similarly, the genus _Sigillaria_ was regarded as a Gymnosperm because
it was believed to be distinguished from _Lepidodendron_ by the power
of forming secondary vascular tissues; the latter genus, originally
thought to be always herbaceous, was classed with the Pteridophytes.
At the time when this unnatural separation was made between stems with
secondary wood and those in which no secondary wood was known to exist,
botanists were not aware of the occurrence of any recent Pteridophyte
which shared with the higher plants the power of secondary growth in
thickness provided by means of a meristematic zone. It is true that the
presence or absence of a cambium does not in practice always coincide
with the division into herbaceous and arborescent plants: no one would
speak of a Date-Palm as a herbaceous plant despite the absence of
secondary wood.
The danger which should be borne in mind, in adopting as a matter of
convenience the term herbaceous as a sectional heading, is that it
should not be taken to imply a complete inability of the so-called
herbaceous types to make secondary additions to their conducting
tissues. The specimens on which the species of _Lycopodites_ and
_Selaginellites_, (genera which may be designated herbaceous,) are
founded are preserved as impressions and not as petrifications; we
can, therefore, base definitions only on habit and on such features
as are shown by fertile leaves and sporangia. We are fully justified
in concluding from evidence adduced by Goldenberg more than fifty
years ago and from similar evidence brought to light by more recent
researches, that there existed in the Palaeozoic era lycopodiaceous
species in close agreement in their herbaceous habit with the lycopods
of present-day floras. It has been suggested[168] that the direct
ancestors of the genera _Lycopodium_ and _Selaginella_ are represented
by the species of _Lycopodites_ and _Selaginellites_ rather than by
_Lepidodendron_ and _Sigillaria_, the arborescent habit of which has
been rendered familiar by the numerous attempts to furnish pictorial
reproductions of a Palaeozoic forest. Until we are able to subject the
species classed as herbaceous to microscopical examination we cannot
make any positive statement as to the correctness of this view, but
such facts as we possess lead us to regard the suggestion as resting on
a sound basis.
Palaeobotanical literature abounds in records of species of
_Lycopodites_, _Lycopodium_, _Selaginella_ and _Selaginites_, which
have been so named in the belief that their vegetative shoots bear
a greater resemblance to those of recent lycopodiaceous plants than
to the foliage shoots of _Lepidodendron_. Many of these records are
valueless: _Lepidodendra_, twigs of _Bothrodendron_[169] species
of conifers, fern rhizomes, and _Aphlebiae_[170] have masqueraded
as herbaceous lycopods. It is obvious that an attempt to identify
fossils presenting a general agreement in habit and leaf-form with
recent species of lycopods must be attended with considerable risk of
error. Recent Conifers include several species the smaller branches
of which simulate the leafy shoots of certain species of _Lycopodium_
and _Selaginella_, and it is not surprising to find that this
similarity has been responsible for many false determinations. Among
Mosses and the larger foliose Liverworts there are species which in
the condition of imperfectly preserved impressions, might easily be
mistaken for lycopodiaceous shoots: an equally close resemblance is
apparent in the case of some flowering plants, such as New Zealand
species of _Veronica_, _Tafalla graveolens_ (a Composite), _Lavoisiera
lycopodiodes_ Gard.[171] (a species of Melastomaceae), all of which
have the habit of Cupressineae among the conifers as well as of certain
lycopodiaceous plants. It may be impossible to decide whether fossil
impressions of branches, which are presumably lycopodiaceous, bear
two kinds of leaves[172] like the great majority of recent species of
_Selaginella_. _Selaginella grandis_, if seen from the under surface,
would appear to have two rows of leaves only and might be confused with
a small twig of such a conifer as _Dacrydium Kirkii_, a New Zealand
species.
The New Zealand conifers _Dacrydium cupressinum_ Soland. and
_Podocarpus dacrydioides_ Rich. closely simulate species of
_Selaginellites_ and _Lycopodites_: in the British Museum a specimen
of the latter species bears a label describing it as _Lycopodium
arboreum_ (Sir Joseph Hooker and Dr Solander; 1769). The twigs of the
Tasmanian conifer _Microcachyrs_ _tetragona_ Hook. f. are very similar
in habit to shoots of the recent _Lycopodium tetragonum_ (fig. 121, C).
In the description of examples of _Lycopodites_ and _Selaginellites_
I have confined myself to such as appear to be above suspicion either
because of the presence of spore-bearing organs or, in a few cases,
because the specimens of sterile shoots are sufficiently large to show
the form of branching in addition to the texture of the leaves. The
two generic names _Lycopodites_ and _Selaginellites_ are employed for
fossil species which there are substantial grounds for regarding as
representatives of _Lycopodium_ and _Selaginella_. The designation
_Selaginellites_ is adopted only for species which afford evidence of
heterospory; the name _Lycopodites_, on the other hand, is used in
a comprehensive sense to include all forms—whether homophyllous or
heterophyllous—which are not known to be heterosporous. This restricted
use of the generic name _Selaginellites_ is advocated by Zeiller[173],
who instituted the genus, and by Halle[174] in his recent paper on
herbaceous lycopods.
Lycopodites.
The generic term _Lycopodites_ was used by Brongniart in 1822[175]
in describing some Tertiary examples of slender axes clothed with
small scale-like leaves which he named _Lycopodites squamatus_. These
are fragments of coniferous shoots. In the _Prodrome d’une histoire
des végétaux fossiles_[176] Brongniart included several Palaeozoic
and Jurassic species in _Lycopodites_ and instituted a new genus
_Selaginites_, expressing a doubt as to the wisdom of attempting to
draw a generic distinction between the two sets of species. In a later
work[177] he recognised only one undoubted species, _Lycopodites
falcatus_. The first satisfactory account of fossils referred to
_Lycopodites_ is by Goldenberg[178] who gave the following definition
of the genus:—“Branches with leaves spirally disposed or in whorls.
Sporangia in the axil of foliage leaves or borne in terminal strobili.”
It was suggested by Lesquereux[179] that Goldenberg’s definition,
which was intended to apply to herbaceous species, should be extended
so as to include forms with woody stems but which do not in all
respects agree with _Lepidodendron_. Kidston[180] subsequently adopted
Lesquereux’s modification of Goldenberg’s definition. We cannot draw
any well-defined line between impressions of herbaceous forms and those
of small arborescent species. We use the name _Lycopodites_ for such
plants as appear to agree in habit with recent species of _Lycopodium_
and _Selaginella_ and which, so far as we know, were not heterosporous:
it is highly probable that some of the species so named had the
power of producing secondary wood, a power possessed by some recent
Pteridophytes which never attain the dimensions of arborescent plants.
It has been shown by Halle[181], who has re-examined several of
Goldenberg’s specimens which have been acquired by the Stockholm
Palaeobotanical Museum, that some of his species of _Lycopodites_
are heterosporous and therefore referable to Zeiller’s genus
_Selaginellites_.
In 1869 Renault described two species of supposed Palaeozoic Lycopods
as _Lycopodium punctatum_ and _L. Renaultii_[182], the latter name
having been suggested by Brongniart to whom specimens were submitted.
These species were afterwards recognised by their author as wrongly
named and were transferred to the genus _Heterangium_[183], a
determination which is probably correct; it is at least certain that
the use of the name _Lycopodium_ cannot be upheld.
We have unfortunately to rely on specimens without petrified tissues
for our information in regard to the history of _Lycopodites_ and
_Selaginellites_. Among the older fossils referred to _Lycopodites_
are specimens from Lower Carboniferous rocks at Shap in Westmoreland
which Kidston originally described as _Lycopodites Vanuxemi_[184],
identifying them with Goeppert’s _Sigillaria Vanuxemi_[185] founded
on German material. In a later paper Kidston transferred the British
specimens of vegetative shoots to a new genus _Archaeosigillaria_[186].
_Lycopodites Stockii_ Kidston[187].
The plant so named was discovered in Lower Carboniferous strata of
Eskdale, Dumfries, Scotland; it is represented by imperfectly preserved
shoots bearing a terminal strobilus and was originally described by
Kidston as apparently possessing two kinds of foliage leaves borne
in whorls. The larger leaves have an ovate cordate lamina with an
acuminate apex, while the smaller leaves, which are less distinct,
are transversely elongated, and simulate sporangia in appearance.
Dr Kidston’s figure of this species has recently been reproduced by
Professor Bower[188] who speaks of the supposed smaller leaves as
sporangia, a view with which the author of the species agrees. It would
appear that this identification is, however, based solely on external
resemblance and has not been confirmed by the discovery of any spores.
Assuming the sporangial nature of these structures, this Palaeozoic
type represents, as Bower points out, a condition similar to that in
some recent species of _Lycopodium_ in which sporangia are not confined
to a terminal strobilus but occur also in association with ordinary
foliage leaves. The strobilus consists of crowded sporophylls which are
too imperfect to afford any definite evidence as to their homosporous
or heterosporous nature. As Solms-Laubach[189] points out, this type
recalls _Lycopodium Phlegmaria_ among recent species.
_Lycopodites Reidii_ Penhallow.
Professor Penhallow[190] instituted this name for a specimen measuring
8 cm. long by 6 mm. in breadth, collected by Mr Reid from the Old Red
Sandstone of Caithness, consisting of an axis bearing narrow lanceolate
leaves some of which bear sporangia at the base.
_Lycopodites Gutbieri_ Goeppert[191].
1894, _Lycopodites elongatus_ Kidston[192] (not Goldenberg).
The species, figured by Geinitz as _Lycopodites Gutbieri_[193], from
the Coal-Measures of Saxony is probably a true representative of the
genus. The Saxon specimens are heterophyllous; the larger lanceolate
and slightly falcate leaves arranged in two rows, are 4–5 mm. long
while the smaller leaves are one half or one third this size; some of
the dichotomously branched shoots terminate in long and narrow strobili
not unlike those of Zeiller’s species _Selaginellites Suissei_[194].
Kidston[195] has included under this specific name some fragments
collected by Hemingway from the Upper Coal-Measures of Radstock,
Somersetshire, but as only one form of leaf is seen the reasons for
adopting Goeppert’s designation are perhaps hardly adequate.
_Lycopodites ciliatus_ Kidston[196].
Under this name Kidston describes a small specimen, obtained by
Hemingway from the Middle Coal-Measures of Barnsley in Yorkshire,
consisting of a slender forked axis bearing oval-acuminate leaves
approximately 5 mm. long with a finely ciliate margin. Associated with
the leaves were found spores which Kidston regards as megaspores.
_Lycopodites macrophyllus_ Goldenberg[197].
This species, originally described by Goldenberg from the Coal-Measures
of Saarbrücken has been re-examined by Halle[198] who is unable to
confirm Goldenberg’s statement as to heterophylly. The shoots closely
resemble _Selaginellites primaevus_[199] (Gold).
[Illustration: FIG. 135. _Selaginellites and Lycopodites._ (After
Halle.)
A. _Selaginellites primaevus_ (Gold.). × 10.
B. Megaspore of _Selaginellites elongatus_ (Gold.). × 50.
C. _Lycopodites Zeilleri_ Halle. (Nat. size.)
D. _Selaginellites elongatus_ (Gold.). × 2.]
_Lycopodites Zeilleri_ Halle[200]. Fig. 135, C.
Halle has founded this species on specimens, from the Coal-Measures
of Zwickau in Saxony, characterised by dimorphic lanceolate leaves in
four rows, the larger being 4–6 mm. long: the smaller leaves have a
ciliate edge. A comparison is made with the recent species _Selaginella
arabica_ Baker, _S. revoluta_ Bak., and _S. armata_ Bak. in which the
leaves are described as ciliate. In the absence of sporangia and spores
the species is placed in the genus _Lycopodites_.
_Lycopodites lanceolatus_ (Brodie). Fig. 136.
1845 _Naiadita lanceolata_, Brodie[201].
_Naiadea acuminata_, Buckman[202].
1850 _Naiadea lanceolata_, Buckman[203].
_Naiadea petiolata_, Buckman[204].
1900 _Naiadites acuminatus_, Wickes[205].
1901 _Naiadita lanceolata_, Sollas[206] (figures showing habit of the
plant).
1904 _Lycopodites lanceolatus_, Seward[207] (figure showing habit of the
plant).
[Illustration: FIG. 136. _Lycopodites lanceolatus_ (Brodie). (After
Miss Sollas. × 40.)
_a_, Sporangium wall; _b_, leaf.
_c_, remains of tubular elements in stem.]
Specimens referred to this species were originally recorded by Brodie
from Rhaetic rocks in the Severn valley, the name _Naiadita_ being
chosen as the result of Lindley’s comparison of the small and delicate
leaves with those of recent species of the Monocotyledonous family
Naiadaceae. The species may be described as follows:
Plant slender and moss-like in habit. The axis, which is delicate and
thread-like, bears numerous linear acuminate or narrow ovate leaves
reaching a length of approximately 5 mm. Under a low magnifying power
the thin lamina of the leaves is seen to have a superficial layer of
polygonal or rectangular cells arranged in parallel series (fig. 136
_b_). There is no trace of midrib or stomata. The sporangia are more or
less spherical and short-stalked, situated at the base of the foliage
leaves and containing numerous tetrads of spores. The spores have a
diameter of 0·08 mm.
Buckman founded additional species on differences in the shape of the
leaves but, as Miss Sollas has pointed out, such differences as he
noticed may be detected on the same axis. It was stated in an earlier
chapter[208] that Starkie Gardner, on insufficient evidence, proposed
to place Brodie’s plant among the Mosses. The discovery by Mr Wickes
of new material at Pylle hill near Bristol afforded an opportunity
for a re-examination of the species: this was successfully undertaken
by Miss Sollas who was able to dissolve out spores from the matrix by
dilute hydrochloric acid, and to recognise the remains of internal
structure in the slender axes by exposing successive surfaces with
the aid of a hone. It was found that sporangia occurred at the base
of some of the leaves containing numerous tetrads of spores, the
individual spores having a diameter of 0·08 mm., apparently twice
as large as those of any recent species of _Lycopodium_. Fig. 136
shows a sporangium, _a_, at the base of a leaf, _b_. Indications of
tubular elements were recognised in the stem and it is noteworthy
that although the outlines of epidermal cells on the leaves are well
preserved no stomata were found. The leaves of the recent American
species _Lycopodium alopecuroides_ Linn. var. _aquaticum_ Spring[209],
which lives under water, possess stomata. It is probable that in
_Lycopodites lanceolatus_ the leaves had a very thin lamina and
may have been similar in structure to those of recent Mosses; the
plant possibly lived in very humid situations or grew submerged.
Miss Sollas’s investigations afford a satisfactory demonstration of
the lycopodiaceous nature of this small Rhaetic species: as I have
elsewhere suggested[210], the generic name _Lycopodites_ should be
substituted for that of _Naiadita_. Examples of this species may be
seen in the British Museum.
The Rhaetic species from Scania, _Lycopodites scanicus_ Nath.[211] (_in
litt._), recently re-described by Halle and originally referred by
Nathorst to _Gleichenia_ affords another example of the occurrence of a
small herbaceous lycopod of Rhaetic age.
[Illustration: FIG. 137. _Lycopodites falcatus_ L. and H. From the
Inferior Oolite of Yorkshire. (Nat. size. M.S.)]
_Lycopodites falcatus_ Lind. and Hutt. Fig. 137.
1831 _Lycopodites falcatus_, Lindley and Hutton[212].
1838 _Muscites falcatus_, Sternberg[213].
1870 _Lycopodium falcatum_, Schimper[214].
In 1822 Young and Bird[215] figured a specimen from the Inferior
Oolite rocks of the Yorkshire coast bearing “small round crowded
leaves,” which was afterwards described by Lindley from additional
material obtained from Cloughton near Scarborough as _Lycopodites
falcatus_. The example represented in fig. 137 shows the dichotomously
branched shoots bearing two rows of broadly falcate leaves. A careful
examination of the type-specimen[216] revealed traces of what
appeared to be smaller leaves, but there is no satisfactory proof of
heterophylly. No sporangia or spores have been found. This British
species has been recorded from Lower Jurassic or Rhaetic rocks of
Bornholm[217] and a similar though probably not identical type,
_Lycopodites Victoriae_[218], has been recognised in Jurassic strata
of Australia (South Gippsland, Victoria). An Indian plant described
by Oldham and Morris[219] from the Jurassic flora of the Rajmahal
hills as _Araucarites_ (?) _gracilis_ and subsequently transferred by
Feistmantel to Schimper’s genus _Cheirolepis_[220] may be identical
with the Yorkshire species. The Jurassic fragments described by Heer
from Siberia as _Lycopodites tenerrimus_[221] may be lycopodiaceous,
but they are of no botanical interest.
Other examples of Mesozoic Lycopods have been recorded, but in the
absence of well-preserved shoots and sporangia they are noteworthy only
as pointing to a wide distribution of _Lycopodites_ in Jurassic and
Cretaceous floras[222].
From Tertiary strata species of supposed herbaceous lycopods have been
figured by several authors, one of the best of which is _Selaginella
Berthoudi_ Lesq.[223] from Tertiary beds in Colorado. This species
agrees very closely in the two forms of leaf with _Selaginella
grandis_, but as the specimens are sterile we have not sufficient
justification for the employment of the generic name _Selaginellites_.
Selaginellites.
This generic name has been instituted by Zeiller[224] for specimens
from the coal basis of Blanzy (France). It is applied to heterosporous
species with the habit of _Selaginella_: Zeiller preferred the
designation _Selaginellites_ to _Selaginella_ on the ground that
the type species differs from recent forms in having more than four
megaspores in each megasporangium. It is, however, convenient to extend
the term to all heterosporous fossil species irrespective of the
spore-output.
_Selaginellites Suissei_ Zeiller.
This species was described in Zeiller’s preliminary note[225] as
_Lycopodites Suissei_, but he afterwards transferred it to the genus
_Selaginellites_. In habit the plant bears a close resemblance
to _Lycopodites macrophyllus_ of Goldenberg; the shoots, 1–3 mm.
thick, are branched in a more or less dichotomous fashion and bear
tetrastichous leaves. The larger leaves reach a length of 4–6 mm.
and a breadth of 2–3 mm.; the smaller leaves are described as almost
invisible, closely applied to the axis, oval-lanceolate and 1–2 mm.
long with a breadth of 0·5–0·75 mm. Long and narrow strobili (15
cm. by 8–10 mm.) terminate the fertile branches; these bear crowded
sporophylls with a triangular lamina and finely denticulate margin.
Oval sporangia were found on the lower sporophylls containing 16–24
spherical megaspores 0·6–0·65 mm. in diameter. The outer membrane
of the spore is characterised by fine anastomosing ridges and thin
plates radiating from the apex and forming an equatorial collarette.
The microspores have a diameter of 40–60μ and the same type of outer
membrane as in the megaspores. The megaspores of the recent species
_Selaginella caulescens_, as figured by Bennie and Kidston[226],
resemble those of the Palaeozoic type in the presence of an equatorial
flange. It is interesting to find that, in spite of the occurrence of
16–24 megaspores in a single sporangium the size of the fossil spores
exceeds that of the recent species.
_Selaginellites primaevus_ (Gold.). Fig. 135, A, fig. 138.
1855 _Lycopodites primaevus_, Goldenberg[227].
1870 _Lycopodium primaevum_, Schimper[228].
1907 _Selaginellites primaevus_, Halle[229].
[Illustration: FIG. 138. _Selaginellites primaevus_ (Gold.). (After
Goldenberg.)]
In habit this species, first recorded by Goldenberg from the
Coal-Measures of Saarbrücken, is similar to _S. Suissei_ Zeill.
The drawing reproduced in fig. 138 is a copy of that of the
type-specimen: another specimen, named by Goldenberg, is figured
by Halle in his recently published paper. The leaves appear to be
distichous: no smaller leaves have been detected, though Halle is
inclined to regard the plant as heterophyllous. The sporophylls, borne
in slender terminal strobili, are smaller than the foliage leaves and
spirally disposed (fig. 138; smaller specimen). Halle succeeded in
demonstrating that some of the sporangia contained a single tetrad of
spores, each spore having a diameter of 0·4–0·5 mm. No microspores
were found, but it is clear that the species was heterosporous and
that it agrees with recent species in having only four spores in the
megasporangium.
_Selaginellites elongatus_ (Gold.). Fig. 135, B, D.
1855 _Lycopodites elongatus_, Goldenberg[230].
1870 _Lycopodium elongatum_, Schimper[231].
The shoots of this species resemble the recent _Lycopodium
complanatum_; they differ from those of _Selaginellites primaevus_ in
their long and narrow branches which bear two forms of leaf. The longer
leaves, arranged in opposite pairs, are slightly falcate; the smaller
leaves are appressed to the axis and have a triangular cordate lamina.
Another peculiarity of this species is the occurrence of sporangia in
the axil of the foliage leaves, a feature characteristic of the recent
_Lycopodium Selago_. In recent species of _Selaginella_ the sporophylls
are always in strobili. No microspores have been found nor the walls
of megasporangia, but tetrads of megaspores were isolated by Halle:
the spores have three radiating ridges (fig. 135, B) connected by
an equatorial ridge. Halle estimates the number of spores (0·45 mm.
in diameter) in a sporangium at 20 to 30. In size as in number the
spores exceed those of recent species and agree more nearly with the
megaspores of _S. Suissei_.
It would seem to be a general rule that the spores (megaspores) of the
fossil herbaceous species exceeded considerably in dimensions those
of recent forms and on the other hand were smaller than those of the
Palaeozoic arborescent species.
There can be little doubt that some of the Mesozoic and Tertiary
species included under _Lycopodites_ agree more closely with the
recent genus _Selaginella_ than with _Lycopodium_, but this does
not constitute an argument of any importance against the restricted
use of the designation _Selaginellites_ which we have adopted. From
a botanical point of view the various records of _Lycopodites_ and
_Selaginellites_ have but a minor importance; they are not sufficiently
numerous to throw any light on questions of distribution in former
periods, nor is the preservation of the material such as to enable us
to compare the fossil with recent types either as regards their anatomy
or, except in a few cases, their sporangia and spores. The Palaeozoic
species are interesting as revealing less reduction in the number of
spores produced in the megasporangia. Among existing Pteridophytes the
genus _Isoetes_ agrees more closely than _Selaginella_, as regards
the number of megaspores in each sporangium, with such fossils as
_Selaginellites Suissei_ and _S. elongatus_.
It would seem that in most Palaeozoic species heterospory had
not reached the same stage of development as in the recent genus
_Selaginella_ in which the megaspores do not exceed four in each
sporangium. In _Selaginellites primaevus_, however, the heterospory
appears to be precisely of the same type as in existing species.
Lycostrobus.
The generic name _Lycostrobus_ has recently been instituted by
Nathorst[232] for certain specimens of a lycopodiaceous strobilus, from
the Rhaetic strata of Scania, which he formerly referred to the genus
_Androstrobus_[233].
_Lycostrobus Scotti_ Nathorst. Fig. 139.
The fossil described under this name is of special interest as
affording an example of a Mesozoic lycopodiaceous cone comparable in
habit and in size with some of the largest examples of Palaeozoic
Lepidostrobi, the cones of _Lepidodendron_. The Swedish fossil from
Upper Rhaetic strata of Helsingborg (Scania) was originally designated
_Androstrobus Scotti_, the generic name being adopted in view of the
close resemblance of the form of the strobilus to the male flower of
a Cycad. A more complete examination has shown that the bodies, which
were thought to be pollen-sacs—though Nathorst recognised certain
differences between them and the pollen-sacs of lycopods—are the
megaspores of a lycopod. Microspores have also been identified. The
axis of the cone has a breadth of 2 cm. with a peduncle which may be
naked or provided with a few small scales; the sporophyll region of
the axis reached a length of at least 12 cm. The spirally disposed
sporophylls terminate in a rhombic distal end which may represent
the original termination or they may have been prolonged upwards as
free laminae. Each sporophyll bears on its upper face a single large
sporangium containing either megaspores or microspores: the megaspores,
0·55–0·60 mm. in diameter, are finely granulate and bear small warty
thorns or more slender pointed appendages. The microspores, after
treatment with eau de Javelle, were found to measure 36–44μ while
others which had been treated with ammonia reached 54μ in diameter.
Nathorst describes the microspores as occurring in spherical groups
or balls, which it is suggested may be compared with the groups of
spores separated by strands of sterile tissue (trabeculae) in the
large sporangia of _Isoetes_ (cf. fig. 133, H). If this comparison is
sound it would point to a more complete septation of the sporangium in
_Lycostrobus_ than in any recent species of _Isoetes_. The size of the
strobilus would seem to indicate the persistence into the Rhaetic era
of an arborescent lycopodiaceous type; but the appearance and manner
of preservation of the axis is interpreted by Nathorst as evidence of
a herbaceous rather than a woody structure. He is disposed to regard
_Isoetes_ as the most nearly allied existing genus.
[Illustration: FIG. 139. _Lycostrobus Scotti_, Nath. (After Nathorst;
⅘ nat. size.)]
The comparison made by Nathorst with _Isoetes_ is based on a
resemblance between the spores of the two genera and on the evidence,
which is not decisive, of the existence of sterile strands of tissue
in the sporangia of _Lycostrobus_. This similarity is however hardly
of sufficient importance to justify the inclusion of the Rhaetic
strobilus in the Isoetaceae. In size and in the arrangement and form
of the sporophylls the cone presents a much closer resemblance to
_Lepidodendron_ than to _Isoetes_. It is probably advisable to regard
this Rhaetic type simply as a lycopodiaceous genus which we are unable,
without additional information, to assign to a particular position.
The opinion expressed by Professor Fliche[234] that the plant described
by Schimper and Mougeot as _Caulopteris tessellata_, a supposed
tree-fern stem, from Triassic rocks of Lorraine, is more probably a
large lycopodiaceous stem, either a _Lepidodendron_ or a new genus, is
worthy of note in reference to Nathorst’s account of _Lycostrobus_.
In habit the fossil strobilus may be compared with the Triassic genus
_Pleuromeia_, but the position of the sporangia on the sporophylls
constitutes a well-marked difference. The most important result of
Nathorst’s skillful treatment of this interesting fossil by chemical
microscopic methods is the demonstration of the existence of a large
heterosporous type of lycopodiaceous cone in a Rhaetic flora.
Poecilitostachys.
Under this generic name M. Fliche[235] has briefly described a fertile
lycopodiaceous shoot from the Triassic rocks of Epinal in France:
the type species _Poecilitostachys Hangi_ consists of a cylindrical
axis, 10 cm. × 5 mm., deprived of leaves and terminating in a rounded
receptacle bearing a capitulum of bracts or fertile leaves. Detached
megasporangia containing small globular bodies found in association
with the capitulum are compared with the megasporangia of _Isoetes_.