Chapter XIV: Lycopodiales (2)

_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.

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).

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).

_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.

_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].

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.

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_.