Chapter IX: PTERIDOPHYTA (Vascular Cryptogams) (1)

The Pteridophytes include plants which vary in size from a few millimetres[482] to several metres in height. The spore on germination gives rise to a small thalloid structure, the _prothallium_, on which the sexual organs are developed; this is the _gametophyte_ or sexual generation. The sexual organs have the form of typical archegonia and antheridia. From the fertilised egg-cell there is developed the Pteridophyte plant or _sporophyte_, which bears the spores. This asexual generation shows a well-marked external differentiation into stem and leaves, and bears true roots. Internally the tissues exhibit a high degree of differentiation into distinct tissue-systems. True vascular bundles occur, which may or may not be capable of secondary thickening by means of a _cambium_, _i.e._ a definitely localised zone of meristematic tissue. The sporangia are borne either on the ordinary foliage leaves or on special spore-bearing leaves called _sporophylls_, which differ in a greater or less degree from the sterile leaves.

The majority of the best known and most important Palaeozoic genera are either true Vascular Cryptogams, or possess certain of the pteridophytic characteristics combined with those of higher plants. It is not merely the commoner and more familiar recent genera with which the student of extinct types must be acquainted, but it is extremely important that he should make himself familiar with the rarer, less known and more isolated recent forms, which often throw most light on the affinities of the older representatives of the group. It is often the case, the more isolated living plants are, the more likely are they to afford valuable assistance in the interpretation of genera representing a class, which reached its maximum development in the earlier periods of the earth’s history. The importance of paying special attention to such recent plants as may be looked upon as survivals of a class now tending towards extinction, will be more thoroughly realised after the extinct vascular cryptogams have been dealt with.

A comparison of the Pteridophyta and Bryophyta brings out certain points of divergence. In the first place, the sporophyte assumes in the former class a much more prominent rôle, and the gametophyte has suffered very considerable reduction. The gametophyte, _i.e._ the structure which is formed on the germination of the asexually-produced spore, is usually short-lived, small, and more or less dependent on the sporophyte for its nutrition. In a few cases only is it capable of providing itself with the essential elements of food. On the other hand, the sporophyte, at a very early stage of its development becomes free from the gametophyte and is entirely self-supporting. Reproduction is effected as in the Bryophyta by sexual reproductive organs and by asexual methods. Not only have we in the Pteridophytes a much more complete external division of the plant-body into definite members, which subserve distinct functions, and behave as well-defined physiological organs adapted for taking a certain share in the life-functions of the individual, but the internal differentiation has reached a much higher stage. True vascular tissue, consisting of xylem and phloem, occurs for the first time in this class. The whole plant is traversed by one or more vascular strands composed of xylem and phloem elements, which are respectively concerned with the distribution of inorganic and organic food substances.

The Pteridophyta include the most important fossil plants. It is from a study of the internal structure of various extinct representatives of this class, that palaeobotanists have been able to contribute facts of the greatest interest and importance towards the advancement of botanical science.

The botanist’s chief aim in the anatomical investigation of Palaeozoic genera is to discover data which point the way to a solution of the problems of plant-evolution. In the abundant material afforded by the petrified remnants of ancient floras we have the means of tracing the past history of existing groups or individual forms, and it is from the Palaeozoic Pteridophytes that our most valuable results have been so far obtained.

In this and the following chapters of Volume I. two divisions of the Pteridophyta are dealt with in such detail as the nature of the book allows. In the earlier chapters of Volume II. the remaining representatives of this class will be described. As in the preceding chapters such recent plants will be described as are most essential for the correct interpretation of the fossil forms.

It is impossible to do more than confine our attention to a few only of the genera of living plants which directly concern us; some acquaintance with the general facts of plant morphology must be assumed. Among the most useful text-books or books of reference on the Pteridophyta the student may consult those mentioned in the footnote[483].

I. EQUISETALES.

Leaves usually small in proportion to the size of the whole plant, arranged in whorls at the nodes. Sporangia borne on specially modified sporophylls or sporangiophores, which are aggregated to form a definite strobilus or spore-bearing cone.

EQUISETACEAE. (Recent Species.)

The leaves are in whorls, coherent in the form of a sheath, and traversed by longitudinal veins which do not fork or anastomose. The stem is divided into comparatively long internodes separated by the leaf-bearing nodes, and the branches arise in the leaf-axils at the nodes. The fertile leaves or sporophylls differ from the sterile leaves, and usually occur in definite aggregations or strobili containing spores of one kind (_isosporous_). In the single living genus _Equisetum_, the outer coat of the mature spore forms two hygroscopically sensitive filamentous structures or _elaters_. On the germination of the spore the gametophyte is developed in the form of a small lobed prothallium 1–2 cm. in length. In most cases there are distinct male and female prothallia.

The genus _Equisetum_ L., the common Horse-tail, is the sole living representative of this Family. It occurs as a common native plant in Britain, and has a wide geographical distribution. Species of _Equisetum_ are abundant in the temperate zones of both hemispheres, and occur in arctic as well as tropical latitudes. Wallace[484] speaks of Horse-tails, “very like our own species,” growing at a height of 5000 feet on the Pangerango mountain in Java. In favourable situations the large British Horse-tail, _Equisetum maximum_ Lam. (= _E. Telmateia_ Erhb.), occasionally reaches a height of about six feet, and growing in thick clusters forms miniature forests of trees with slender erect stems and regular circles of long and thin branches. A tropical species, _Equisetum giganteum_ Linn.[2] living in the marshes of Mexico and Cuba[485], and extending southward to Buenos Ayres and Chili, reaches a height of twenty to forty feet, but the stem always remains slender, and does not exceed an inch in diameter. Groves of such tall slender plants on the eastern slopes of the Andes[486] suggest to the palaeobotanist an enfeebled forest-growth recalling the arborescent Calamites of a Palaeozoic vegetation. The twenty-five existing species of _Equisetum_ are remnants of various generic types of former epochs, and possess a special interest from the point of view of the geological history of plants. A brief description of the main characters of the recent genus will enable the student to appreciate the points of difference and agreement between the extinct and present representatives of the Equisetales.

. B. Sporophyll bearing open sporangia (after
Luerssen; slightly enlarged). C. Part of a transverse section
(diagrammatic); _v_, vallecular canals, _e_, endodermis, _c_,
carinal canals (after Luerssen; × 20). D. _Equisetum arvense_ L.
Part of a transverse section of an internode of a sterile shoot.
_v_, cortex, _e_, endodermis, _x_, xylem tracheids, _a_ remains
of annular tracheids of the protoxylem, _c_, carinal canal (after
Strasburger; × 90).]

_Equisetum._

The plant consists of a perennial underground creeping rhizome, branching into secondary rhizomes, divided into well-marked nodes and internodes. From the nodes are given off two sets of buds, which may develope into ascending aerial shoots or descending roots. At each node is a leaf-sheath more or less deeply divided along the upper margin into teeth representing the tips of coherent leaves (fig. 52, A).

In some species one or more internodes of underground branches become considerably swollen and assume the form of ovate or elliptical starch-storing tubers, which are capable of giving rise to new plants by vegetative reproduction. Tubers, either singly or in chains, occur in _E. arvense_ Linn., _E. sylvaticum_ Linn., _E. maximum_ Lam., among British species.

.]

In the example shown in fig. 53 (_Equisetum palustre_ L.[487]) the stout rhizome R gives off from its node, marked by a small and irregular leaf-sheath, two thin roots and a single shoot. The latter has a leaf-sheath at its base, and from the second node, with a larger leaf-sheath, there have been developed branches with tuberous internodes; the constrictions between the tubers and the tips of the terminal tubers bear small leaf-sheaths. Branched roots are also given off from the upper node of the erect shoot.

Near the surface of the ground the buds on the rhizome nodes develope into green erect shoots. The shoot axis is marked out into long internodes separated by nodes bearing the leaf-sheaths. The surface of each internode is traversed by regular and more or less prominent longitudinal ridges and grooves; each ridge marking the position of an internal longitudinal vascular strand. In the axil of each leaf, that is in the axil of each portion of a leaf-sheath corresponding to a marginal uni-nerved tooth, there is produced a lateral bud which may either remain dormant or break through the leaf-sheath and emerge as a lateral branch. At the base of each branch an adventitious root may be formed from a cell immediately below the first leaf-sheath, but in aerial shoots the roots usually remain undeveloped. The lateral branches repeat on a smaller scale the general features of the main axis. In some species, the shoots are unbranched, and in others the slender branches arise in crowded whorls from each node. Leaves, roots and branches are given off in whorls, and the whorls from each node alternate with those from the node next above and next below.

[Sidenote: ANATOMY OF EQUISETUM.]

In some species of _Equisetum_ the aerial stem terminates in a conical group of sporophylls, while in others the strobilus is formed at the apex of a pale-coloured fertile shoot, which never attains any considerable length and dies down early in the season of growth (fig. 52, A). Below the terminal cone or strobilus there occur one or two modified leaf-sheaths. Such a ring of incompletely developed leaves intervening between the cone of sporangiophores and the normal leaves, is known as the _annulus_. The annulus is seen in fig. 52, A, immediately below the lowest whorl of sporophylls; it has the form of a low sheath with a ragged margin. In the region of the cone the internodes remain shorter, and the whorls of appendages, known as sporophylls or sporangiophores, have the form of stalked structures terminating distally in a hexagonal peltate disc, which bears on its inner face a ring of five to ten oval sporangia (fig. 52, B). Each sporangium contains numerous spores which eventually escape by the longitudinal dehiscence of the sporangial wall. The opening of the sporangia is probably assisted by the movements of the characteristic elaters formed from the outer wall of each spore.

The spores, which are capable of living only a short time, grow into aerial green prothallia, 1–2 cm. in length; these have the form of irregularly and more or less deeply lobed structures. On the larger and more deeply lobed prothallia the archegonia or female reproductive organs are borne, and the smaller or male prothallia bear the antheridia. On the fertilisation of an egg-cell, the _Equisetum_ plant is gradually developed. For a short time parasitic on the female prothallium or gametophyte, the young plant soon takes root in the ground and becomes completely independent.

As seen in transverse section through a young stem near the apex, the axis consists of a mass of parenchyma, in which may be distinguished a central larger-celled tissue, surrounded by a ring of smaller-celled groups marking the position of a circle of embryonic vascular strands. In each young vascular strand, a few of the cells next the pith may be seen to have thicker walls and to be provided with a ring-like internal thickening; these have passed over into the condition of annular tracheids and represent the _protoxylem_ elements. At a later stage, a transverse section through the stem shows a central hollow pith, formed by the tearing apart and subsequent disappearance of the medullary parenchymatous cells, which were unable to keep pace with the growth in thickness of the stem. The pith cavity is bridged across at each node by a multi-layered plate of parenchyma, which forms the so-called nodal _diaphragm_. The inner edge of each vascular strand is now found to be occupied by a small irregularly circular canal (fig. 52, C, c, and D, c) in which may be seen some of the rings of protoxylem tracheids (D, a) which have been torn apart and almost completely destroyed. These canals, known as _carinal canals_, have arisen by the tearing and disruption of the thin-walled cells in the immediate neighbourhood of the protoxylem. Each carinal canal is bounded by a layer of elongated parenchymatous cells which form part of the xylem of the vascular bundle, and is succeeded internally by the general ground-tissue of the stem. The xylem parenchyma next a carinal canal is succeeded externally by phloem tissue, consisting of short protoplasmic cells and longer elements, without nuclei and poor in contents; the latter may be regarded as sieve-tubes. On either side of the phloem, the xylem occurs in two separate bands or groups of annular and reticulately thickened tracheids. In some species, _e.g._ _Equisetum xylochaetum_ Metten.[488] and _E. giganteum_[489] L. a native of South America, the xylem has the form of two bands composed of fairly numerous tracheids, but in most species the xylem tracheids occur in small groups, as shown in the figure of _E. maximum_ (fig. 52, D). In the shape of the vascular bundle, and in the formation of the carinal canal, there is a distinct resemblance between the vascular bundles of _Equisetum_ and those of a monocotyledonous stem. These collateral stem-bundles of xylem and phloem traverse each internode as distinct strands, and at the nodes each strand forks into two branches (fig. 54, A), which anastomose with the alternating bundles passing into the stem from the leaf-sheath. Thus the vascular strands of each internode alternate in position with those of the next internode.

There are certain points connected with the vascular bundles in the nodal region of a shoot, which have an important bearing on the structure of fossil equisetaceous stems. Fig. 54 B represents a diagrammatic longitudinal section through the node of a rhizome of _Equisetum arvense_ from which a root _h_ is passing off in a downward direction, and a branch in an upward direction. The black band _c_ in the parent stem shows the position of the vascular strands; in the region of the node the vascular tissue attains a considerable thickness, as seen at _d_ in the figure. The bands passing out to the left from _d_ go to supply the branch and root respectively. The increased breadth of the xylem strands at the node is due to the intercalation of a number of short tracheids. Fig. 55, 4 shows a transverse section through a mature node of _Equisetum maximum_; _px_ marks the position of the protoxylem and _e_ that of the endodermis. On comparing this section with that of the internodal vascular bundle in fig. 52, D, the much greater development of wood in the former is obvious; the carinal canal of the internodal bundle is absent in the section through a node. The disposition of the xylem tracheids in fig. 55, 4 shows a certain regularity which, though not very well marked, suggests the development of wood elements as the result of cambial activity. Longitudinal sections through the nodal region demonstrate the existence of “cells similar to those of an ordinary cambium, and a cell-formation resulting from their division which is similar to that in an ordinary secondary thickening.”[490] The short tracheids which make up this nodal mass of xylem differ from those in the internodal bundle in their smaller size, and in being reticulately thickened. There is, therefore, evidence that in the nodes of some _Equisetum_ stems additional xylem elements are produced by a method of growth comparable with the cambial activity which brings about the growth in thickness of a forest-tree[491]. The significance of these statements will be realised when the structure of the extinct genus _Calamites_ is described and compared with that of _Equisetum_.

The small drawing in fig. 55, 3 shows part of the ring of thick nodal wood; the section cuts through two bundles about their point of bifurcation, the strand _x_ is passing out in a radial direction to a lateral branch, the strand to the right of _x_ and the separate fragment of a strand to the left of _x_ are portions of leaf-trace bundles on their way to the leaf-sheath. Reverting to fig. 54, B, the other structures seen in the section are the leaf-sheaths (_l_ and _m_), the vallecular canal (_f_), the epidermis, cortex and pith (_k_, _e_ and _a_) of the stem. The epidermis which has been ruptured by the root and branch is indicated at _i_, _i_; the dotted lines traversing the upper part of the pith of the lateral branch mark the position of a nodal diaphragm.

Immediately external to each vascular strand, as seen in transverse section, there is a layer of cells containing starch, and this is followed by a distinct endodermis, of which the cells show the characteristic black dot in the cuticularised radial walls (fig. 52, D). Beyond the endodermis there is the large-celled parenchyma of the rest of the cortex. Tannin cells occur here and there scattered among the ground tissue. On the same radius on which each vascular strand occurs, the cortical parenchyma passes into a mass of sub-epidermal thick-walled mechanical tissue or stereome. Alternating with the ridges of stereome, the grooves are occupied by thin-walled chlorophyll-containing tissue which carries on most of the assimilating functions, and communicates with the external atmosphere by means of stomata arranged in vertical rows down each internode. The continuity of the cortical tissue is interrupted by the occurrence of large longitudinal _vallecular canals_ alternating in position with the stem ridges and vascular strands (fig. 52, C, _v_). The epidermis consists of a single layer of cells, containing stomata, and with the outer cell-walls impregnated with silica.

In certain species of _Equisetum_, _e.g._ _E. palustre_ L., the whole circle of vascular strands is enclosed by an endodermis, and has the structure typical of a monostelic stem. In others _e.g._ _E. litorale_ Kühl. each vascular strand is surrounded by a separate endodermis, and in some forms _e.g._ _E. sylvaticum_ L. there is an inner as well as an outer endodermal layer[492]. Without discussing the explanation given to this variation in the occurrence of the endodermis, it may be stated that in all species of _Equisetum_ the stem may be regarded as monostelic[493].

In the rhizome the structure agrees in the main with that of the green shoots, but the vallecular canals attain a larger size, and the pith is solid. A slightly enlarged transverse section of a rhizome of _Equisetum maximum_ is shown in fig. 55, 2, the small circles surrounding the pith mark the position of the vascular bundles and carinal canals; the much larger spaces between the central cylinder and the surface of the stem are the vallecular canals.

The central cylinder or stele of the root is of the diarch, triarch or tetrarch type; _i.e._ there may be 2, 3 or 4 groups of protoxylem in the xylem of the root stele. The axial portion is occupied by large tracheids, and the smaller tracheids of the xylem occur as radially disposed groups, alternating with groups of phloem. External to the xylem and phloem strands there occur two layers of cells, usually spoken of as a double endodermis, but it has been suggested that it is preferable to describe the double layer as the _phloeoterma_[494], of which the inner layer has the functions of a pericycle, and the outer that of an endodermis. A transverse section of a root is seen in fig. 55, 1, the dark cells on the left are part of a thick band of sclerenchyma in the cortex of the root, the layer e is the outer layer of the phloeoterma.

Without describing in detail the development[495] of the sporangia, it should be noted that the sporangial wall is at first 3 to 4 cells thick, but it eventually consists of a single layer. The cells have spiral thickening bands on the ventral surface, and rings on the cells where the longitudinal splitting takes place. Each sporangium is supplied by a vascular bundle which is given off from that of the sporangiophore axis. The strobili are isosporous.

╭ A. EQUISETITES.
│ B. PHYLLOTHECA.
II. FOSSIL EQUISETALES. ┤ C. SCHIZONEURA.
│ D. CALAMITES.
╰ E. ARCHAEOCALAMITES.

In dealing with the fossil Equisetales, we will first consider the genera _Equisetites_, _Phyllotheca_ and _Schizoneura_, and afterwards describe the older and better known genera _Calamites_ and _Archaeocalamites_. A thoroughly satisfactory classification of the members of the Equisetales is practically impossible without more data than we at present possess. It has been the custom to include _Equisetites_, _Phyllotheca_ and _Schizoneura_ in the family Equisetaceae, and to refer _Calamites_ and _Archaeocalamites_ to the Calamarieae; such a division rests in part on assumption, and cannot be considered final. When we attempt to define the Equisetales and the two families Equisetaceae and Calamarieae, we find ourselves seriously hampered by lack of knowledge of certain important characters, which should be taken into account in framing diagnoses. There is little harm in retaining provisionally the two families already referred to, if we do not allow a purely arbitrary classification to prejudice our opinions as to the affinities of the several members of the Equisetales.

The Equisetaceae might be defined as a family including plants which were usually herbaceous but in some cases arborescent, bearing verticils of leaves in the form of sheaths more or less deeply divided into segments or teeth. The strobili were isosporous and consisted of a central axis bearing verticils of distally expanded sporophylls with sporangia, as in _Equisetum_. The genus _Equisetites_ might be included in this family, but it must be admitted that we know next to nothing as to its anatomy, and we cannot be sure that the strobili were always isosporous.

The genus _Schizoneura_ is too imperfectly known to be defined with any approach to completeness, or to be assigned to a family defined within certain prescribed limits. _Phyllotheca_ is another genus about which we possess but little satisfactory knowledge; we are still without evidence as to its structure, and the descriptions of the few strobili that are known are not consistent. Recent work points to a probability of _Phyllotheca_ being closely allied to _Annularia_, a genus included in the Calamarieae, and standing for a certain type of Calamitean foliage-shoots.

In comparing the Calamarieae with the Equisetaceae, the alternation of sterile and fertile whorls in the strobilus, and the free linear leaves at the nodes instead of leaf-sheaths are two characters made use of as distinguishing features of the genus _Calamites_ as the type of the Calamarieae. On the other hand, the strobili of _Phyllotheca_ appear to agree with those of _Calamites_ rather than with those of _Equisetum_, and strobili of _Archaeocalamites_ have been found exhibiting the typical _Equisetum_ characters. The sheath-like form of the leaves is not necessarily peculiar to the Equisetaceae, and we have evidence that leaf-sheaths occurred on the nodes of Calamitean plants. In _Archaeocalamites_ the leaves possess characteristic features, and can hardly be said to agree more closely with those of Calamites than with the leaves of _Phyllotheca_ or _Sphenophyllum_, a genus belonging to another class of Pteridophytes.

On the whole, then, without discussing further the possibilities of a subdivision of the Equisetales, we may regard the genera _Calamites_, _Archaeocalamites_, _Equisetites_, _Equisetum_, _Phyllotheca_ and _Schizoneura_ as so many members of the Equisetales, without insisting on a classification which cannot be supported by satisfactory evidence.

Our knowledge of _Calamites_ is fairly complete. Abundant and well-preserved material from the Coal-Measures of England, and from Permo-Carboniferous rocks of France, Germany and elsewhere, has enabled palaeobotanists to investigate the anatomical characters of both the vegetative and reproductive structures of this genus. We are in a position to give a detailed diagnosis of Calamitean stems, roots and strobili, and to determine the place of this type of plant in a system of classification. _Calamites_ not only illustrates the possibilities of palaeobotanical research, but it demonstrates the importance of fossil forms as foundations on which to construct the most rational classification of existing plants. The close alliance between _Calamites_ and the recent Equisetaceae has been clearly established, and certain characteristics of the former genus render necessary an extension and modification of the definition of the class to which both _Calamites_ and _Equisetites_ belong. The Calamites broaden our conception of the Equisetaceous alliance, and by their resemblance to other extinct Palaeozoic types they furnish us with important links towards a phylogenetic series, which the other members of the Equisetales do not supply.

From the Upper Devonian to the Permian epoch _Calamites_ and other closely related types played a prominent part in the vegetation of the world. We have no good evidence for the existence of _Calamites_ in Triassic times; in its place there were gigantic Equisetums which resembled modern Horse-tails in a remarkable degree. In the succeeding Jurassic period tree-like Equisetums were still in existence, and species of _Equisetites_ are met with in rocks of this age in nearly all parts of the world. A few widely distributed species are known from Wealden rocks, but as we ascend the geologic series from the Jurassic strata, the Equisetums become less numerous and the individual plants gradually assume proportions practically identical with those of existing forms.

A. _Equisetites._

The generic name _Equisetites_ was proposed by Sternberg in 1838[496] as a convenient designation for fossil stems bearing a close resemblance to recent species of _Equisetum_. Some authors have preferred to apply the name _Equisetum_ to fossil and recent species alike, but in spite of the apparent identity in the external characters of the fossil stems with those of existing Horse-tails, and a close similarity as regards the cones, there are certain reasons for retaining Sternberg’s generic name. It is important to avoid such nomenclature as might appear to express more than the facts admit. If the custom of adding the termination -_ites_ to the root of a recent generic term is generally followed, it at once serves to show that the plants so named are fossil and not recent species. Moreover, in the case of fossil Equisetums we know nothing of their internal structure, and our comparisons are limited to external characters. Stems, cones, tubers, and leaves are often very well preserved as sandstone casts with distinct surface-markings, but we are still in want of petrified specimens. There is indeed evidence that some of the Triassic and Jurassic species of _Equisetites_, like the older Calamites, possessed the power of secondary growth in thickness, but our deductions are based solely on external characters.

In the following pages a few of the better known species of _Equisetites_ are briefly described, the examples being chosen partly with a view to illustrate the geological history of the genus, and partly to contribute something towards a fuller knowledge of particular species. One of the most striking facts to be gleaned from a general survey of the past history of the Equisetaceae is the persistence since the latter part of the Palaeozoic period of that type of plant which is represented by existing Equisetums. There is perhaps no genus in existence which illustrates more vividly than _Equisetum_ the survival of an extremely ancient group, which is represented to-day by numerous and widely spread species. The Equisetaceous characteristics mark an isolated division of existing Vascular Cryptogams, and without reference to extinct types it is practically impossible to do more than vaguely guess at the genealogical connections of the family. When we go back to Palaeozoic plants there are indications of guiding lines which point the way to connecting branches between the older Equisetales and other classes of Pteridophytes. The recently discovered genus _Cheirostrobus_[497] is especially important from this point of view.

[Sidenote: LEAF-SHEATHS OF EQUISETITES.]

The accurate description of species, and the determination of the value of such differences as are exhibited in the surface characters of structureless casts, are practically impossible in many of the fossil forms. In certain living Horse-tails we find striking differences between fertile and sterile shoots, and between branches of different orders. The isolated occurrence of fragments of fossil stems often leads to an artificial separation of ‘species’ largely founded on differences in diameter, or on slight variations in the form of the leaf-sheaths. It is wiser to admit that in many cases we are without the means of accurate diagnosis, and that the specific names applied to fossil Equisetums do not always possess much value as criteria of taxonomic differences.

The specimens of fossil Equisetums are usually readily recognised by the coherent leaf-segments in the form of nodal sheaths resembling those of recent species. The tissues of the cortex and central cylinder are occasionally represented by a thin layer of coal pressed on to the surface of a sandstone cast, or covering a flattened stem-impression on a piece of shale. It is sometimes possible under the microscope to recognise on the carbonised epidermal tissues the remains of a surface-ornamentation similar to that in recent species, which is due to the occurrence of siliceous patches on the superficial cells. Longitudinal rows of stomata may also be detected under favourable conditions of preservation. The nodal diaphragms of stems have occasionally been preserved apart, but such circular and radially-striated bodies may be misleading if found as isolated objects. Casts of the wide hollow pith of _Equisetites_, with longitudinal ridges and grooves, and fairly deep nodal constrictions, have often been mistaken for the medullary casts of _Calamites_.

Several species of _Equisetites_ have been recorded from the Upper Coal-Measures and overlying Permian rocks, but these present special difficulties. In one instance described below, (_Equisetites Hemingwayi_ Kidst.), the species was founded on a cast of what appeared to be a strobilus made up of sporophylls similar to those in an _Equisetum_ cone. In other Permo-Carboniferous species the choice of the generic name _Equisetites_ has been determined by the occurrence of leaf-sheaths either isolated or attached to the node of a stem. The question to consider is, how far may the Equisetum-like leaf-sheath be regarded as a characteristic feature of _Equisetites_ as distinct from _Calamites_? In the genus _Calamites_ the leaves are generally described as simple linear leaves arranged in a whorl at the nodes, but not coherent in the form of a sheath (fig. 85). The fusion of the segments into a continuous sheath or collar is regarded as a distinguishing characteristic of _Equisetites_ and _Equisetum_. The typical leaf-sheath of a recent Horse-tail has already been described. In some species we have fairly large and persistent free teeth on the upper margin of the leaf-sheath, but in other Equisetums the rim of the sheath is practically straight and has a truncated appearance, the distal ends of the segments being separated from one another by very slight depressions, as in a portion of the sheath of _Equisetum ramosissimum_ Desf. of fig. 58, _C_. In other leaf-sheaths of this species there are delicate and pointed teeth adherent to the margin of the coherent segments; the teeth are deciduous, and after they have fallen the sheath presents a truncated appearance. This difference between the sheaths to which the teeth are still attached and those from which they have fallen is illustrated by fig. 58, _B_ and _C_; it is one which should be borne in mind in the description of fossil species, and has probably been responsible for erroneous specific diagnoses. In some recent Horse-tails the sheath is occasionally divided in one or two places by a slit reaching to the base of the coherent segments[498]; this shows a tendency of the segments towards the free manner of occurrence which is usually considered a Calamitean character. In certain fossils referred to the genus _Annularia_, the nodes bear whorls of long and narrow leaves which are fused basally into a collar (fig. 58, _D_). There are good grounds for believing that at least some Annularias were the foliage shoots of true Calamites. Again, in some species of _Calamitina_, a sub-genus of _Calamites_, the leaves appear to have been united basally into a narrow sheath. We see, then, that it is a mistake to attach great importance to the separate or coherent character of leaf-segments in attempting to draw a line between the true _Calamites_ and _Equisetites_. Potonié[499] while pointing out that this distinction does not possess much value as a generic character, retains the genus _Equisetites_ for certain Palaeozoic Equisetum-like leaf-sheaths.

Fig. 56 represents a rather faint impression of a leaf-sheath and nodal diaphragm. The specimen is from the Coal-Measures of Ardwick, Manchester. The letter _a_ probably points to the attachment of the sheath to the node of the stem. The flattened sheath is indistinctly divided into segments, and at the middle of the free margin there appears to be a single free tooth. The lower part of the specimen, as seen in the figure, shows the position of the nodal diaphragm. Between the diaphragm and the sheath there are several slight ridges converging towards the nodal line; these agree with the characteristic ridges and grooves of Calamite casts which are described in detail in Chapter X. There is another specimen in the British Museum which illustrates, rather more clearly than that shown in fig. 56, the association of a fused leaf-sheath with a type of cast usually regarded as belonging to a Calamitean stem. Some leaf-sheaths of Permian age described by Zeiller[500] as _Equisetites Vaujolyi_ bear a close resemblance to the sheath in fig. 58 E. The nature of the true Calamite leaves is considered more fully on a later page.

[Sidenote: PALAEOZOIC EQUISETITES.]

The examples of supposed _Equisetites_ sheaths referred to below may serve to illustrate the kind of evidence on which this genus has been recorded from Upper Palaeozoic rocks. I have retained the name _Equisetites_ in the description of the species, but it would probably be better to speak of such specimens as ‘Calamitean leaf-sheaths’ rather than to describe them as definite species of _Equisetites_. We have not as yet any thoroughly satisfactory evidence that the _Equisetites_ of Triassic and post-Triassic times existed in the vegetation of earlier periods.

In Grand’Eury’s _Flore du Gard_[501] a fossil strobilus is figured under the name _Calamostachys tenuissima_ Grand’Eury, which consists of a slender axis bearing series of sporophylls and sporangia apparently resembling those of an _Equisetum_. There are no sterile appendages or bracts alternating with the sporophylls; and the absence of the former suggests a comparison with _Equisetites_ rather than _Calamites_. Grand’Eury refers to the fossil as “parfois à peine perceptible,” and a recent examination of the specimen leads me to thoroughly endorse this description. It was impossible to recognise the features represented in Grand’Eury’s drawing. Setting aside this fossil, there are other strobili recorded by Renault[502] and referred by him to the genus _Bornia_ (_Archaeocalamites_), which also exhibit the Equisetum-like character; the axis bears sporophylls only and no sterile bracts. It would appear then that in the Palaeozoic period the Equisetaceous strobilus, as we know it in _Equisetum_, was represented in some of the members of the Equisetales.

1. _Equisetites Hemingwayi_ Kidst. Fig. 57, _A_.

Mr Kidston[503] founded this species on a few specimens of cones found in the Middle Coal-Measures of Barnsley in Yorkshire. The best example of the cone described by Kidston has a length of 2·5 cm., and a breadth of 1·5 cm.; the surface is divided up into several hexagonal areas 4 mm. high and 5 mm. wide. Each of these plates shows a fairly prominent projecting point in its centre; this is regarded as the point of attachment of the sporangiophore axis which expanded distally into a hexagonal plate bearing sporangia. An examination of Mr Kidston’s specimens enabled me to recognise the close resemblance which he insists on between the fossils and such a recent Equisetaceous strobilus as that of _Equisetum limosum_ Sm. Nothing is known of the structure of the fossils beyond the character of the superficial pattern of the impressions, and it is impossible to speak with absolute confidence as to their nature. The author of the species makes use of the generic name _Equisetum_; but in view of our ignorance of structural features it is better to adopt the more usual term _Equisetites_.

Since Kidston’s description was published I noticed a specimen in the British Museum collection which throws some further light on this doubtful fossil. Part of this specimen is shown in fig. 57, _A_. The stem is 21 cm. in length and about 5 mm. broad; it is divided into distinct nodes and internodes; the former being a little exaggerated in the drawing. The surface is marked by fine and irregular striations, and in one or two places there occur broken pieces of narrow linear leaves in the neighbourhood of a node. Portions of four cones occurring in contact with the stem, appear to be sessile on the nodes, but the preservation is not sufficiently good to enable one to speak with certainty as to the manner of attachment. Each cone consists of regular hexagonal depressions, which agree exactly with the surface characters of Kidston’s type-specimen. The manner of occurrence of the cones points to a lateral and not a terminal attachment. The stem does not show any traces of Equisetaceous leaf-sheaths at the nodes, and such fragments of leaves as occur appear to have the form of separate linear segments; they are not such as are met with on _Equisetites_. It agrees with some of the slender foliage-shoots of Calamitean plants often described under the generic name _Asterophyllites_. As regards the cones; they differ from the known Calamitean strobili in the absence of sterile bracts, and appear to consist entirely of distally expanded sporophylls as in _Equisetum_. The general impression afforded by the fossil is that we have not sufficient evidence for definitely associating this stem and cones with a true _Equisetites_. We may, however, adhere to this generic title until more satisfactory data are available.

2. _Equisetites spatulatus_ Zeill. Fig. 58, _A_.

This species is chosen as an example of a French _Equisetites_ of Permian age. It was recently founded by Zeiller[504] on some specimens of imperfect leaf-sheaths, and defined as follows:—

Sheaths spreading, erect, formed of numerous uninerved coherent
leaves, convex on the dorsal surface, spatulate in form, 5–6 cm.
in length and 2–3 mm. broad at the base, and 5–10 mm. broad at the
apex, rounded at the distal end.

The specimen shown in fig. 58, _A_, represents part of a flattened sheath, the narrower crenulated end being the base of the sheath. The limits of the coherent segments and the position of the veins are clearly marked. Zeiller’s description accurately represents the character of the sheaths. They agree closely with an Equisetaceous leaf-sheath, but as I have already pointed out, we cannot feel certain that sheaths of this kind were not originally attached to a Calamite stem.

The portion of a leaf-sheath and a diaphragm represented in fig. 57, _B_, agrees closely with Zeiller’s examples. This specimen is from the English Coal-Measures, but it is not advisable to attempt any specific diagnosis on such fragmentary material. It is questionable, indeed, if these detached fossil leaf-sheaths should be designated by specific names. Another similar form of sheath, hardly distinguishable from Zeiller’s species, has recently been described by Potonié from the Permian (Rothliegende) of Thuringia.

_A._ _Equisetites spatulatus_, Zeill. Leaf-sheath. ⅘ nat. size.
(After Zeiller.)
_B._ _E. columnaris_, Brongn. From a specimen in the British Museum.
¾ nat. size.
_C._ _Equisetum ramosissimum_, Desf. × 2.
_D._ _Annularia stellata_ (Schloth.). Leaf-sheath. Slightly enlarged.
(After Potonié.)
_E._ _Equisetites zeaeformis_ (Schloth.). Leaf-sheath. ⅘ nat. size.
(After Potonié.)
_F._ _E. lateralis_, Phill. From a specimen in the Scarborough
Museum. Nat. size.]

3. _Equisetites zeaeformis_ (Schloth.)[505] Fig. 58, _E_.

The sheaths consist of linear segments fused laterally as in _Equisetum_. In some specimens the component parts of the sheath are more or less separate from one another, and in this form they are apparently identical with the leaves of _Calamites_ (_Calamitina_) _varians_, Sternb. The example shown in fig. 58, _E_ is probably a young leaf-sheath; the segments are fused, and each is traversed by a single vein represented by a dark line in the figure. The regular crenulated lower margin is the base of the sheath, and corresponds to the upper portion of fig. 58, _A_. This species affords, therefore, an interesting illustration of the difficulty of separating _Equisetites_ leaves from those of true _Calamites_. Potonié has suggested that the leaf-sheath of a young Calamite might well be split up into distinct linear segments as the result of the increase in girth of the stem.

• • • • •

Other Palaeozoic species of _Equisetites_ have been recorded, but with one exception these need not be dealt with, as they do not add anything to our knowledge of botanical importance. The specimen described in the _Flore de Commentry_ as _Equisetites Monyi_, by Renault and Zeiller[506], differs from most of the other Palaeozoic species of _Equisetites_, in the fact that we have a stem with short internodes bearing a leaf-sheath at each node divided into comparatively long and distinct teeth. This species presents a close agreement with specimens of _Calamitina_, but Renault and Zeiller consider that it is generically distinct. They suggest that the English species, originally described and figured by Lindley and Hutton[507] as _Hippurites gigantea_, and now usually spoken of as _Calamitina_, should be named _Equisetites_. It would probably be better to adopt the name _Calamitina_ for the French species. The type-specimen of this species is in the Natural History Museum, Paris.

[Sidenote: EQUISETITES PLATYODON.]

When we pass from the Permian to the Triassic period, we find large casts of very modern-looking Equisetaceous stems which must clearly be referred to the genus _Equisetites_. The portion of a stem represented in fig. 59 known as _Equisetites platyodon_ Brongn.[508] affords an example of a Triassic Equisetaceous stem with a clearly preserved leaf-sheath. The stem measures about 6 cm. in diameter. One of the oldest known Triassic species is _Equisetites Mougeoti_[509] (Brongn.) from the Bunter series of the Vosges.

The Keuper species _E. arenaceus_ is, however, more completely known. The specimens referred to this species are very striking fossils; they agree in all external characters with recent Horse-tails but greatly exceed them in dimensions.

4. _Equisetites arenaceus_ Bronn.

This plant has been found in the Triassic rocks of various parts of Germany and France; it occurs in the Lettenkohl group (Lower Keuper), as well as in the Middle Keuper of Stuttgart and elsewhere. The species may be defined as follows:—

Rhizome from 8–14 cm. in diameter, with short internodes,
bearing lateral ovate tubers. Aerial shoots from 4–12 cm. in
diameter, bearing whorls of branches, and leaf-sheaths made up
of 110–120 coherent uni-nerved linear segments terminating in an
apical lanceolate tooth. Strobili oval, consisting of crowded
sporangiophores with pentagonal and hexagonal peltate terminations.

The casts of branches, rhizomes, tubers, buds and cones enable us to form a fairly exact estimate of the size and general appearance of this largest fossil Horse-tail. The Strassburg Museum contains many good examples of this species, and a few specimens may be seen in the British Museum. In the École des Mines, Paris, there are some exceptionally clear impressions of cones of this species from a lignite mine in the Vosges.

It is estimated that the plant reached a height of 8 to 10 meters, about equal to that of the tallest recent species of _Equisetum_, but in the diameter of the stems the Triassic plant far exceeded any existing species.

It is interesting to determine as far as possible, in the absence of petrified specimens, if this Keuper species increased in girth by means of a cambium. There are occasionally found sandstone casts of the pith-cavity which present an appearance very similar to that of Calamitean medullary casts[510]. The nodes are marked by comparatively deep constrictions, which probably represent the projecting nodal wood. The surface of the casts is traversed by regular ridges and grooves as in an ordinary Calamite, and it is probable that in _Equisetites arenaceus_, as in _Calamites_, these surface-features are the impression of the inner face of a cylinder of secondary wood (_cf._ p. 310). Excellent figures of this species of _Equisetites_ are given by Schimper in his Atlas of fossil plants[511], also by Schimper and Koechlin-Schlumberger[512], and by Schoenlein and Schenk[513].

5. _Equisetites columnaris_ Brongn. Figs. 11 and 58, _B_.

This species, which is by far the best known British _Equisetites_, was founded by Brongniart[514] on some specimens from the Lower Oolite beds of the Yorkshire coast. Casts of stems are familiar to those who have collected fossils on the coast between Whitby and Scarborough; they are often found in an erect position in the sandstone, and are usually described as occurring in the actual place of growth. As previously pointed out (p. 72), such stems have generally been deposited by water, and have assumed a vertical position (fig. 11). Young and Bird[515] figured a specimen of this species in 1822, and in view of its striking resemblance to the sugar-cane, they regarded the fossil as being of the same family as _Saccharum officinarum_, if not specifically identical.

A specimen was described by König[516] in 1829, from the Lower Oolite rocks of Brora in the north of Scotland under the name of _Oncylogonatum carbonarium_, but Brongniart[517] pointed out its identity with the English species _Equisetites columnaris_.

Our acquaintance with this species is practically limited to the casts of stems. A typical stem of _E. columnaris_ measures 3 to 6 cm. in diameter and has fairly long internodes. The largest stem in the British Museum collection has internodes about 14 cm. long and a diameter of about 5 cm. In some cases the stem casts show irregular lateral projections in the neighbourhood of a node, but there is no evidence that the aerial shoots of this species gave off verticils of branches. In habit _E. columnaris_ probably closely resembled such recent species as _Equisetum hiemale_ L., _E. trachyodon_ A. Br. and others.

The stems often show a distinct swelling at the nodes; this may be due, at least in part, to the existence of transverse nodal diaphragms which enabled the dead shoots to resist contraction in the region of the nodes. The leaf-sheaths consist of numerous long and narrow segments often truncated distally, as in fig. 58, _B_, and as in the sheath of such a recent Horse-tail as _E. ramosissimum_ shown in fig. 58, _C_. In some specimens one occasionally finds indications of delicate acuminate teeth extending above the limits of a truncated sheath. Brongniart speaks of the existence of caducous acuminate teeth in his diagnosis of the species, and the example represented in fig. 58, _B_, demonstrates the existence of such deciduous appendages. There is a very close resemblance between the fossil sheath of fig. 58, _B_, with and without the teeth, and the leaf-sheath of the recent _Equisetum_ in fig. 58, _C_. In some specimens of _E. columnaris_ in which the cast is covered with a carbonaceous film, each segment in a leaf-sheath is seen to be slightly depressed in the median portion, which is often distinctly marked by numerous small dots, the edges of the segment being flat and smooth. The median region is that in which the stomata are found and on which deposits of silica occur.

6. _Equisetites Beani_ (Bunb.). Figs. 60–62.

Bunbury[518] proposed the name _Calamites Beani_ for some fossil stems from the Lower Oolite beds of the Yorkshire coast, which Bean had previously referred to in unpublished notes as _C. giganteus_. The latter name was not adopted by Bunbury on account of the possible confusion between this species and the Palaeozoic species _Calamites gigas_ Brongn. The generic name _Calamites_ must be replaced by _Equisetites_ now that we are familiar with more perfect specimens which demonstrate the Equisetean characters of the plant.

]

Schimper[519] speaks of this species as possibly the pith-cast of _Equisetites columnaris_, but his opinion cannot be maintained; the species first described by Bunbury has considerably larger stems than those of _E. columnaris_. It is not impossible, however, that _E. columnaris_ and _E. Beani_ may be portions of the same species. The chief difference between these forms is that of size; but we have not sufficient data to justify the inclusion of both forms under one name. Zigno[520], in his work on the Oolitic Flora, figures an imperfect stem cast of _E. Beani_ under the name of _Calamites Beani_, but the species has received little attention at the hands of recent writers. In 1886 Starkie Gardner[521] figured a specimen which was identified by Williamson as an example of Bunbury’s species; but the latter pointed out the greater resemblance, as regards the external appearance of the Jurassic stem, to some of the recent arborescent Gramineae[522] than to the Equisetaceae. Williamson, with his usual caution, adds that such appearances have very little taxonomic value. Fig. 60 is reproduced from the block used by Gardner in his memoir on Mesozoic Angiosperms; he quotes the specimen as possibly a Monocotyledonous stem. The fossil is an imperfect cast of a stem showing two clearly marked nodal regions, but no trace of leaf-sheaths. A recent examination of specimens in the museums of Whitby, Scarborough, York and London has convinced me that the plant named by Bunbury _Calamites Beani_ is a large _Equisetites_. As a rule the specimens do not show any indications of the leaf-sheaths, but in a few cases the sheaths have left fairly distinct impressions.

In the portion of stem shown in fig. 61 the impressions of the leaf segments are clearly marked. This specimen affords much better evidence of the Equisetaceous character of the plant than those which are simply internal casts. The narrow projecting lines extending upwards from the nodes in the figured specimen probably represent the divisions between the several segments of each leaf-sheath.

In the museums of Whitby and Scarborough there are some long specimens, in one case 44 cm. in length, and 33 cm. in circumference, which are probably casts of the broad pith-cavity. These casts are often transversely broken across at the nodes, so that they consist of three or four separate pieces which fit together by clean-cut faces. This manner of occurrence is most probably due to the existence of large and resistant nodal diaphragms which separated the sand-casts of adjacent internodes. In the York museum there are some large diaphragms, 10 cm. in diameter, preserved separately in a piece of rock containing a cast of _Equisetites Beani_. The nodal diaphragms of some of the Carboniferous Calamites were the seat of cork development[523], and it may be that the frequent preservation of Equisetaceous diaphragms in Triassic and Jurassic rocks is due to the protection afforded by a corky investment.

The stem shown in fig. 62 appears to be a portion of a shoot of _E. Beani_ not far from its apical region. From the lower nodes there extend clearly marked and regular lines or slight grooves tapering gradually towards the next higher node; these are no doubt the impressions of segments of leaf-sheaths. The sheaths themselves have been detached and only their impressions remain. The flattened bands at the node of the stem in fig. 60, and shown also in fig. 61, mark the place of attachment of the leaf-sheaths. On some of these nodal bands one is able to recognise small scars which are most likely the casts of outgoing leaf-trace bundles.

Some of the internal casts of this species are marked by numerous closely arranged longitudinal lines, which are probably the impressions of the inner face of a central woody cylinder. In the smaller specimen shown in fig. 62 we have the apical portion of a shoot in which the uppermost internodes are in an unexpanded condition.

It is impossible to give a satisfactory diagnosis of this species without better material. The plant is characterised chiefly by the great breadth of the stem, and by the possession of leaf-sheaths consisting of numerous long and narrow segments. _Equisetites Beani_ must have almost equalled in size the Triassic species, _E. arenaceus_, described above.

7. _Equisetites lateralis_ Phill. Figs. 58, _F_, 63, and 64.