II. Zygoptereae.

Corda[1132] proposed the generic name Zygopteris for petrified petioles from the Permian of Saxony, included by Cotta in his genus Tubicaulis, which he named T. primarius. Corda’s genus has been generally used for petioles of Palaeozoic ferns characterised by a vascular strand having the form of an H in transverse section ([fig. 308], D). Since the generic name was instituted, information has been obtained in regard to the nature of the stems which bore some of the petioles of the Zygopteris type; and for other species of Zygopteris, the stems of which are still unknown, new generic names have been proposed. P. Bertrand[1133] retains Zygopteris for one species only, Z. primaria. Fig. 308, D, shows the character of the petiolar vascular strand; the chief points are the comparatively long cross-pieces (antennae of P. Bertrand) inclined at an angle of 45° to the plane of symmetry of the petiole axis, and the groups of protoxylem elements shown by the white patches in fig. D. In this as in other members of the Zygoptereae the main rachis of the leaf gives off four sets of branches in pairs alternately from the right and left side of the primary vascular axis. This method of branching of the stele in the primary rachis of several members of the Coenopterideae shows that the fronds bore pinnae laterally disposed, in some cases in one row and in others in two rows on each side of the rachis. In a typical fern frond, as represented by recent and most fossil species, branching of the rachis occurs in the plane of the frond, that is in the plane represented by the horizontal arm of xylem in Zygopteris primaria connecting the two antennae or cross-pieces. In the Zygoptereae the branches from the petiole vascular axis lie in a plane at right angles to that of the frond; they lie in the transverse and not in the horizontal plane. The two strands shown in [fig. 308], B, 4, have been formed by the division of a single strand, 3, in the transverse plane (i.e. in the plane of the paper). As Tansley[1134] points out, a type of branching superficially similar to, though not identical with this, is seen in some recent species of Gleichenia and Lygodium. In this connexion it is worthy of note that a fern figured by Unger from Thuringia as Sphenopteris petiolata Goepp[1135] bears pinnae in two rows on the rachis which are characterised by repeated branching and by a very narrow lamina or by slender naked axes; the occurrence of this form of frond in rocks containing Clepsydropsis antiqua ([fig. 308], A) suggests a possible connexion between the petrified rachis and the impressions of the leaves.

Fig. 308.

The white patches in the xylem in figs. B–G mark the position of protoxylem elements.
(A, after Unger; B–G, after P. Bertrand.)

Fig. 309.

(A, B, after Renault; C, D, after Zeiller; E, after Renault and Zeiller.)

The vascular strand of the rachis of Zygopteris primaria ([fig. 308], D) is simpler than that of most of the Zygoptereae and exhibits a close resemblance to the type of strand described by Renault as Diplolabis ([fig. 308], C).

Diplolabis.

Renault[1136] instituted this genus for two species from the Culm beds and Coal-Measures of France based on the structure of the petioles. The stems are unknown. The main rachis has a stele similar to that of Zygopteris primaria, but distinguished by its greater similarity, in transverse section, to an X rather than to the letter H: the long transverse bar in Zygopteris is here much reduced in size. The petiole of Diplolabis forensis[1137] Ren. ([fig. 308], C) has a diameter of 1·5–2 cm. From the antennae a pair of small bundles is given off alternately from the right and left side, as in Zygopteris; the members of each pair coalesce after leaving the antennae and then separate to pass into the lateral branches of the frond. The position of the protoxylem and the formation of the lateral xylem strands previous to their separation are shown in [fig. 308], C. On the side of the vascular strand shown in fig. C, 2, the two lateral extensions of the antennae are converging towards one another previous to their separation and subsequent union. The ovoid sporangia occur in groups of three to six and are coalescent below with a central receptacle; they have no annulus, but the cells on the side next the receptacle are smaller than those on the external wall ([fig. 309], A). The synangial form of the sorus suggests comparison with Marattiaceae.

The species described by Renault from the Culm of Esnost is regarded by P. Bertrand as identical with that described by Solms, from the Culm of Falkenberg, as Zygopteris Roemeri[1138]. Diplolabis is compared by P. Bertrand with Metaclepsydropsis, the generic name given to the Lower Carboniferous petiole described by Williamson as Rachiopteris duplex[1139].

Mr Gordon has recently described in a preliminary note a new type of stem stele under the name Zygopteris pettycurensis from the Lower Carboniferous plant bed of Pettycur[1140]: he regards the petioles attached to the stem as identical with Zygopteris Roemeri Solms-Laubach[1141]. This species, founded by Solms-Laubach on petioles only, is placed by Bertrand[1142] in the genus Diplolabis and regarded as identical with D. esnostensis Ren. The stele found by Mr Gordon may therefore be assigned to the genus Diplolabis: it includes two regions composed exclusively of tracheae and is cylindrical in transverse section. The inner xylem zone consists of short, square-ended, reticulately pitted elements and the outer zone is composed of long and pointed conducting tracheae. The scalariform protoxylem elements are situated between the two metaxylem zones. As Mr Gordon says: this type of stem occupies a position “in the Zygopteroid alliance” corresponding to that which Thamnopteris Schlechtendalii (p. 329) occupies in the Osmundaceous series. The discovery of this stem supplies another link between the two fern groups, Osmundaceae and Coenopterideae. Pelourde[1143] has described an imperfectly preserved vascular strand from a locality near Autun as the type of a new genus Flicheia esnostensis. Mr Gordon has pointed out to me that this is a partially rotted petiole of Diplolabis esnostensis (= Zygopteris Roemeri).

In their recent account of fossil Osmundaceous genera, Kidston and Gwynne-Vaughan[1144] speak of the central parenchyma of the existing medullated stele as being derived from tracheal tissue. They add that if the Zygopteroid line of descent is at all close to the Osmundaceous, we must be prepared for the existence of a Zygopteris with a solid xylem like that of Thamnopteris: “such a discovery, in fact, we hopefully anticipate[1145].” The new Pettycur stem amply justifies this prophecy. It is noteworthy that Mr Gordon’s stem affords an instance of the occurrence of a type of stele, similar in its cylindrical form and in the absence of parenchyma to that of Botryopteris, in a plant bearing leaves characterised by the Zygopteris type of vascular strand.

Metaclepsydropsis duplex (Will.) [fig. 310], A[1146]. [= Rachiopteris duplex, Williamson 1874. Asterochlaena (Clepsydropsis) duplex, Stenzel 1889. Clepsydropsis, Renault 1896.]

The vascular axis of the main axis of the frond is characterised by the hour-glass shape of the xylem which consists entirely of tracheae, most of which are reticulately pitted. In a transverse section ([fig. 310], A) the two ends of the stele are dissimilar; at one end of the long axis is a small bay of thin-walled tissue (phloem) enclosed by a narrow band of xylem, and at the other the bay is open and has two protoxylem groups. The latter represents the earliest stage in the production of secondary bundles: at a later stage the bay is closed by the elongation of the edges, the enclosed group of phloem is vertically extended, and the protoxylem strands are more widely separated. The curved band of xylem becomes detached as a curved arc and divides into two ([fig. 310], A). In a single section of this species one often sees several strands of xylem enclosed in a common cortex with the main vascular axis; these are the xylem bundles of lateral pinnae. Metaclepsydropsis duplex shows the method of branching of the petiole vascular axis which has already been noticed in Diplolabis and Zygopteris. In reference to this feature, Williamson wrote in 1872—“I know of no recent fern in which the secondary branches of the petiole are thus given off in pairs, which pairs are distichously arranged on the primary axis, and each of which secondary petioles sustains ternary ones arranged distichously.” By slightly altering the primary stele of this type of frond, by narrowing of the constricted portion of the hour-glass and extending the lateral groups of xylem obliquely upwards, the form of stele shown in [fig. 310], A, would be converted into the Diplolabis type ([fig. 308], C).

Clepsydropsis.

Unger[1147] instituted this genus as a subdivision of Corda’s family Rhaciopterideae[1148], the name having reference to the hour-glass form of the vascular axis[1149]. The type-species C. antiqua ([fig. 308], A) is spoken of as the commonest fossil plant in the Devonian rocks of Thuringia. In some sections the xylem has the form seen in [fig. 308], A, in which an invagination of thin-walled tissue occurs at each end; in other sections ([fig. 308], A′) the bays become islands in the xylem. Solms-Laubach speaks of Unger’s species as Rachiopteris (Clepsydropsis) antiqua. P. Bertrand[1150], who has recently described Unger’s plant, while recognising that C. antiqua and Metaclepsydropsis duplex closely resemble one another, draws attention to certain differences in the structure of the xylem which he regards as sufficient to justify a generic separation. The leaf-traces of Clepsydropsis are described by Bertrand as almost circular closed rings of xylem instead of an arc as in Metaclepsydropsis.

Fig. 310.

[A, from a section in Dr Kidston’s Collection (Lower Carboniferous); B, C, from sections in the Cambridge Botany School; D, after Stenzel.]

Ankyropteris.

Stenzel adopted this name for a subdivision of Corda’s genus Zygopteris, applying it to a species described by Renault as Z. Brongniarti, to a Permian species described by himself as Z. (Ankyropteris) scandens, and to Z. Lacattii Ren.; Rachiopteris Grayi Will. and Rachiopteris corrugata Will. are also included in this genus. The characters emphasised by Stenzel[1151] are (i) the double anchor-like form of the H-shaped petiole strand in which the lateral arms (antennae) are curved like the flukes of an anchor, and (ii) the emission of four rows of branches instead of two. The latter distinguishing feature no longer holds good, as Z. primaria also gives off four rows of bundles and not two as Stenzel supposed. P. Bertrand has adopted Stenzel’s genus in a narrower sense[1152].

Fig. 311. Ankyropteris Grayi. Stele. (From a section in Dr Kidston’s Collection, × 18.)

Ankyropteris scandens Stenzel[1153]. Fig. 310, D.

This Lower Permian species is very similar to or perhaps identical with Ankyropteris Grayi (Williamson). The stem of

A. scandens was found in association with the roots of a Psaronius stem evidently petrified in situ as it burrowed, like Tmesipteris, tropical aroids, and other recent plants, among the living roots of the tree-fern. The stem, 10–11 mm. in diameter, bore fronds with an H-shaped vascular strand, small scale-leaves, and adventitious roots. The stele consists of a five-angled cylinder of scalariform tracheae surrounding an axial strand of parenchyma containing scattered tracheae of smaller diameter. This axial tissue extends as a narrow strip into each of the short and obtusely truncated arms (cf. [fig. 311]). A striking feature is the production of a shoot in the axil of the foliage-leaves ([fig. 310], D), a manner of branching characteristic of Trichomanes (see [page 365]).

Ankyropteris Grayi (Will.). Fig. 311.

In describing this species, Williamson wrote—“That no classification of these fossil ferns based solely upon transverse sections of the petiolar bundles is or can be of much value, is clearly shown when tested amongst those living ferns the classification of which is chiefly based upon the sporangial reproductive organs[1154].” This is a view entirely opposed to that which inspires P. Bertrand’s recent monograph. Whether the value attached to the vascular structure of petioles as a basis of classification is upheld or not, it is noteworthy that since Williamson expressed his opinion, our knowledge of the anatomy of ferns and of the value of anatomical evidence has enormously increased. The slender stem[1155] of this Lower Coal-Measures species agrees closely with that of A. scandens; it bore spirally disposed fronds, scale-leaves, and roots. The stele has the form of an irregular five-rayed star ([fig. 311]) in which the relative length of the arms varies in different sections owing to the separation of the distal ends to form leaf-traces. The axial region is composed of parenchyma and associated narrow tracheae, as in A. scandens. The xylem, with protoxylem elements at the ends and especially at the angles of the arms, is completely surrounded by phloem. The cortex consists internally of parenchyma which becomes thicker-walled towards the periphery and bears multicellular epidermal hairs. A leaf-trace is detached in the form of a triangular strand and is formed by the tangential extension of the distal end of an arm of the stele. The trace, on its way through the cortex, divides into two; the outer branch gradually changes from a slightly curved band to an H-shaped meristele; the inner branch, which supplied an axillary shoot, is similar to the stele of the stem, but smaller. Scott[1156] has recently recorded the occurrence of scale-leaves (aphlebiae) in this species like those described by Stenzel in A. scandens.

Fig. 312.

  1. Thamnopteris Schlechtendalii. Leaf-trace: px, protoxylem; s, island of parenchyma. (After Kidston and Gwynne-Vaughan.)
  2. Ankyropteris corrugata. Single trachea with tyloses.
  3. A. bibractensis. Part of foliar strand. (After P. Bertrand.)

Bertrand includes in Ankyropteris Renault’s species Zygopteris bibractensis[1157] and Williamson’s species Rachiopteris corrugata[1158]: the former he names A. bibractensis var. westphaliensis. The fossil described by Williamson as R. irregularis or inaequalis[1159] are the secondary branches of A. bibractensis.

Ankyropteris bibractensis, var. westphaliensis. Figs. [312], C; [313].

The rachis stele of this species, which is represented by portions of fronds only, has the form of a double anchor ([fig. 313]); the antennae are continued at the outer edge of their distal ends into a narrow band (“filament” of P. Bertrand) ([fig. 312], C, and 313, a) composed of smaller tracheae and separated from the xylem of the antennae by a strip of thin-walled tissue (phloem?). A group of protoxylem occurs at the junction of the filament and antennae. The whole of the xylem is surrounded by phloem.

Fig. 313. Ankyropteris bibractensis: s, stigmarian rootlet; a, narrow loop of xylem. (Cambridge Botany School; × 6).

The section reproduced in [fig. 313] shows the characteristic form of the petiolar vascular axis, consisting of a horizontal band of metaxylem with groups of much smaller tracheae on both the upper and lower margins. At the junction between the antennae, curved like the flukes of an anchor, and the horizontal band of xylem, the latter is only one trachea in breadth. The narrow loops of smaller xylem elements are shown on the outer edge, a ([fig. 313]), of the antennae separated from the arcs of larger tracheae by a dark line which represents a crushed band of delicate tissue. The spaces enclosed by the incurved antennae are largely occupied by parenchymatous ground-tissue. The cylinder of outer cortex consists internally of comparatively thin-walled parenchyma succeeded externally by a zone of dark and thicker-walled cells characterised by a fairly regular arrangement in radial series, as if formed by a secondary meristem; there is, however, no indication of a meristematic layer. Below the small-celled epidermis are a few layers of thinner-walled cells which are not arranged in radial series. The structure of the outer part of the cortex is similar to that in the petiole of recent species of Angiopteris ([fig. 243], p. 319) and Marattia, in which a more delicate hypoderm is succeeded by a band of mechanical tissue.

The rachis of this type of frond gives off two rows of lateral branches from the vascular axis, the plane of symmetry being at right angles to the primary rachis. Each pinna bore at its base two aphlebiae supplied with vascular strands from the leaf-traces.

We have no certain information in regard to the sporangia of this species, but Scott points out that “pear-shaped sporangia, with a very broad and extensive annulus, are commonly found associated with Zygopteris bibractensis and Z. corrugata in the petrifactions of the English Lower Coal-Measures[1160].”

Ankyropteris corrugata (Will.). Figs. [312], B; [314–317].

The stem of this type of Zygoptereae was described by Williamson from the Lower Coal-Measures of Lancashire as Rachiopteris corrugata and included by him in the sub-group Anachoropteroides. The stele ([fig. 314], B) is oval in transverse section; it consists of a cylinder of xylem tracheae enclosing a central region occupied by parenchymatous tissue and scattered narrow scalariform tracheae. The central tissue extends radially in the form of narrow arms which reach almost to the outer edge of the tracheal tissue and divide it up into 5–7 groups. A cylinder of thin-walled tissue encloses the xylem and in this occur groups of large sieve-tubes ([fig. 314], D, Sv).

In a section of this species in the Williamson Collection[1161] the long axis of the stele has a length of 5 mm. and the diameter of the stem as a whole is 2·5 cm. The greater part of the extra-stelar tissue consists of large parenchymatous cells passing near the periphery into a band of darker and thicker-walled tissue.

Reniform vascular strands traverse the cortex in an obliquely ascending course on their way to the leaves, also smaller bundles, some of which are given off directly from the stele, while others are branches of the petiole vascular strands. The petioles described by Williamson as Rachiopteris insignis[1162] were afterwards recognised by him as those of Ankyropteris corrugata, though this conclusion was not published[1163]. Williamson’s species R. insignis must not be confused with Unger’s Culm species Arctopodium insigne, which Solms-Laubach[1164] refers to as Rachiopteris insignis. The leaf-bundle of Ankyropteris corrugata is at first reniform in contour ([fig. 314], C, P), but as it becomes free from the stem it gradually assumes the H-shaped form ([figs. 315–317]). This petiolar strand differs from that of Ankyropteris bibractensis ([fig. 313]) in the shorter and less strongly curved antennae; and, as Williamson first noticed, the tracheae are frequently filled with thin-walled parenchyma ([fig. 312], B). The existence of scale-leaves or aphlebiae like those of Ankyropteris scandens and A. Grayi has been recorded by Scott in A. corrugata[1165].

The section represented in [fig. 314], C, shows the relatively small size of the stele S in the stem of Ankyropteris corrugata. The main mass of the cortex consists of uniform parenchyma passing near the surface into darker and stronger tissue: two vascular bundles are shown in the cortex, one of which forms the conducting strand of a petiole, P, which has nearly freed itself from the stem: the other bundle, as shown by the examination of a series of sections, eventually passes into another leaf-stalk. A root of another plant has invaded the cortex at R, [fig. 314], C.

Fig. 314. Ankyropteris corrugata. R, intruded root; P, petiole; S, stele, Sv sieve-tubes.

The form and structure of the stele is diagrammatically represented in [fig. 314], B. The outer portion (black) consists of a cylinder of scalariform tracheae in which the position of groups of smaller elements (protoxylem) is shown by the white patches. The xylem is thus seen to be mesarch. The prominent group of xylem on the lower right-hand side of the section consists of tracheae, cut across in an oblique direction, which are about to pass out as a separate strand. The centre of the stele is occupied by parenchymatous tissue in which are included scattered tracheae, either singly or in small groups. These medullary tracheae are rather narrower than those of the main xylem cylinder. A characteristic feature is the radial outward extension into the xylem of the medullary parenchyma, which tends partially to divide the tracheal cylinder into broad groups.

Fig. 315. Ankyropteris corrugata (Will.). Petiolar vascular strand. [From a section in the University College (London) Collection; after Tansley × 35.]

Fig. 314, A, enlarged from fig. B, a, shows the mesarch position of a protoxylem group, and a few of the parenchymatous cells of one of the narrow arms of the axial tissue. At Sv in fig. D a group of large sieve-tubes is seen separated from the xylem by a few parenchymatous cells, and beyond the sieve-tubes are some tangentially elongated cells. Both the sieve-tubes, Sv, and the flattened cells resemble tissues in a corresponding position in the steles of modern Osmundaceae.

In a section of Ankyropteris corrugata in the Williamson Collection the radial arrangement of the more external metaxylem elements suggests the addition of secondary tracheae[1166]. This suggestion of secondary thickening, a point which requires much more thorough investigation, is interesting in relation to a new type of stem named by Scott Botrychioxylon[1167], but not yet fully described. This generic name has been given to a stem stele which closely resembles that of Ankyropteris corrugata except in the regular radial arrangement of the peripheral xylem elements. The name Botrychioxylon was chosen by Scott because of the secondary xylem characteristic of the recent genus Botrychium ([fig. 247], p. 322).

Fig. 316. Ankyropteris corrugata. Petiole. a, narrow xylem loop; b, spaces in cortex. From a section in the Cambridge Botany School Collection. (× 10.)

In the petiolar vascular strand represented in [fig. 315] the narrow band of tracheae which forms a loop external to the antennae is clearly seen, also the small-celled parenchyma between the loops and the larger metaxylem elements of the antennae. The crushed tissue lying on the outer face of each of the loops probably represents the phloem and pericycle; the thin-walled elements above and below the horizontal band of metaxylem are probably sieve-tubes.

Fig. 316 shows a transverse section of a petiole of this species: the loops, a, of small tracheae are seen bending round the outer edge of the antennae. The inner and more delicate cortical tissue is partially preserved and spaces, b, have been formed in it as the result of contraction previous to petrifaction. In the petiole represented in [fig. 317] the tracheae of the horizontal band are considerably crushed; the section is, however, of interest because of the presence of Lyginodendron roots, l, in the space originally occupied by the inner cortex.

Fig. 317. Ankyropteris corrugata. From a section in the Cambridge Botany School Collection. (× 9.)

In a paper on the tyloses of Rachiopteris corrugata, Weiss[1168] draws attention to the fact that similar inclusions have not been found in the tracheae of recent ferns. The occurrence of thin-walled parenchymatous cells in the large tracheae of Ankyropteris corrugata petioles and of other species is a striking feature. Williamson[1169] compared these cells with the tyloses in the vessels of recent flowering plants, and in a later paper[1170] he suggested that the included cells may belong to saprophytic or parasitic fungi. It is, as Weiss points out, difficult to explain the occurrence of tyloses in tracheae not immediately in contact with living parenchyma. It may be that the pits in the tracheae of Ankyropteris were open spaces as in the xylem of recent ferns described by Gwynne-Vaughan, and if so this would facilitate the invasion of the conducting elements by growing cells. A comparison is made by Weiss between certain cell-groups found by him in the tracheae of Ankyropteris and by Miss Jordan[1171] in the vessels of the recent dicotyledon Cucumis sativus. In a more recent paper on tyloses Miss McNicol[1172] expresses the opinion that pseudoparenchyma in the tracheae of the fossil petioles owes its origin to fungal hyphae.

Williamson compared the petiole bundles of Ankyropteris corrugata with those of recent Osmundaceae, a comparison based on the structure of the leaf-trace before its separation from the stem and its assumption of the H-form. It is noteworthy, however, that this comparison has acquired a greater significance as the result of recent work. The stele of Ankyropteris bears a fairly close resemblance to that of Zalesskya described by Kidston and Gwynne-Vaughan; in both types the xylem is represented by two kinds of tracheal tissue. In the Permian Osmundaceous genus the centre of the stele consists of short storage tracheids, while in Ankyropteris we may regard the central parenchyma and scattered tracheae as derivatives of the solid xylem core of some ancestral type. Moreover, the appearance and arrangement of the phloem and the tangentially elongated elements external to it ([fig. 314]) remind one of the extra-xylem zone in recent Osmundaceae. That the Osmundaceae and Zygoptereae are closely related groups there can be little doubt; of this affinity and common origin[1173] Ankyropteris corrugata affords striking evidence.

The difference between the steles of Ankyropteris Grayi and A. scandens (figs. [310], D; [311]) and that of Ankyropteris corrugata is comparatively small. In the two former species the cylindrical form has become stellate owing to the radial extension of the xylem arms. It may be that this more elaborate style of vascular construction is connected with the climbing habit of A. scandens and possibly A. Grayi. The radial extension of the xylem and the consequent alternation of the yielding parenchymatous cortex and the more rigid tracheal arms would probably render the water-conducting elements less liable to injury in a twisting axis[1174]. In Anachoropteris Decaisnii[1175], described by Renault, and more especially in Asterochlaena laxa[1176] Stenzel, a Lower Permian type from Saxony ([fig. 324]), the xylem of the stele is much more deeply lobed than in Ankyropteris Grayi or A. scandens.

Fig. 318. Etapteris Scotti, P. Bert. (From Tansley, after Renault.)

Etapteris Scotti. Figs. [308], B; [309], E; [318].

P. Bertrand has proposed this name for a species of petiole from the Lower Coal-Measures of England referred by Binney[1177] to Zygopteris Lacattii Ren., and included by Williamson[1178] in his comprehensive genus Rachiopteris. Bertrand[1179] regards the English species, which is recorded also from Germany[1180], as distinct from Renault’s type[1181] and therefore proposes a new name. The petiole stele has the H-form, but its structure is simpler than that of the Ankyropteris petiole.

The horizontal band of xylem has at each end two oval groups of tracheae connected with it by a single row of xylem elements ([fig. 318]). From the lower part of each oval group a small strand is detached; the two strands from one side of the stele coalesce and then separate to pass into two pinnae. Fig. 308, B, shows four stages in the giving-off of the secondary branches. This species, therefore, produces four rows of branches in alternate pairs from the right and left sides of the petiole.

The first stage is shown at 0, 0, [fig. 308], B; the two projecting groups of protoxylem mark the points of departure of a pair of small strands. At 1, the projections are more prominent, and at 2 a pair of strands has become detached: at a later stage, 3, these two strands unite to divide later (4) into two slightly curved bundles.

Fig. 319.

Our knowledge of the fructification of Etapteris is based on Renault’s account of sporangia, which he regarded as belonging to Zygopteris (Etapteris) Lacattii. They have the form of elongated slightly curved sacs (2·5 × 1·3 mm.) borne in clusters ([fig. 319], A–C) on slender ramifications of the fertile frond, which is characterised by the absence of a lamina. Each sporangium has a pedicel, and three to eight sporangia are attached to a common peduncle; the walls of the sporangia are at least two cell-layers in thickness and the annulus consists of a band of thick-walled cells passing from the crest down each side (figs. B and C), thus differing from the sporangia of Botryopteris ([fig. 319], D, F) in which the broad annulus is confined to one side.

Fig. 320. Stauropteris oldhamia. (After Tansley. From a section in Dr Scott’s Collection. × 60.)

It is practically certain that the fronds described by Grand’Eury[1182] as Schizopteris pinnata ([fig. 309], E) and Schizostachys frondosus represent respectively the sterile and fertile leaves of Etapteris. Zeiller[1183] gives expression to this by substituting the generic name Zygopteris for Schizopteris, and we may now speak of the leaves as Etapteris. Dr White[1184] has referred to a new genus, Brittsia, some impressions of pinnate fronds from the Coal-Measures of Missouri which, as he points out, bear a close resemblance to Schizopteris pinnata Grand’Eury ([fig. 309], E). No sporangia have been found; it is, however, probable that Brittsia problematica represents fragments of a leaf borne by a plant closely allied to Etapteris (Zygopteris). The broad rachis bears crowded pinnae given off at a wide angle; the small pinnules are rather deeply lobed or pinnatifid (3–10 mm. long by 1·5–3 mm. broad). The lamina is traversed by irregularly lobed and occasionally anastomosing veins. In the fertile pinnae the segments have no lamina but bear bundles of pedicellate sporangia.

It should be noticed that the sporangia described by Renault and by other authors as those of Zygopteris (fig. 319, A–C) have not been found in organic continuity with a frond showing a well-preserved vascular strand. It is, however, certain that this characteristic annulate sporangium, borne on branched and slender pedicels, was produced on fronds with a much reduced lamina belonging to some species of the Zygoptereae, Etapteris and probably also Ankyropteris.

Stauropteris.

This genus was instituted by Binney for petioles from the Lower Coal-Measures of Oldham (Lancashire).

Stauropteris oldhamia Binney[1185] is characterised by a stele (figs. [308], E–G; [310], C; [320]; [321]) composed of four groups of xylem which Bertrand regards as homologous with the antennae of Diplolabis, Ankyropteris, and Etapteris, the horizontal cross-piece of these genera being absent in Stauropteris. Williamson spoke of this species as “one of the most beautiful and also one of the most perplexing of the plants of the Coal-Measures”; he discussed its possible affinity with both Lycopods and ferns, deciding in favour of the latter group[1186]. In transverse section the petiolar vascular axis is approximately square, the xylem groups forming the ends of the diagonals; the tracheal groups are separated by phloem and the centre of the stele in the primary rachis is also occupied by that tissue, which is connected by four narrow strips with the external phloem. The structure of the petiolar vascular axis is very clearly shown in the drawing by Mrs Tansley reproduced in [fig. 320]. Protoxylem elements occur close to the surface of each of the four arms of the xylem; the bays between the two lateral and the two lower xylem groups contain large sieve-tubes. Portions of the inner cortex are seen in places abutting on the small-celled pericyclic tissue.

The right and left halves of the stele are not absolutely identical ([fig. 320]; [fig. 308], E); this is due to the fact that secondary branches are given off in four rows, two alternately from the right and left sides. The preparation for the departure of the lateral strands alters the configuration of the stelar xylem groups. The protoxylem groups are not external but separated from the surface by one or two layers of metaxylem. In [fig. 308], E, the occurrence of two protoxylem strands in the right-hand groups of metaxylem marks an early stage in the detachment of branches. These two protoxylems are the result of division of single protoxylem strands like those in the left-hand half of the stele. At a later stage the petiolar stele assumes the form shown in [fig. 308], F, and two small bundles are detached to supply aphlebiae: this is followed by the stage shown in fig. G, where two four-armed strands are passing out to a pair of branches of the leaf axis. The separation of these two meristeles leaves the right-hand half of the stele in the condition seen on the left-hand side of fig. E. The diagrammatic sketch represented in [fig. 310], C, shows one pair of branches in organic connexion with the rachis, and each of these arms contains an obliquely cut vascular strand like those in [fig. 308], G.

The cortex consists for the most part of fairly thick-walled parenchyma ([fig. 321]) which in the hypodermal region is replaced by a zone of thin-walled lacunar tissue. A few stomata have been recognised in the epidermis[1187]. The lower left-hand branch seen in [fig. 310], C, has been shaved by the cutting wheel so that the aerenchymatous tissue, l, is shown in surface-view: a portion of this tissue is enlarged in fig. C′. The same delicate chlorophyllous tissue forms a folded and shrivelled layer with an uneven margin on the surface of the rachis and lateral branches. This hypha-like tissue, which was discovered by Scott[1188] and figured by Bertrand[1189], doubtless represents the much reduced lamina of the highly compound leaves; it may be compared with the green outer cortex of Psilotum shoots and with the lacunar tissue in the capsule of the common moss, Funaria hygrometrica.

Fig. 321. Stauropteris oldhamia: a, sections of pinnae. (× 10. From a section in the Cambridge Botany School Coll.)

The rachis reproduced in [fig. 321] is surrounded by an enormous number of sections, some transverse, others more or less vertical, of branchlets of various sizes. Fig. 310, B, shows the three-rayed vascular axis of a branch of a lower order than those seen in fig. C, and the single vascular strands of still finer ramifications of the leaf. The extraordinary abundance of axes of different sizes, many of which are cut in the plane of branching, in close association with the rachises of Stauropteris affords a striking demonstration of the extent to which the subdivision of the frond was carried in a small space. The leaves must have presented the appearance of a feathery plexus of delicate green branchlets devoid of a lamina, some of which bore terminal sporangia. It may be that the delicate fronds were borne on a slender rhizome which lived epiphytically in a moist atmosphere on the stouter stems of a supporting plant.

The sporangia[1190] of Stauropteris oldhamia are exannulate and nearly spherical, with a wall of more than a single row of cells; they occur at the tips of slender and doubtless pendulous branchlets. The discovery by Scott[1191] of germinating spores ([fig. 323]) in a sporangium of this type supplies an interesting piece of evidence in favour of the fern nature of these reproductive organs. Similar germinating spores have been described by Boodle[1192] in sporangia of Todea.

Fig. 322. Sporangia of Stauropteris oldhamia. St, stomium: p, palisade tissue. (From Tansley, after D. H. Scott, from a drawing by Mrs D. H. Scott.)

Stauropteris burntislandica.

This Lower Carboniferous plant identified by Williamson with the Oldham plant from the Lower Coal-Measures is referred by Bertrand to a distinct species. In the structure of the rachis stele it agrees closely with Stauropteris oldhamia; the main vascular strand gives off four rows of branches, two from each side, and aphlebiae were present at the common base of each pair of pinnae. Mrs Scott[1193], who has recently described the sporangia of this species, speaks of one specimen in which germinating spores were found. The same author gives an account of some curious spindle-shaped bodies which she found in association with S. burntislandica. The nature of these organs is uncertain; Mrs Scott inclines to regard them as glands borne in pairs on lateral pedicels of the frond: she adopts for these the name Bensonites fusiformis proposed by Dr Scott. If there is a reasonable probability, as there certainly seems to be, in favour of connecting these organs with Stauropteris, it is legitimate to question the desirability of adding to the long list of names included in the group Coenopterideae.

Fig. 323. Germinating spores from a sporangium of Stauropteris. (From Tansley, after D. H. Scott.)

Corynepteris. Fig. 309, C, D.

This genus was founded by Baily[1194] on fragments of a fern from Carboniferous rocks in County Limerick, Ireland, characterised by a peculiar type of fructification which he named Corynepteris stellata. More complete examples of the same genus have been described by Zeiller[1195] from the Coal-field of Valenciennes. The sporangia are large, ovoid, and sessile; the annulus ([fig. 309], D) has the form of a complete vertical band several cells in breadth: five to ten sporangia are grouped round a receptacle. Zeiller describes two species as Sphenopteris (Corynepteris) coralloides Gutb. and S. (Corynepteris) Essinghii And.; in both the fronds are quadripinnate and bear aphlebiae at the base of the pinnae. The former species is recorded by Kidston[1196] from the South Wales Coal-field. A single pinnule of C. coralloides is shown in [fig. 309], C. Potonié[1197] refers this frond to his genus Alloiopteris: the portion of a pinna represented in [fig. 354], G shows the characteristic modified pinnule next the rachis. Zeiller draws attention to the occurrence of two parallel lines on the rachis of a specimen of Corynepteris coralloides which he figures[1198], and suggests that these may indicate the existence of an H-shaped form of vascular strand like that of Etapteris and Ankyropteris. The sorus of Corynepteris is comparable with that of the Marattiaceae, but the broad annulus is a difference which suggests affinity to Etapteris. The sorus is similar to that in Diplolabis ([fig. 309], A), but in that genus the sporangia are exannulate.

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The vascular axis in the stems of different members of the Coenopterideae assumes a variety of types. In Botryopteris antiqua the xylem forms a solid protostele in which no protoxylem strands have been recognised; in other species, e.g. B. ramosa, the cylindrical stele is similar to that of Trichomanes radicans (Hymenophyllaceae) in the more or less central position of the protoxylem. In Botryopteris forensis the protostele is said to be exarch. The probability is that the central Botryopteris type is the endarch protostele, a form of vascular axis which may be regarded as primitive. The leaf-traces of the Lower Carboniferous Botryopteris antiqua are simple oval strands differing but slightly from the cylindrical stele of the stem. In the Upper Carboniferous British species the petiolar vascular strand has become more specialised and farther removed from that of the stem; in B. forensis the distinction between leaf and stem steles is still more pronounced. It is perhaps legitimate to regard these types as representing an ascending series, the more primitive of which are distinguished by the greater similarity between leaf and stem, organs differentiated from a primitive thallus[1199], that is from a vegetative body. Portions of this ultimately became specialised as lateral members or leaves, while a portion acquired the character of a radially constructed supporting axis or stem.

ANKYROPTERIS, ETC.

The vascular strand characteristic of the Zygoptereae is represented by the H-shaped form as seen in Ankyropteris corrugata or in a more complex form in A. bibractensis. This style of strand may be regarded as a development from the simple strands of Grammatopteris and Tubicaulis or Botryopteris antiqua along other lines than those followed by B. forensis. The extension of the xylem in two symmetrically placed arms at the ends of the cross-piece of the H is correlated with the habit of branching of the leaf-system which forms one of the striking peculiarities of many of the Zygoptereae. The solid type of stele characteristic of the Botryoptereae is closely matched by that in the Lower Carboniferous stem discovered by Mr Gordon[1200]. By the partial transformation of the central xylem region into parenchymatous tissue and the concentration of water-conducting elements in the peripheral region the style of Ankyropteris corrugata was developed. The vascular strand of the older plant, which is of the Diplolabis type, may be regarded as a more primitive style than that of the H-form of petiole strand represented by Ankyropteris corrugata. A further stage in evolution is seen in the stem stele of Ankyropteris Grayi and A. scandens, both of which have the H-form of meristele. This step in increasing complexity of stem stele, though probably connected with the increasing specialisation of the leaf-traces, as held by Mr Tansley, may also be associated with the development of a climbing habit. In Asterochlaena laxa Stenzel ([fig. 324]) and A. ramosa (Cotta)[1201] the tendency towards a stellate expansion of the originally cylindrical form of stele reaches a higher degree, with the result that a style is evolved which agrees closely with that of the conducting tissue of some existing Dicotyledonous Lianes.

Attention has already been drawn to the generalised features exhibited by the Coenopterideae both in the anatomy of the steles and in the structure of the sporangia. The conclusion arrived at is that while the Coenopterideae foreshadow in some of their characters more than one group of more recent ferns, some at least of their members afford convincing evidence of the correctness of the view—which is also that of Dr Kidston and Mr Gwynne-Vaughan—that the Osmundaceae and the Coenopterideae are offshoots of a common stock.

Fig. 324. Asterochlaena laxa: part of stem with petiole and a few roots. From Tansley, after Stenzel.