Glossopteris.

The name Glossopteris was proposed by Brongniart in 1822[1295] for an imperfect leaf-impression which he called Filicites (Glossopteris) dubius, but the specimen so named has since been identified as part of a sporophyll of a Lepidostrobus. The author of the genus afterwards published[1296] a diagnosis, based on well-preserved leaves from Permo-Carboniferous rocks in Australia and India, of the type-species Glossopteris Browniana, the Indian examples being distinguished as G. Browniana var. indica while the Australian form was named G. Browniana var. australasica. Schimper[1297] afterwards raised the Indian fossils to specific rank as G. indica though some authors[1298] have continued to consider the two forms as insufficiently distinct to be regarded as different species.

The genus Glossopteris may be defined as follows:

Leaves simple, varying considerably in size, shape, and venation characters, but almost without exception characterised by repeatedly anastomosing lateral veins. The leaves are of two kinds: (i) foliage leaves; apparently always sterile, usually spathulate, with an obtuse apex, a well-marked midrib which may persist to the apex or die out in the upper half of the lamina, characterised by its slight prominence and comparatively great breadth especially in the basal part of the frond. In most cases the lamina extends as a narrow margin to the leaf-base, but in a few forms there is a short petiole ([fig. 334]). Though usually spathulate, the frond may be linear-lanceolate, or ovate; the apex is sometimes acute. Leaves vary in length from 3 to 40 cm. and may in larger forms have a breadth of 10 cm. Numerous lateral veins curve upwards and outwards to the margin of the lamina or pursue a straight course almost at right-angles to the midrib. (ii) Scale-leaves[1299] which differ from the foliage-leaves in their much smaller size and in the absence of a midrib; they are deltoid, oval or cordate in shape and generally terminate in an acute apex; the edge of the lamina may be slightly incurved so that the leaf presents a convex upper surface supplied with anastomosing veins. The scale-leaves, which vary in length from about 1 to 6 cm., probably acted as sporophylls. The only evidence as to the nature of the fructification so far obtained is represented by empty sporangium-like organs (1·2–1·5 mm. long by 0·6–0·8 mm. broad) frequently associated with the scale-leaves[1300].

The leaves, in some cases at least, were borne near together on a cylindrical stem or rhizome which produced branched adventitious roots[1301]. The fossils long known as Vertebraria were recognised by Zeiller[1302] and by Oldham[1303] as the stems of Glossopteris.

The systematic position of Glossopteris must for the present be left an open question. Though usually spoken of as a fern, it is noteworthy that despite the enormous abundance of its foliage leaves in the Permo-Carboniferous strata of India, Australia, South Africa, and South America, no single example has been discovered which shows undoubted remains of sori or sporangia. Many authors have described fertile leaves of Glossopteris; but it was not until Arber’s discovery of sporangia in close association with the scale-leaves that any light was thrown on the nature of the reproductive organs.

The probability is that Glossopteris was not a true fern but a member of that large and ever-increasing class, the Pteridosperms. This opinion is based largely on negative evidence. Such sporangia as have been described may have contained microspores and the plant may have been heterosporous. The occurrence of seeds in association with Glossopteris fronds recorded by more than one writer[1304], though by no means decisive and possibly the result of chance association, is favourable to this view. Dr White[1305] has suggested that the small leaves described by Zeiller[1306] as Ottokaria bengalensis from Lower Gondwana (Permo-Carboniferous) rocks of India, and similar fossils recorded by himself from Brazil as O. ovalis, may represent “sporangiferous” organs of Glossopteris or Gangamopteris, “both of which are probably pteridospermic.” There is, however, no conclusive evidence in support of this suggestion.

The genus, whatever its position may be, has a special interest for the geologist and for the student of plant distribution; it is a characteristic member of a Permo-Carboniferous flora which flourished over an enormous area, including India, South Africa,—extending from Cape Colony to Rhodesia and German East Africa[1307],—Australia, and South America[1308]. This flora, known as the Glossopteris flora, differed considerably in its component genera from that which overspread Europe and North America and some more southern regions in the Upper Carboniferous and Permian periods.

The discovery by Amalitzky[1309] of Glossopteris, and other genera characteristic of the Glossopteris flora, in the Upper Permian rocks in Vologda (Russia) demonstrates the existence of a northern outpost of the southern botanical province, and Zeiller’s discovery of the genus in the Rhaetic flora of Tonkin[1310] shows that Glossopteris persisted beyond the limits of the Palaeozoic epoch. Dr David White[1311] has recently proposed to re-christen the Glossopteris flora the Gangamopteris flora on the ground that Gangamopteris is strictly Palaeozoic in its range, whereas Glossopteris persisted into the Mesozoic era; this is perhaps hardly a sufficient reason for giving up so well established a title as the Glossopteris flora. A fuller account of this southern flora must be reserved for another volume.

Glossopteris Browniana, Brongniart[1312]. [Figs. 334–36].

The specific name Browniana is now applied to obtusely pointed leaves which sometimes reach a length of 15 cm., but are usually rather shorter. In form and venation they closely resemble the leaves of the recent genus Antrophyum and species of Acrostichum. The comparatively broad midrib may be replaced in its proximal portion by several parallel veins: from it are given off numerous lateral veins which form a reticulum characterised by meshes approximately equal in size and elongated in a direction parallel to the general course of the secondary veins ([fig. 334]).

Fig. 334. Glossopteris Browniana, Brongn. A. Nat. size: B × 3½.

The drawings, originally published by Zeiller[1313], reproduced in [fig. 335] illustrate the venation and its range of variation; the meshes are usually hexagonal and arranged as shown in figs. A and B, but occasionally ([fig. 335], C) they follow a more steeply inclined course.

Small leaves with a more or less distinct midrib, 2–3 cm. in length, supply transitional stages between foliage- and scale-leaves. In the true scale-leaves spreading and occasionally anastomosing veins take the place of the midrib and lateral veins of the ordinary frond. McCoy[1314] in describing some Australian specimens of Glossopteris in 1847 spoke of scale-like appendages of the rhizome which he compared with the large ramenta of Acrostichum and other ferns. It was, however, Zeiller[1315] who first recognised the leaf-nature of these scales and adequately described them; additional figures of scale-leaves have been published by Mr Arber[1316] and by myself[1317]. The importance of these small leaves has been considerably increased by Mr Arber’s discovery of associated sporangia which, as he suggests, were probably borne on their lower concave surface.

Fig. 335. Glossopteris Browniana, Brongn. (After Zeiller. × 2.)

The sporangia ([fig. 336]) are compared by Arber with the microsporangia of recent Cycads and with the Palaeozoic sporangia described by Zeiller as Discopteris Rallii ([fig. 256], D); the latter are distinguished by the well-defined group of thicker walled cells representing the annulus of true fern sporangia. We know nothing as to the contents of the Glossopteris sporangia, whether they contained microspores or whether they are the spore-capsules of a homosporous plant.

Fig. 336. Glossopteris Browniana, Brongn. Sporangia. (× 30). After Arber.

The rhizome of Glossopteris Browniana has been described in detail by Zeiller, who first demonstrated that the fossils originally assigned by Royle[1318] to the genus Vertebraria represent the stem of this and, as we now know, of some other species of Glossopteris. Vertebraria occurs in abundance in Permo-Carboniferous strata in association with Glossopteris; the differences between Australian, Indian, and South forms, though expressed by specific names, are insignificant. The stems are usually preserved in the form of flattened, single or branched, axes sometimes bearing slender branched roots and characterised by one or two, or less frequently three, longitudinal grooves or ridges ([fig. 337]) from which lateral grooves or ridges are given off at right angles, dividing the surface into more or less rectangular areas 1 cm. or more in length. The surface of these areas is often slightly convex and in some specimens the outlines of cells may be detected. Mr Oldham has described some interesting examples of Vertebraria from India in which the longitudinal and transverse grooves are occupied by a dark brown ferruginous substance or by the carbonised remains of plant-tissues ([fig. 338], C, D). In transverse section, a Vertebraria cast appears to be divided into a number of wedge-shaped segments radiating from a common centre. Prof. Zeiller[1319] has figured specimens of Vertebraria with portions of Glossopteris fronds still attached.

Fig. 337. Vertebraria indica, Royle. Nat. size. (After Feistmantel.)

The rhizome of Glossopteris, as represented by the Vertebraria casts, is aptly compared by Zeiller[1320] with that of the recent Polypodiaceous fern Onoclea struthiopteris. Sections of the recent stem ([fig. 338], E, F) show that the form is irregularly stellate owing to the presence of prominent wings which anastomose laterally at intervals as shown by the examination of a series of sections. The leaf-traces are derived from the steles of adjacent wings. Fig. 338 (B and A) represents somewhat diagrammatically a longitudinal and transverse view of a Vertebraria; the radiating arms represented in the transverse section (fig. A) are the stem ribs or wings and the segments between them are intrusions of sedimentary material. The rectangular areas characteristic of the surface of a Vertebraria are the intruded segments of rock: these are separated at intervals by transverse grooves, which mark the course of vascular strands given off at each anastomosis of the longitudinal wings to supply the leaves.

Fig. 338.

Mr Oldham, who discovered the connexion between Glossopteris and Vertebraria independently of Dr Zeiller, does not agree with the interpretation of the structural features of the rhizome which Zeiller bases on a comparison between Vertebraria and Onoclea struthiopteris. Oldham[1321] describes Vertebraria as consisting of a central axis “joined to an outer rind by a series of radial septa,” the spaces between the septa being divided into chambers by transverse partitions. His view is that the rhizome of Glossopteris was a cylindrical organ and not an irregularly winged axis like the stem of Onoclea. Zeiller[1322] has replied in detail to Oldham’s interpretation and adheres to his original view, that the rhizome consisted of a solid axis with radial wings or flanges which at intervals anastomosed transversely in pairs at the nodes. It may, however, be possible that the spaces between the longitudinal and transverse grooves on a Vertebraria axis, which have been filled with the surrounding rock, were originally occupied in part at least by secondary wood, and the transverse strips of carbonaceous material[1323] lying in the grooves may represent medullary-ray tissue and accompanying leaf-traces. The longitudinal striations seen in some specimens of Vertebraria on the areas between the grooves may be the impressions of woody tissue. It is impossible without the aid of more perfectly preserved material to arrive at a satisfactory conception of the structural features of a complete Glossopteris rhizome.

Fig. 339. Glossopteris fronds attached to rhizome. (From a specimen lent by Dr Mohlengraaff. Considerably reduced.)

In the specimen of Glossopteris Browniana shown in [fig. 339] several leaves are attached to an axis which shows none of the surface-features of Vertebraria. I am indebted to the kindness of Dr Mohlengraaff of Delft for the loan of this specimen which was obtained from Permo-Carboniferous rocks in the Transvaal. An axis figured by Etheridge[1324] from an Australian locality bears a tuft of Glossopteris leaves, possibly G. Browniana; in place of the rectangular areas characteristic of Vertebraria it shows transversely elongated leaf-scars or, on the internal cast, imbricate rod-like projections which Etheridge suggests represent vascular bundles.

Glossopteris indica, Schimper. Figs. [340], A, [341].

It is a question of secondary importance whether or not the fronds which Brongniart spoke of as a variety of Glossopteris Browniana should be recognised as specifically distinct. The careful examination by Zeiller of the venation characters has, however, afforded justification for separating G. Browniana and G. indica. We must admit that the slight and not very constant differences in the size and form of the meshes produced by the anastomosing of the lateral veins are characters which cannot be recognised as having more than a secondary value, though, as a matter of convenience, we employ them as aids to determination. The arbitrary separation of sterile leaves, which differ by small degrees from one another in form and in the details of venation, by the application of specific names is a thankless task necessitated by custom and convenience; it is, however, idle to ignore the artificial basis of such separation. Mr Arber has recently published, in his valuable Glossopteris Flora, an analytical key which serves to facilitate the description and determination of different types of frond[1325].

Fig. 340.

  1. Glossopteris indica, Schimper. (½ nat. size.)
  2. Glossopteris angustifolia, Brongniart. (Nat. size.) From Arber, after Feistmantel.

The large leaves of Glossopteris indica, reaching a length in extreme cases of 40 cm. and a breadth of 10 cm., are characterised by a rather greater regularity in the arrangement of the meshes and by the greater parallelism of the upper and lower sides of each mesh ([fig. 341]) and by less difference in size between the venation meshes than in G. Browniana, the leaves of which are usually smaller. The relatively thick epidermis consists of rectangular cells with stomata in depressions[1326]. The scale-leaves[1327], rather larger than those of G. Browniana, are more or less rhomboidal with rounded angles and reach a length of 1·5–6 cm. and a breadth of 1·5–2·5 cm. The rhizome is practically identical with that of G. Browniana[1328].

Fig. 341. Glossopteris indica, Schimp. (× 1½.) From Arber, after Zeiller.

This species occurs in great abundance in the Permo-Carboniferous rocks of India, Australia, and in various parts of South Africa, and elsewhere. It has been recognised also by Amalitzky[1329] in Upper Permian beds in Russia and by Zeiller in the Rhaetic series of Tonkin[1330].

Fig. 342. Glossopteris angustifolia var. taeniopteroides. (× 3½.)

Glossopteris angustifolia, Brongniart. Figs. [340], B; [342].

It is convenient to retain this designation for linear fronds with an acute or obtuse apex and a venation-reticulum composed of long and narrow meshes ([fig. 340], B). It is by no means unlikely, as Arber suggests, that the same plant may have produced leaves of the G. indica type and narrower fronds which conform to G. angustifolia. In his description of some Indian specimens of G. indica, Zeiller draws attention to the variation exhibited in regard to the extent of anastomosing between the secondary veins: some examples with very few cross-connexions agree more closely with Taeniopteris than with Glossopteris as usually defined[1331]. The venation shown in [fig. 342] illustrates an extreme case of what is almost certainly a Glossopteris leaf of the G. angustifolia type. This specimen, which was discovered by Mr Leslie in the Permo-Carboniferous sandstone of Vereeniging (Transvaal), has been referred to a variety of Brongniart’s species as G. angustifolia var. taeniopteroides[1332] on account of the almost complete absence of any cross-connexions. The reference to Glossopteris, which my friend Dr Zeiller suggested, is amply justified by the form of the leaf as a whole, by the angle at which the lateral veins leave the midrib, a feature in contrast to the wider angle at which the lateral veins are usually given off in Taeniopteris (figs. [329], [332]), and by the similarity to the Indian specimens already mentioned. Several authors have described leaves or leaflets under the generic name Megalopteris[1333] from Carboniferous and Permian rocks which bear a close resemblance to the South African variety, but in some cases at least Megalopteris is known to be a pinnate and not a simple leaf. The leaf figured by Jack and Etheridge as Taeniopteris sp.[1334] from Queensland may also be an example of Glossopteris. Comparison may be made also with the Palaeozoic leaves described in the first instance by Lesquereux and more recently by Renault and Zeiller as species of Lesleya[1335] ([fig. 347]).

Fig. 343. Blechnoxylon talbragarense, Eth.: s, scale-leaves; x, secondary xylem. (After Etheridge. A × 2; B × 3; C much enlarged.)

Blechnoxylon talbragarense, Etheridge. Fig. 343.

Under this name Etheridge[1336] described some specimens from the Permo-Carboniferous Coal-Measures of New South Wales, which he regards as a fern, comparable, in the possession of a cylinder of secondary xylem, with the recent genus Botrychium and with Lyginodendron and other members of the Cycadofilices. The slender axis (1–3 mm. in diameter) appears to consist of a zone of radially disposed tissue ([fig. 343], C, x), which is probably of the nature of secondary xylem, enclosing a pith and surrounded externally by imperfectly preserved remnants of cortex. Unfortunately no anatomical details could be made out, but the general appearance, if not due to inorganic structure, certainly supports Etheridge’s determination. The stem bore at intervals clusters of linear-lanceolate leaves (reaching 12 mm. in length) in close spirals ([fig. 343], A and B); the leaves are characterised by a strong midrib and forked secondary veins. Small “pyriform” bodies of the nature of scale-leaves occur in association with the fronds ([fig. 343], B, s).

In his description of this interesting plant, Etheridge quotes an opinion which I expressed in regard to the comparison of the stem with those of Botrychium, Lyginodendron, and other genera. No satisfactory evidence has been found as to the nature of the fructification. Although the leaves of Blechnoxylon are much smaller than those of Glossopteris, I am now disposed to regard the genus as closely allied or even generically referable to Glossopteris. The crowded disposition of the leaves is like that in Glossopteris, shown in [fig. 339] and in the figures published by Etheridge and by Oldham; the association of scale-leaves and foliage-leaves is another feature in common. The absence of a reticulum of anastomosing veins can no longer be considered a fatal objection to the suggestion that the Australian type may be a species of Glossopteris. If the view that Blechnoxylon is not a distinct genus is correct, the occurrence of secondary xylem is favourable to the opinion already expressed that Glossopteris is more likely to be a Pteridosperm than a true fern. The data at present available render it advisable to retain Mr Etheridge’s name: the comparison with Glossopteris lacks confirmation.

BLECHNOXYLON

Fig. 344. Glossopteris retifera. (Nat. size. From Arber, after Feistmantel.)

Glossopteris retifera, Feist. Fig. 344.

In some Glossopteris leaves the anastomosing secondary veins form a coarser reticulum, as in the example represented in [fig. 344]. The name G. retifera was given by Feistmantel[1337] to Indian fronds of this type; similar forms have been described as G. conspicua and G. Tatei. The type illustrated by G. retifera is recorded also from Permo-Carboniferous rocks in Zululand[1338], Natal, the Transvaal, Cape Colony, and the Argentine.