Chapter XXXVI: Cycadophyta (2)

McBride[1157] first described this Lower Cretaceous species from the Black Hills of South Dakota as _Bennettites dacotensis_, the generic name _Cycadeoidea_ being adopted by Ward[1158] who distinguished some of McBride’s specimens as _Cycadeoidea McBridei_. The stem is elliptical in section, 32 cm. long and 45 cm. in girth: owing to the partial decay of the petiole-bases the ramental reticulum forms a prominent feature. Numerous flowers, all of which are approximately at the same stage of development, project like conical buttons above the general level of the stem-surface[1159]. The bisporangiate flowers consist of a conical receptacle bearing interseminal scales and megasporophylls, the whole being surrounded by spirally disposed bracts (fig. 529). A whorl of 18 or 19 nearly mature microsporophylls is attached to the base of the receptacle (figs. 528, 530); each microsporophyll has a strong rachis extending beyond the apex of the flower-axis and bent inwards and downwards with the distal end tucked between the ascending rachis and the receptacle. The slender pinnules, in two alternate series, are bent downwards and bear synangia in rows. A transverse section just below the ovulate portion of the cone shows a continuous ring of tissue encircling the receptacle with vascular bundles parallel to the surface (fig. 530, _A_, _m_); at a higher level the bundles fall into groups preparatory to the breaking up of the disc into separate microsporophylls. Immediately above the apex of the central cone the coherent basal portion of the verticil of sporophylls is replaced by the separate rachises which in section have the form of isosceles triangles (fig. 530, _B_). Each microsporophyll is approximately 10 cm. long; the longest pinnule being 1·5 cm. in length and bearing two lateral series of synangia, 10 in each row. At maturity the microsporophylls probably spread out and the whole whorl was thrown off leaving an annular rim (fig. 514, _d_) such as is often seen at the base of older ovulate strobili. The immaturity of the ovules in this (fig. 528) and other specimens led to the suggestion that the strobili of _Cycadeoidea_ were protandrous. The synangia (fig. 531) are several times broader than long and similar in size to those of the Fern _Marattia_. In each synangium are two rows of elongated loculi (fig. 532) containing oval microspores usually with a smooth exine and rather larger than those of recent Cycads. The synangium-wall is composed of a palisade-layer of thick-walled cells and a layer of smaller and thinner cells: the palisade-layer is narrowest at the apex where dehiscence occurs and broadest at the base where the hypodermal tissue is more abundant and forms a short thick pedicel. The two rows of loculi are bounded by flattened cells and there are 10–20 loculi in each of the two rows: dehiscence of the individual loculi occurs in the middle of the inner wall.

_Cycadeoidea Marshiana_ Ward.

This Lower Cretaceous Black Hills species[1160] affords an admirable example of a method of branching exhibited by some stems: as shown in Wieland’s photographs[1161] there may be five or six large and massive trunks all in the same stage of fructification, differing but little in size and forming a cluster resembling independent plants growing in close proximity to one another. There is apparently no central or major axis and the habit is therefore very different from that of a branching _Bucklandia_. _Cycadeoidea Marshiana_ bore large ovulate pyriform strobili projecting slightly beyond the leaf-bases; they are 5 cm. long with a maximum diameter of 3·5 cm. and have an elongated receptacle bearing short scales and sporophylls. The apical bud of one of the trunks shows particularly well in transverse section the dense ramental packing between the bud-scales[1162]. Similar clusters of short and thick trunks occur in _Cycadeoidea superba_ and other American species.

In the small bisporangiate flowers of this species[1163] the microsporophylls, 11 or 12 in the verticil, are characterised by the presence of a ventral furrow which, as Wieland suggests, may be comparable on a much simpler scale with the winged sporophylls of _Cycadeoidea colossalis_ (fig. 533). The disc splits up at a distance of 1 cm. above the apex of the receptacle into the microsporophylls, each of which is 5·5 cm. long and bears two rows of pinnules which in the middle region have a length of 1 cm.; a microsporophyll is elongate, elliptical, and acuminate. In this species as in some others there is a brush of sterile scales at the apex of the receptacle.

_Cycadeoidea colossalis_ Ward.

Wieland[1164] has recently described some interesting features in the microsporophylls of a bisporangiate flower assigned to this Lower Cretaceous species from the Black Hills illustrating a departure from the usual type. The hairy bracts extend considerably above the apex of the flower-proper; in fig. 533, A, a transverse section above the receptacle, they are shown grouped about a circle of V-shaped structures, converging towards a central point, which are the sterile prolongations of the ten rachises of the free portions of the staminate disc. Each rachis is divided by a deep ventral furrow into a pair of wings (fig. 533, B, C), and it is these pairs of wings that form the V-shaped structures in fig. 533, A. The wings form a dome-like group above the flower-apex (fig. 533, D, E). The synangia are borne in two rows on the concrescent disc and on the free sporophylls, which in this type are much simpler than in other species and agree in the absence of pinnules with some forms of _Williamsonia_ (_cf._ fig. 556). The receptacle is pyriform and bears very short scales and seed-stalks; from its apex several interseminal scales are prolonged as a terminal brush, a feature of interest in connexion with flowers of _Williamsonia_. Wieland compares the wings of the microsporophylls to the two horns on the distal surface of the corresponding organs of _Ceratozamia_ and draws a comparison between them and the canopy of some Palaeozoic seeds, but it is doubtful whether homologies can be established between these elaborate sporophylls and the integuments of Pteridosperm seeds[1165].

_Cycadeoidea Reichenbachiana_ (Goeppert).

Goeppert referred this species to his genus _Raumeria_[1166], a generic name retained by Carruthers though, as Solms-Laubach[1167] points out, the abundance of lateral fertile shoots among the leaf-bases shows it to be essentially similar to _Cycadeoidea Gibsoniana_. The type-specimen in the Dresden Museum is a splendid example of a fossil cycadean stem; it is probably of Lower Cretaceous age, but was found in Galicia as an isolated specimen. It consists of a portion of a cylindrical trunk similar in habit to _C. gigantea_, ·5 met. high and about the same in diameter. The pith, according to Schuster[1168], is 13 cm. in diameter and the xylem 8 cm. broad. There are many flowering shoots some of which are represented by cup-like depressions, the base of the cup consisting of the cylindrical axis and the sides bearing the impress of the encircling bracts (fig. 534). Goeppert compared these lateral shoots with the buds frequently produced on the stems of _Cycas_. It is interesting to find that some of the cups correspond very closely in size with _Williamsonia_ flowers, another indication of the very close agreement between the _Williamsonia_ and _Bennettites_ types of strobili. Some flowers still in place show 16 bipinnate microsporophylls that were petrified before the synangium-bearing fronds unfolded[1169]. The ramental scales and other tissues figured by Schuster are of the usual type. A second specimen found as a boulder in Silesia and named by Goeppert _Raumeria Schulziana_ may, as he suggests, be a younger example of _C. Reichenbachiana_. Another Galician stem, probably also Lower Cretaceous in age, is described by Raciborski[1170] as _Cycadeoidea Niedźwiedzkii_.

_Cycadeoidea gigantea_ Seward.

This species is founded on a large stem from the Upper Purbeck series of the Isle of Portland[1171] where it was discovered in a shaly clay 17 ft above the great Dirt bed which yielded the trunks described by Buckland and other authors (fig. 535). The stem (fig. 554) is 1 met. 18 cm. high and has a maximum diameter of 1 met. 7 cm. It is the tallest fossil Cycadean stem so far found in a single piece though _Cycadeoidea Jenneyana_ probably reached a greater height. The stem is elliptical in section (fig. 537), a form due in part at least to compression. The only tissues preserved are in the superficial region of the persistent leaf-bases. As the result of decay before mineralisation many of the petiole-bases are represented by cavities or meshes in a prominent reticulum of silicified ramental scales. Towards the apex the leaf-bases are smaller and a conical bud surrounded by linear scale-leaves occupies the summit: an irregular cap of ramental scales forms the apex of the terminal bud. In section the bud would no doubt present an appearance like that shown in Wieland’s photograph of the terminal cone of _C. Marshiana_[1172]. There is a striking resemblance both in the ramental cap and in the form of the protective scales between the fossil stem and such a Cycad as _Encephalartos Altensteinii_[1173]. A remarkable feature of _C. gigantea_ is the absence of any fertile shoots among the leaf-bases. In one tangential section a small bud was found, but it affords no evidence of the presence of any reproductive organs. A similar bud is described by Lignier[1174] in the stem of _C. micromyela_. The absence of lateral flowers is, however, hardly a sufficient reason for separating this stem generically from other species of _Cycadeoidea_: negative evidence in this case is of doubtful value. While it is possible that the strobili were terminal as in most recent Cycads, it is more probable that they were lateral. The surface-features, though not perfect, are for the most part sufficiently well preserved to enable one to recognise the bract-encircled axes of strobili were any present. If as Wieland believes, and he is probably correct, the production of flowers was the culminating event in the life of these Cycadean plants, the absence of fertile shoots is merely an expression of immaturity. It is, however, difficult to understand how lateral branches could find their way to the surface among the crowded and cork-covered leaf-bases of the stem. The absence of flowers may be due to some unfavourable external conditions. The petioles consist of parenchymatous ground-tissue with many secretory sacs and in some cases twelve partially preserved vascular bundles: the xylem consists of radial rows of centrifugal tracheids and medullary rays, but it is not possible to say whether any centripetal xylem was present. A fairly thick band of phelloderm and periderm, apparently subepidermal in origin, forms the peripheral tissue, and in places epidermal cells with attached ramenta are clearly preserved. The ramenta are of the type characteristic of the majority of fossil Cycadean stems. Stomata were found showing a pair of guard-cells and apparently two subsidiary cells: the epidermal cells have straight walls.

_Cycadeoidea micromyela_ Morière.

This Liassic species from Normandy has been fully investigated by Lignier[1175]; originally referred to the Conifers, it was afterwards described by Morière[1176] as _Cycadeoidea micromyela_. As regards external features the stem is of the usual _Cycadeoidea_ type: Saporta[1177] assigned it to the genus _Platylepis_. The secondary xylem consists of tracheids with bordered pits on the radial walls arranged as single contiguous rows, resembling scalariform pitting, or as 1–2 rows of separate circular pits (fig. 538, A), a type different from that of the great majority of Mesozoic species. The uniseriate medullary rays are 7–20 cells deep, a character recalling the compact wood of Indian _Williamsonia_ stems. Ramental scales are for the most part replaced by long unicellular hairs like those of _Williamsonia scotica_ and some Indian stems. Lignier states that true ramenta occur on the young leaves and in older fronds become transformed by separation of the cells into long tubes. Each leaf-trace arises as a single bundle dividing into three as it leaves the stele and eventually splits up into several collateral bundles (fig. 538, B, C). The stele is elliptical. No fertile shoots are preserved: a single axillary bud is described which agrees very closely with that in _Cycadeoidea gigantea_.

_Cycadeoidea Jenneyana_ Ward.

The stems of this species, from the Black Hills of Dakota[1178], resemble those of _Cycadeoidea gigantea_ and _C. Reichenbachiana_ in their thick columnar form but differ from the former in the presence of bisporangiate flowers associated with the leaf-bases. This species probably reached a length of 3–4 met. A striking feature[1179] is the unusual thickness of the xylem-cylinder, which shows clear indications of concentric rings, probably the expression of seasonal changes during the production of secondary conducting tissue by a single cambium. Similar concentric rings have been described in the stems of the recent Cycad _Dioon spinulosum_[1180]. Fig. 539 represents an ovulate strobilus referred to this species by Wieland.

_Cycadeoidea pumila_ Fliche and Zeiller.

This French Portlandian species[1181] affords a good example of a very small bulbiform trunk, 3·5–4 by 2·5–3 cm., superficially resembling a Pine-cone: the leaf-bases are, however, less uniform in size and shape than Abietineous cone-scales; there are also indications of interpetiolar ramenta and a small lateral, presumably fertile, branch. A stem of similar form is described by Saporta[1182] from Corallian beds in France as _Bulbopodium micromerum_, and _Cycadeoidea nana_ Ward[1183] from Dakota is another example of a small Zamia-like Mesozoic stem. A small tuberous stem, 8 × 7·5 cm., was described by Lindley and Button from Lyme Regis as _Cycadeoidea pygmaea_[1184]; no flowers are shown in the drawing, but occasional irregularities in the arrangement of the leaf-bases may be due to the presence of lateral fertile shoots.

=Cycadella.= Ward.

This generic name was instituted by Lester Ward[1185] for some petrified stems from Jurassic beds in Wyoming of relatively small size, bulbous or more or less spherical, and characterised by the presence of a thick layer (5–15 mm.) of dense ramental tissue. The unusually vigorous development of the ramental scales is an interesting feature, though it is perhaps questionable whether it is worthy of generic recognition. The flowers of _Cycadella_, though less than those of most _Cycadeoidea_ stems, are of the same type. In a later account of the genus Ward[1186] describes a few additional species and in _Cycadella ramentosa_ he notes the preservation of young fronds. The fronds are small and bear a few pinnae: the xylem is said by Wieland[1187] to be mesarch, but in the absence of more details the foliar bundles cannot be fully described. In some Cycadellas young flowers, less than 1 cm. in diameter, are preserved, those of _Cycadella wyomingensis_ (7 mm. in diameter) being the smallest bisporangiate strobili so far recorded. The French species, _Cycadeoidea micromyela_ Mor., resembles _Cycadella_ in the profuse development of ramental tissue which may cover the surface of the stem.

=Amphibennettites.= Fliche.

This generic name was instituted[1188] for two species founded on very imperfectly preserved ‘fruits’ of Lower Cretaceous (Albian) age from the Argonne which, though in close agreement with _Bennettites_ as represented by _B. Gibsonianus_ and _B. Morierei_, are referred to a separate genus on the ground that the preservation is not such as to establish their generic identity with those species and because of certain distinctive features.

_Amphibennettites Bleicheri_ Fliche. The ovulate strobili are sub-elliptical, 3·5 × 3 cm., with several elliptical pits close to the surface originally occupied by seeds borne on short stalks and larger than the seeds of other species of _Cycadeoidea (Bennettites)_. Interseminal scales occur between the seeds. The second species, _Amphibennettites Renaulti_, is rather larger and more conical: the seeds reach a length of 11 mm. greater than that of any of the American seeds described by Wieland. In neither species is there any indication of an involucre of bracts. The preservation of the specimens leaves much to be desired, but it would seem that the Argonne fossils agree in their main characters with Bennettitean strobili and it is doubtful whether a distinctive generic name is necessary. The occurrence of seeds over the whole surface, a feature that suggested the name _Amphibennettites_, may be only apparent and the result of cutting the cone transversely. It is worthy of note that one of Fliche’s sections[1189] bears a close resemblance to an Araucarian cone, and in the absence of structure the two types of cone might easily be confused.

=Vectia.= Stopes. Genus _incertae sedis_.

_Vectia luccombensis_ Stopes.

The generic name _Vectia_ has been given by Dr Stopes[1190] to some petrified secondary phloem discovered by her at Luccomb Chine in the Isle of Wight: the fossil is from Aptian beds. The mass of phloem is 26 mm. in breadth and consists of regularly alternating bands of thin-walled sieve-tubes and very thick fibres associated with a little parenchyma (fig. 540). To the naked eye the specimen presents an appearance suggestive of rings of growth but this is due to the presence of bands of 2–3 narrow cells which are probably cork. The whole is penetrated by uniseriate medullary rays. A striking feature is the regular alternation of single rows of fibres with two bands of sieve-tubes; in places the two bands of sieve-tubes are separated by 2–4 rows of very flat, presumably, cork-cells, and similar bands may be adjacent to or pass obliquely across the fibres. The elongated elements described as sieve-tubes, though thin in comparison with the fibres, have thickened walls and on their radial faces are single rows of circular pits, often in pairs; these are almost certainly sieve-areas which have lost the finer pitting of the sieve-plates. The fibres are more or less square in transverse section and have excessively thick walls, the lumen being reduced to a small hole. Parenchymatous cells (fig. 540, _a_) occupy the angles between the sieve-tubes and occasionally stretch tangentially between a pair of tubes: these are compared by Dr Stopes to companion-cells, but their manner of occurrence hardly justifies the interesting suggestion that they may be precursors of the Angiosperm companion-cells. The medullary rays may be as wide as the elements which they traverse; they are characterised by their wavy walls, as seen in radial longitudinal sections; no pitting was noticed. A remarkable feature of the specimen is its considerable breadth: it is pointed out that in a giant stem of _Sequoia_ with a girth of over 40 ft the secondary phloem does not exceed 3–4 mm. in breadth. The great thickness of the phloem in the fossil suggests comparison with the corresponding tissue in recent and fossil Cycadean stems, and the alternation of hard and soft phloem is a feature exhibited also by _Cycadeoidea Gibsoniana_ (fig. 518, B). Dr Stopes concludes that the phloem is in the main similar to that in some Cupressineae, Taxineae, and Taxodineae. It is noteworthy that similar phloem with sieve-tubes and fibres associated with cork is described by Graf Solms-Laubach[1191] from Upper Jurassic beds in Franz Josef Land. The systematic position of _Vectia_ cannot be definitely determined, but I believe that it is more closely allied to Cycadean than to Coniferous phloem.

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+Note.+ It is unfortunate that Mr Wieland’s second volume dealing with American Cycads [Wieland (16)] did not come into my hands until nearly the whole of this volume was in type. Students will find in it many additions to our knowledge of _Cycadeoidea_ and _Williamsonia_, much theoretical discussion that is suggestive and interesting, useful summaries of our knowledge of fossil Cycads, and many beautiful photographic plates illustrating the morphology of American species of _Cycadeoidea_.