DICTYOZAMITES. Oldham.
This generic name was instituted[1567] for pinnate fronds from the Rajmahal series of India, assigned by Feistmantel to a Lower Jurassic horizon, but probably nearer in age to the Middle Jurassic series. Morris, joint author with Oldham of the Memoir on the Rajmahal plants, adopted the name Dictyopteris. Dictyozamites agrees very closely with Otozamites but is readily distinguished by the anastomosing veins (fig. 609), while the absence of a midrib differentiates it from the Palaeozoic genus Dictyopteris. The genus is recorded also from Jurassic strata in England, Bornholm, Japan[1568], Korea[1569], and from a locality 60 nautical miles N.W. of Cape Horn in Tierra del Fuego[1570]: the specimens from these localities differ only in unimportant details from the Indian type. Dictyozamites is confined to Jurassic floras and appears to be especially characteristic of Middle Jurassic floras. In India[1571] fronds occur in close association with stems of the Bucklandia type and, although there is no proof of any connexion with stems or flowers, it is probable that the fronds of this generic type were borne on stems similar to those of Bucklandia and that the flowers were constructed on the Williamsonia plan.
Dictyozamites falcatus (Morris).
Morris[1572] described this Rajmahal species under the names Dictyopteris falcata and D. falcata var. obtusa; the specimens were included by Feistmantel under the designation Dictyozamites indicus, the original term falcata being discarded without adequate reason. An examination of several of the figured fronds enables me to confirm the accuracy of the published drawings and to extend the description in a few particulars. Fronds pinnate of the habit of Otozamites; pinnae comparatively thick, broadly linear, alternate, varying considerably in size and in the form of the apex which may be obtuse or acute. The pinnae are attached by the middle part of the lamina only; the upper edge of the base is slightly rounded or auriculate; the lower angle is generally more definitely lobed. The imbricate bases completely cover the upper face of the comparatively slender rachis. The longest pinna seen on an Indian frond is 6 cm. with a breadth of 2 cm. Some of the pinnae on magnification exhibit a finely tuberculate appearance, due no doubt to the presence of papillae on the epidermal cells like those on the pinnae of English and Bornholm specimens. This species is recorded by Yokoyama[1573] from Japan as D. falcatus var. distans, and a similar form, referred by him to a distinct species, D. grossinervis, may be more appropriately named D. falcatus var. grossinervis[1574]. The Indian type occurs also in Jurassic rocks of Korea, and Halle has discovered some Dictyozamites leaves in Tierra del Fuego[1575] which he describes as Dictyozamites sp. cf. D. falcatus: this record is of special interest from a phytogeographical point of view.
Dictyozamites Johnstrupi Nathorst.
This species[1576], from rocks usually spoken of as Liassic but not improbably Middle Jurassic in age, differs from D. falcatus in the slightly more falcate form of the pinnae and in the more strongly auriculate upper angle of the base of the lamina. The stomata[1577] are confined to the lower epidermis and the strongly cuticularised guard-cells frequently lie at right-angles to the course of the veins. The epidermal cells have very sinuous walls (fig. 609, C) and in the middle of each is a prominent papilla.
Dictyozamites Hawelli Seward.
This Middle Jurassic type from Marske[1578], Yorkshire, differs from D. falcatus in the relatively broader segments, their straighter form (fig. 609, B), and in the attachment to the rachis being slightly below the middle of the pinna base. Fig. 609, A, shows the base of a pinna and an oval scar similar to those on the rachis of some recent Cycads (cf. fig. 387). The epidermis is like that in D. Johnstrupi and as in that species the stomata are confined to the lower surface. There are approximately 120 stomata per square millimetre; each with two subsidiary cells (fig. 609, C) of elongated form, and the guard-cells have small thickened outgrowths or papillae as in D. Johnstrupi.
The epidermal features are described under D. Johnstrupi and D. Hawelli (fig. 609, C): a fuller account of those in the latter species will be found in the paper by Mr Thomas and Miss Bancroft.
PTEROPHYLLUM. Brongniart.
The name Pterophyllum was first applied by Brongniart[1579] to some fronds from the Lias of Höör, which he named P. majus and P. minus, species subsequently removed by Nathorst[1580] to Schimper’s genus Anomozamites. Brongniart[1581] defined Pterophyllum fronds, e.g. P. Jaegeri (fig. 610), as pinnate, bearing pinnules of almost equal breadth attached by the whole width of the base and with a truncate apex; veins slender, equal, simple and slightly arched. There has been considerable confusion and laxity in regard to the application of the name Pterophyllum and in many cases no clear distinction has been recognised between this genus and Nilssonia. In Nilssonia the distinctive feature is the complete absence of any rachis uncovered by the lamina on the upper face of well-preserved specimens, while in Pterophyllum the continuity of the lamina is broken by a greater or less breadth of rachis in the middle line of the frond; the lamina does not cover the rachis but is attached laterally, or at least the two laminae of the frond, whether entire or deeply dissected, do not meet in the middle of the rachis. A specimen from the Cretaceous of Greenland described by Heer as Pterophyllum concinnum[1582] and now in the Stockholm Museum is probably a piece of a Nilssonia; the rachis is not exposed on the surface of the frond. In Nilssonia the veins are with few exceptions simple; in Pterophyllum they are often branched especially near their origin from the rachis: in Nilssonia the segments are of unequal breadth; in Pterophyllum they are usually equal. It has been the practice of several authors to follow Schimper[1583] in the employment of the generic name Anomozamites for fronds with a more or less entire or irregularly pinnatisect lamina which bear a very close resemblance to Nilssonia except that a portion of the rachis is exposed on the upper face. Potonié[1584] used Pterophyllum in a wider sense including both fronds with equal pinnae and those of the Anomozamites type: this more extended use of Pterophyllum is adopted by Zeiller[1585] who prefers to retain Anomozamites only as a sub-genus. It is in this sense that the following definition is framed.
Fronds pinnate; pinnae linear, attached by the whole base, which may be enlarged; the apex is truncate, rounded, or occasionally pointed; the veins are simple or dichotomously branched and parallel to the edge of the lamina. In some fronds (sub-genus Anomozamites) the segments are unequal (fig. 615), short, broad and truncate, and the lamina may be entire or divided into a few very unequal segments. The surface of the rachis is never completely covered by the lamina on the upper face of the frond and is often characterised by transverse wrinklings, possibly due to the presence of ramental scales.
Some interesting xerophilous features have recently been described by Halle[1586] in the lamina of Pterophyllum (Anomozamites) marginatum, a Rhaetic species figured by Schenk and Nathorst from Franconia and Scania. The pinnae were thick and succulent. Stomata are confined to the lower surface and the edges of the upper face of the lamina form a rounded curve causing the thick upper cuticle to abut on the thinner stomatiferous lower cuticle, thus producing in impressions the appearance of thickened margins.
There is no satisfactory evidence as to the nature of the reproductive organs or stems of the plants which bore fronds of the type represented by P. Jaegeri (fig. 610) and other forms with equal segments. A stem referred by Heer[1587] and by Leuthardt[1588] to Pterophyllum was not found in connexion with leaves, and there is the same absence of any convincing evidence in the case of the imperfect specimens from the Keuper of Basel believed by Leuthardt to be male and female inflorescences. On the other hand leaves of Anomozamites are known to have been borne on comparatively slender stems with dichasial branching, and the reproductive shoots are of the same general type as those known as Bennettites. This discovery is due to Nathorst who founded the genus Wielandiella[1589] on fertile stems with Anomozamites fronds. Mr Thomas[1590] has recently obtained evidence from Yorkshire Jurassic material pointing to a similar connexion between Anomozamites fronds and Wielandiella stems. Pterophyllum is further distinguished from Nilssonia by the folded walls of the epidermal cells and by the presence of two crescentic subsidiary cells (fig. 611, A) instead of the ring of cells which surrounds the guard-cells in Nilssonia (fig. 611, B). Schenk[1591] figured a few pieces of cuticle from fronds referred to Pterophyllum in which the walls are more or less sinuous, but the cells of Pterophyllum crassinerve Goepp.[1592] appear to have straight walls: this species is, however, probably a Nilssonia. The stoma represented in fig. 611, A, is from the lower epidermis of Pterophyllum (Anomozamites) Nilssoni (Phill.)[1593].
Pterophyllum extends from Upper Carboniferous to Lower Cretaceous strata and is especially characteristic of Keuper and Rhaetic floras. A possible representative of the sub-genus Anomozamites is described by Ettingshausen from Tertiary beds in New South Wales as Anomozamites Muelleri[1594].
Pterophyllum Fayoli Renault and Zeiller.
This species[1595] was founded on a large frond from the Commentry coalfield with a stout rachis bearing linear pinnae, attached along a narrow groove on its outer edge, reaching a length of 12 cm. and a breadth of 1 cm.; adjacent pinnae are united at the base. The apices are pointed and there are 10–12 unbranched veins in the lamina. Fig. 612 shows a small piece of the large specimen in the Natural History Museum, Paris.
Pterophyllum sp. Arber.
The only representative of a Carboniferous frond of Cycadean habit discovered in Britain is that recorded by Arber[1596] from Westphalian strata at a depth of 1834 ft at Barfreston in Kent. A fairly stout rachis bears incomplete pinnae the longest of which is slightly more than 1 cm. in length and 1–1·5 cm. broad; each segment, very slightly contracted at the base, has 3–4 occasionally forked veins. Arber compares the fragments with Pterophyllum blechnoides Sand. from Germany[1597].
Pterophyllum Cambryi Renault.
This Permian species from Autun[1598] is similar to P. Fayoli, but differs in the more slender rachis and in the finer venation of the linear, acute, pinnae. The pinnae in Renault’s specimen reach a length of 4·8 cm. and are from 3·5–4 mm. broad; they are slightly confluent at the base and the veins occasionally fork.
Among other Palaeozoic species are Pterophyllum Grand’Euryanum Sap. and Mar.[1599] discovered by Grand’Eury in the Coal Measures of France, P. blechnoides Sand.[1600] from the Upper Coal Measures of Oppenau and recorded by Geinitz[1601] from the Middle Permian of Germany, P. Cottaeanum Gutb.[1602], a similar type, also from Permian rocks, but with coarser venation. The specimens figured by Geinitz as P. blechnoides and P. Cottaeanum are in the Dresden Museum: the former species is represented by a good impression on shale which shows the generic characters very clearly.
Pterophyllum Jaegeri Brongniart.
Brongniart[1603] founded this species (fig. 610) on material from the Keuper of Stuttgart. The fronds are characterised by the relatively narrow parallel-sided pinnae with rounded or truncate apices and little or no basal contraction; the pinnae may be contiguous or separate. There are 14–16 veins in each pinna. Leuthardt[1604], who figures several examples of this species from Keuper strata at Basel, speaks of the breadth-index of the pinnae as 12, a term employed to denote the relation of the breadth to the length of a segment. Pterophyllum longifolium[1605] is a very similar Keuper type: in this species the pinnae are rather shorter and broader than in P. Jaegeri and contracted at the base; the apex is truncate or obtuse. The differences between these species is, however, very trifling. The imperfectly preserved fossils figured by Leuthardt[1606] as the inflorescences of Pterophyllum have not been found in connexion with fronds and their nature is problematical. This and other species are recorded by Krasser[1607] from the Lunz beds in Austria.
Pterophyllum Bronni Schenk.
A Keuper species[1608] from Raibl in Carinthia distinguished by its large pinnae, 15 cm. or more in length and 0·5 cm. broad, and by their almost digitate disposition on the rachis. The fronds appear to be relatively short: the upper pinnae are highly inclined while the lower are given off at an obtuse angle; they are obcuneate or more or less oblanceolate, with a truncate asymmetrical apex and rounded angles; the base is slightly contracted. Arber[1609] has figured some specimens from the Munich Museum in one of which there are five spreading pinnae and a portion of the rachis; one pinna is undoubtedly attached and the others, though probably in place, are not in actual connexion with the axis. There is a close resemblance between this species and Sphenozamites Rogersianus Font.[1610]: in the American type the fronds must have been 1 met. long; the leaflets, 19 × 9 cm., are elliptical, broadest near the apex and attached by the middle of the rounded base. Between the divergent and forked veins there is a fine granulation probably due to the presence of papillae. Pterophyllum Bronni and P. Rogersianum might be placed in Schimper’s genus Macropterygium[1611], though another Raibl species, originally described by Schenk as Pterophyllum giganteum[1612] and referred by Schimper to Macropterygium, has much longer and relatively narrow pinnae and agrees more closely with Pterophyllum as the genus is usually defined. In order to emphasise the distinctive features of Pterophyllum Bronni and Fontaine’s Virginian species they may conveniently be spoken of as Pterophyllum (Macropterygium) Bronni and Pterophyllum (Macropterygium) Rogersianum.
Pterophyllum Braunianum Goeppert.
In this Rhaetic species[1613] (fig. 613) the narrow linear pinnae, contiguous or more or less distant, are 1–2 mm. broad and may exceed 5 cm. in length though they are usually shorter; they are attached more or less at right-angles to the sides of a transversely wrinkled rachis. The base of the lamina is broadened and the veins are parallel, simple or forked. The epidermal cells have sinuous walls.
Pterophyllum Tietzei Schenk.
A Rhaetic species founded[1614] on specimens from Persia and described also by Zeiller[1615] from Tonkin represented by fronds 1 met. long and at least 15 cm. broad. The segments, 3–7 cm. long and 8–18 mm. broad, are often set at right-angles to the rachis; the edges are parallel but the lower edge bends upwards at the apex. The veins are simple or forked and approximately 0·3 mm. apart.
Pterophyllum Bavieri Zeiller.
This Rhaetic species from Tonkin[1616] and Persia[1617] is interesting as affording an example of a frond with exceptionally narrow and contiguous pinnae. The frond is narrow and lanceolate, 15–20 cm. long with pinnae normal to the rachis, ·5–1 mm. broad and 7–25 mm. long attached by the whole base which is sometimes slightly enlarged. The veins are simple or forked, ·15–·25 mm. apart: the apices of the segments are rounded or truncate and the form of the whole is similar to that of Pterophyllum aequale but narrower.
Pterophyllum (Anomozamites) inconstans Braun.
In habit this Rhaetic species[1618] resembles Nilssonia compta. The linear-lanceolate fronds, as described by Zeiller[1619] in his Tonkin memoir, attain a length of 20 cm. and a breadth of 4 cm. The rachis, characterised by numerous transverse ridges, bears alternate or sub-opposite leaflets often at right-angles; they are almost rectangular and hardly longer than broad, attached by the whole of the base, which may be broadened, to the upper face of the rachis, but the lamina does not cover the whole of the rachis as in Nilssonia. The pinnae vary in breadth on the same frond and occasionally the lamina is continuous as in some forms of Nilssonia. Schenk[1620] figures examples from the Rhaetic of Franconia in which the lamina is very irregularly dissected and may be entire for a considerable distance. The epidermal cells have slightly sinuous walls. A similar type is represented by Pterophyllum (Anomozamites) Schenki Zeill.[1621], but the segments are contracted basally and become wider towards the apex where, as in P. inconstans, the edges are parallel. Feistmantel’s species Platypterygium Balli[1622], though superficially similar to Pterophyllum, appears to agree more closely with Pseudoctenis.
Pterophyllum Nathorsti Seward.
This species, founded on a specimen in the Sedgwick Museum, Cambridge, from the Middle Jurassic beds of Yorkshire, was formerly referred to the genus Dioonites[1623], but since the Yorkshire specimens were supplemented by material from Kimmeridge strata in Scotland[1624] affording more definite information with regard to the attachment of the pinnae it has been transferred to Pterophyllum. Pinnae laterally attached to the rachis which in some specimens shows a transverse wrinkling, narrow, linear, 3–4 mm. wide and reaching a length of 9 cm. The base of the segments is slightly expanded; the apex is acuminate or bluntly pointed: there are approximately three veins in 1 mm. of lamina. This type (fig. 614, A, B) is similar to Pterophyllum aequale Brongn. as figured by Zeiller[1625] from the Rhaetic of Tonkin and agrees closely with some of the large Rhaetic specimens referred by Schenk to P. Braunianum. The Jurassic fronds from Oregon described by Fontaine[1626] as Ctenophyllum angustifolium are similar forms and a specimen from the Wealden of Germany included by Schenk in P. Dunkerianum[1627] should probably be included in this species. Other closely allied forms are represented by P. distans Old. and Morr.[1628] and P. Footeanum from India.
Pterophyllum (Anomozamites) Nilssoni (Phillips).
The type-specimen, from the Jurassic beds on the Yorkshire coast and now in the York Museum, was described by Phillips[1629] as Aspleniopteris Nilssoni and placed in Pterophyllum by Lindley and Hutton and many other authors[1630]. The fronds are linear; the lamina may be almost entire or divided into unequal deep truncate segments; the veins are simple or dichotomously branched (fig. 615). The leaves are very similar to those of Nilssonia; they were probably borne on stems similar to those described by Nathorst as Wielandiella. Reference is made to the structure of the epidermal cells in the account of the genus.
Pterophyllum (Anomozamites) Lyellianum (Dunker).
This Wealden species, originally referred by Dunker[1631] to Pterophyllum, was afterwards transferred to Anomozamites[1632]. The fronds, in habit resembling the larger Rhaetic species P. Braunianum Goepp., are characterised by the narrow linear pinnae of almost equal breadth attached approximately at right angles; there is a narrow line of rachis separating the two rows of segments (fig. 616).
{
CYCADITES. Sternberg.
PSEUDOCYCAS. Nathorst.
The generic name Cycadites was first used by Sternberg[1633] though it has since been shown that the specimens for which he proposed it belong to other genera. As employed by Brongniart[1634] and other authors Cycadites stands for fossil fronds agreeing in habit with the pinnate leaves of recent species of Cycas (fig. 387, A); the presence of a single median in the linear pinnae is generally regarded as an essential feature. Cycadites may be defined as follows: Fronds pinnate; pinnae alternate or opposite, linear entire, with a midrib and no lateral veins, attached to the rachis by the entire base, the lower margin of which may be slightly decurrent or narrowed near the point of attachment. Epidermal cells with straight walls and stomata agreeing in structure with those of the recent genus. As thus defined, including the characters of the epidermal cells, the genus has no representatives; but the generic name should also be applied, as Miss Holden[1635] has recently suggested, to fronds of the Cycas form which afford no information with regard to the structure of the epidermis. The resemblance of Cycadites fronds to those of Cycas, coupled with the fact that impressions superficially resembling megasporophylls of the recent genus are recorded from Mesozoic strata, was considered sufficient justification for the employment of a name implying close affinity to or even generic identity with Cycas. A fuller knowledge of the epidermal characters of some of the fronds hitherto included in Cycadites has necessitated the substitution of a new genus for Cycadites. An examination by Nathorst[1636] of Heer’s Lower Cretaceous Greenland species Cycadites Steenstrupi and of other fronds showed that the epidermal cell-walls are sinuous and the stomata arranged differently from those of Cycas. For these and other reasons Nathorst proposed a new name Pseudocycas. He also pointed out that the pinnae in the fossil fronds have as a rule a double ‘midrib.’ Nathorst’s discovery raised the question whether other fronds described as Cycadites should be transferred to Pseudocycas. Miss Holden’s investigation of other supposed Mesozoic representatives of Cycas leads to the conclusion that ‘it is not impossible that all Mesozoic specimens of Cycadites will turn out to be Pseudocycas.’ This observer shows that Cycadites Saportae Sew. and C. Roemeri Schk. agree in the sinuous walls of the epidermal cells with Pseudocycas; and so far no species of Cycadites, in which it was possible to examine the cuticular membrane, has been found that does not conform to Nathorst’s definition of Pseudocycas at least as regards the structure of the epidermal cells. Nathorst defined Pseudocycas in the following sense: Fronds similar to those of Cycadites and Cycas but with pinnae characterised by a double ‘midrib,’ the stomata being confined or almost confined to the groove between the two ‘midribs.’ Nathorst also notes that in Pseudocycas the bases of the pinnae are not contracted as is often the case in Cycadites and Cycas, but this is a feature of secondary importance. Miss Holden has, however, expressed the opinion that the double ‘midrib’ is an untrustworthy character and that the stomata are not always arranged as in the fronds examined by Nathorst. Nathorst states that in Pseudocycas Steenstrupi and P. insignis the midrib though usually double may be single: the same variation occurs in P. Saportae. Miss Holden found that an impression made from a pinna of Cycas siamensis showed a double line on the upper face and a single rib on the lower; that is Pseudocycas and Cycas characters occur on a single pinna. On drying, the ridge on the lower face of the lamina collapses and a groove is formed bounded by two ridges.
In a recently published and interesting paper on ‘Some Xerophytic Leaf-structures in Mesozoic Plants,’ Dr Halle[1637] makes some important additions to our knowledge of Pseudocycas. He agrees with Miss Holden’s views with regard to the use of the generic names Pseudocycas and Cycadites, but dissents from her interpretation of the median groove as the result of shrinkage of a midrib. Part of the evidence presented by Halle is based on the structure of the leaf-cuticle as shown in microtome sections[1638]. He found that a distinct median groove is always present on the pinnae of Pseudocycas insignis and other species. If the groove were due to collapse of the tissue of a true median rib the presence of stomata—which in P. insignis are confined to the middle line of the lamina—below the vein would be an unusual feature. He found no indication of a median vein or of any vascular tissue. All that is revealed by an examination of the cuticle is that the lamina has a well-defined groove bounded by prominent ridges, and in Pseudocycas Roemeri the groove is deeper and is formed by the bending-back of the lamina. The stomata are practically confined to the groove and in P. insignis and P. Steenstrupi an additional protection against excessive transpiration was afforded by elongated papillae.
In the sinuous walls of the epidermal cells and in the structure of the stomata Pseudocycas agrees with the Bennettitalean type as defined by Thomas and Bancroft[1639]. The ‘midrib’ of the pinnae marks the position of a pronounced groove and we have no information with regard to the venation. As Halle says, protection against drought is afforded by ‘other means than those usual among existing Cycads,’ which provide against excessive transpiration by the sinking of the individual stomata below the surface. The epidermal features of Pseudocycas afford a striking example of the danger of basing conclusions on mere impressions and they further emphasise the great difference between Mesozoic Cycadean fronds and those of recent genera.
We have no knowledge of the nature of the reproductive organs of the plants which bore Pseudocycas fronds, but the agreement of the stomata and epidermal cells with those in some Bennettitalean types favours the inclusion of the genus in that class. It has previously been pointed out that the impression figured by Heer[1640] in close association with the Greenland specimens of Pseudocycas Steenstrupi as a carpellary leaf of the Cycas type is much too imperfect to be identified and has no claim, as an examination of the actual specimen in the Copenhagen Museum convinced me, to be compared with a megasporophyll of Cycas. This view is shared by Nathorst.
In some cases the name Cycadites has been applied to fragments that might equally well belong to Conifers or other plants, and not infrequently a careful examination of fossils referred to Cycadites shows that the pinnae afford no evidence of a true midrib. Casts of revolute pinnae like those of the recent species Encephalartos Ghellinckii (fig. 382) would present an appearance closely resembling a strong midrib. Heer’s Siberian Jurassic species Cycadites sibiricus[1641] is probably a piece of a Taeniopteris or Nilssonia frond, and C. gramineus Heer[1642] should be referred to Taxites. An examination of the Indian fronds described by Oldham and Morris and Feistmantel as species of Cycadites leads me to discard all of them as trustworthy records of the genus: in Cycadites confertus[1643] and C. Blandfordianus[1644] there appear to be several veins in the pinnae and not a single midrib. Cycadites constrictus[1645] is almost certainly a Conifer as Nathorst has also suggested. No satisfactory example of Cycadites has been described from Palaeozoic strata; Goeppert’s Cycadites gyrosus[1646] is founded on material too imperfect to determine, and an examination of his Culm species C. taxodinus in the Breslau Museum led me to refer it to a Conifer. The specimens figured[1647] by Zeiller from Lower Gondwana rocks in India as Cycadites? sp. are portions of linear leaves or leaflets 5 mm. broad with a median rib but they can hardly be accepted as evidence of the presence of a Cycadean plant.