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Originally appearing in Volume V20, Page 551 of the 1911 Encyclopedia Britannica.
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MAP B.—M,–M... D. G. Distribution of the r:tz alac a E1 ~0 Matnnineae. 1)ible7•irlinae(; D1–D6, Distribution of the Dipteridinae. G5 (urassic ; G12 Cretaceous-Tertiary) ; G1–G17, Distribution of the Ginkgoales G6 (Jurassic and Tertiary) ; G16 (Tertiary, Alaska) ; during the Mesozoic and Tertiary G7 (Jurassic); G14 (Cretaceous-Tertiary) ; Periods. G8 (Rhaetic-Jurassic); G15 (Jurassic) ; G1 (Trias-Tertiary) ; G9 (Trias-Rhaetic) ; G10 (Jurassic, Spitsbergen) ; G2, Ge (Rhaetic-Jurassic); G10 (Rhaetic, Chile); Gu (Jurassic, Franz Josef Land). G4 (Tertiary, Sakhalin I.) ; G11 (Trias) ; as Ctenozamites, Ctenis, and Podozamites, the position of which is ductive organs and stems less certain. Ctenozamites occurs chiefly in the Rhaetic coal-bearing of these fossil Cycads? beds of Scania, and has been found also in the Liassic clays of Cycadean stems have re- cently been found in great abund- ance in Jurassic pants become much more abundant, especially in the Keuper period; as well in North America, from Rhaetic rocks a still greater number of types have been re- throughout Europe. It is corded, among which may be mentioned Nilssonia (fig. To), Anorno- noteworthy that Tertiary zamites, Pterophyllum, Otozamites, Cycadites (fig. II). The species plant-beds have yielded of Nilssonia shown in fig. io (N. compta) is a characteristic member hardly any specimens that of the Jurassic flora, practically identical with a form from Rhaetic can be recognized as rocks described as Nilssonia polymorpha. The large frond of Cycads. Cycadites represented in fig. 11 (C. Saportae) is from the Wealden A more important quesstrata of Sussex, and possibly identical with Cycadites tenuisectus tion is, What knowledge from Portugal. In addition to these genera there are others, such have we of the repro- and possibly higher strata in Wyoming, South Dakota, and other parts of the United States. Cycadean stems have been found also in the upper-most Jurassic, Wealden and Greenland, and other Arctic lands and Dorsetshire and in the Inferior Oolite beds of Yorkshire, as well as in Rhaetic strata in Persia and elsewhere; it is characterized by its bipinnate fronds, and may be compared with the recent Australian genus Bowenia—peculiar among living Cycads in having bipinnate fronds. Ctenis has been incorrectly placed among the ferns by some authors, on account of the occurrence of supposed sporangia on its pinnae; but there is reason to believe that these so-called sporangia are probably nothing more than prominent papillose cells ofythe epidermis. Podozamites (fig. 12) is usually considered to be a Cycad, but the broad pinnae (or leaves) and their arrangement on the axis suggests a possible relationship with the southern coniferous genus Agathis, represented by the Kauri pine and other recent species. The considerable variation in the size of the pinnae of Podozamites, as represented by species from the Jurassic rocks in the Arctic regions and various European localities, recalls the variation in length and breadth of the leaves of Agathis. With regard to the distinguishing features and the distribution of the numerous Cycatean leaves of Mesozoic age, the most striking fact is the abundance of xx. 18 a more compact structure. It is in- FIG. i i.—Cycadites Saporteresting to find that G. R. Wieland of tae: Wealden, England. Lower Cretaceous rocks of England, India and other parts of the world. An example of an Indian Cycadean stem from Upper Gondwana rocks is represented in fig. 13; the surface of the trunk is covered with persistent bases (fig. 13, A) of the fronds known as Ptilophyllum cutchense, which are practically the same as the European species Williamsonia pecten (fig. 17). In a section of the stem (fig. 13, B) a large pith is seen to occupy the axial region, and this is surrounded by a zone of secondary wood, which appears to differ from the characteristic wood of modern Cycads (see GYMNOSPERMS) in having II Yale University has noticed in some of the Cycadean stems from the Black hills of Dakota and Wyoming that the wood appears to possess a similar structure, differing in its narrower medullary rays from the wood of modern Cycads. The lozenge-shaped areas external to the axis of the stem represent the sections of petioles, some of which are shown in fig. 13, A, attached to the stem. The majority of Mesozoic stems agree in external appearance with those of recent species of Encephalartos, Macrozamia, and some other genera; the trunk is encased in a mass of persistent petiole-bases separated from one another by a dense felt or packing of scaly ramenta. The structure of the leaf-stalks is like that of modern A B Cycads, but the ramenta, instead of having the form of long unicellular hairs like those on the petioles and bud-scales of existing species are exactly like the paleae or ramental scales characteristic of the majority of ferns. This fern-like character affords an interesting survival of the close relationship between Cycads and Ferns. Some examples of Jurassic Cycadean stems from Wyoming are characterized by an unusually rich development of ramental scales; the ramenta from the old leaf-bases form an almost complete covering over the surface of the -trunk. Professor Lester Ward has instituted a new generic name, Cycadella, for these woolly forms. In a few cases the fossil stems show no trace of any lateral flowering shoots, and in that respect agree with modern forms: an instance of this is afforded by a large Cycadean trunk discovered some years ago in one of the Portland quarries, and named Cycadeoidea gigantea (fig. 14). In this stem the flowers may have been terminal, as in existing Cycads. As a rule, however, the fossil stems show a marked difference from modern forms in the possession of lateral shoots given off from the axils of leaves, and terminating in a flower of complex structure containing numerous orthotropous seeds. These reproductive shoots differ in many important respects from the flowers of recent Cycads, and chiefly on this account it is customary to include the plants in a separate genus, Bennettites, and in a separate group—the Bennettitales—distinct from that of the Cycadales including the existing Cycads. The best preserved specimens of the true Bennettites type so far described are from the Lower Green-sand and Wealden of England, and from Upper Mesozoic strata in North America, Italy and France. A study of the anatomical structure of the FIG. 1 — Cycadeoidea vegetative stem, which on the whole eigantea. Portland rocks, is very similar to that 9f recent England. Cycads (fig. 15, i and 2), reveals certain characters which are not met with in modern Cycads. The chief distinguishing feature is afforded by the leaf-traces; in recent species (ssee GYMNOSPERMS)these pursue a somewhat complicated course as they pass from the petiole towards the vascular cylinder of the stem, but in Bennettites the vascular bundles from the leaves followed a more direct course through the cortex of the stem (fig. 15, 3). Among existing types the genus Macrozamia appears to show the nearest approach to this simpler structure of the leaf-traces. In a Floridan species of Zamia the leaf-traces are described as characterized by a more direct course from the stele of the stem to the leaves than in most modern genera, thus agreeing more closely with the extinct Bennettites. The typical Bennettites female flower (fig. 15, 4 and 7), as investigated in English. French, Italian, and American specimens, may be briefly described as a short lateral shoot or peduncle, arising in a leaf-axil and terminating in a bluntly rounded apex, bearing numerous linear bracts enclosing a central group of appendages, some of which consist of slender pedicels traversed by a vascular strand and bearing a single terminal ovule enclosed in an integument, which forms a distal canal or micropyle. Associated with these seminiferous pedicels occur sterile appendages consisting of slender stalks, terminating in distal expansions, which form a fleshy covering over the surface of the flower, leaving small apertures immediately above the micropyles for the entrance of the pollen-grains. It has been suggested by some authors that the almost complete investment of the small Bennettites seeds by the surrounding swollen ends of the interseminal scales (fig. 15, 7) represents an approach to the angiospermous ovary. In Bennettites the ovules are left exposed at the apex, but they are by no means so distinctly gymnospermous as in recent Cycads and Conifers. The seeds have in some cases been preserved in wonderful perfection, enabling one to make out the structure of the embryo, with its bluntly conical radicle and two fleshy cotyledons filling the exalbuminous seed (fig. 1 II). Our knowledge of the reproductive organs of the Bennettitaceae has until recently been confined to the female flowers, as described by Carruthers, Solms-Laubach, Lignier, and others. The fortunate discovery of several hundred Cycadean stems in the United States, of Lower Cretaceous and Upper Jurassic age, has supplied abundant material which has lately been investigated and is still receiving attention at the hands of Mr Wieland. This investigator has already published a well-illustrated account of his discoveries, which give valuable information as to the morphology of the male organs, and lead us to expect additional results in the future of the greatest importance and interest. On some of the American stems flowers have been found, borne at the apex of lateral shoots, which possess fully developed male organs consisting of sporangia with spores (pollen-grains), surrounding a conical central receptacle bearing numerous small and probably functionless or immature ovules (fig. 15, lo). The structure of this type of flower may be briefly described as follows. In shape and size the flower is similar to that long known as the female flower of Bennettites and Williamsonia. A number of hairy linear bracts enclose the whole; internal to these occur 12 to 20 crowded pinnate leaves (sporophylls), with their apical portions bent over towards the axis of the flower, the bases of the petioles being fused laterally into a disk surrounding the base of the conical receptacle. Numerous pairs of pinnules are attached to the rachis of each sporophyll, and the larger pinnules bear 20 to 30 synangia (sori or plurilocular sporangia) (fig. 15, 8 and 9). The synangia consist of a stout wall composed of thick-walled cells, succeeded by a layer of more delicate and smaller elements; and internal to the wall occur two rows of sporangial loculi containing microspores. When the synangia are ripe dehiscence takes place along a median line between the two rows of loculi. In size, position, arrangement, and manner of dehiscence the sporangia bear a striking resemblance to those of Marattia and Danaea among recent Marattiaceae. The most important point elucidated by this discovery is the very close correspondence of the male organs of the Bennettites flower with the sporophylls and synangia of Marattiaceous ferns—a further relic of the common origin of Cycads and Ferns. It remains to be seen if the ovuliferous cone in the centre of the flower represents simply a functionless gynoecium, as in Welwitschia and abnormal cones of certain Coniferae, or if the flowers were hermaphrodite, with both male and female organs fully developed. We have a combination in the same flower of stalked ovules, the structure of which has already been described, and interseminal scales constituting a complex gynoecium, which exhibits in certain features an approach to the angiospermous type, and differs in structure from other Gymnosperm flowers, associated with male organs constructed on a plan almost identical with that of the sporophylls in Marattiaceae. In many of the flowers de-scribed by Mr Wieland the structure is identical in essential features with that of the female flowers of Bennettites Gibsonianus described by Carruthers and by Solms-Laubach, and with that of a French Liassic species described by Lignier: the whole consists of a convex receptacle bearing mature seeds at the tips of pedicels associated with interseminal scales (fig. 15, 7) as already described. Mr Wieland's researches have, however, demonstrated the existence in flowers of this type of the remains of a disk at the base of the receptacle, between the receptacle and the surrounding bracts, to which staminate leaves were originally attached. The flowers hither-to regarded as female were in some cases at least hermaphrodite, but the male organs had been thrown off before the complete development of the gynoecium. This fact suggests the possibility that the flowers described by Mr Wieland, in which the male organs are mature and the gynoecium is composed of very short and immature ovuliferous stalks and interseminal scales, are not essentially distinct from those which have lost the staminate leaves I, Bennettites stem: portion of transverse section of stem; a, vascular cylinder; b, leaf-traces; c, pith; d, cortex. 2, Bennettites stem, tangential section; e, flower-peduncles. 3, Bennettites stem, leaf-traces attached to the vascular cylinder and passing as simple strands through the cortex; d, cortex. 4, Williamsonia, Wealden, England. 5, Young leaf of Bennettites. 6, Ramenta of Bennettites in transverse section. 7, Bennettites, female flower in longitudinal section; f, apex of peduncle; g, bracts (shown in surface view in 4); h, seeds and seminiferous pedicels; i, interseminal scales. 8, Bennettites, synangium of male flower, showing line of dehiscence, k, and microspores, 1. 9, Synangium, in transverse section, showing sporangial groups, m, and microspores, 1. I0, Bennettites flower in vertical section, showing the central female portion, n, two sporophylls bearing synangia (male), o, and hairy bracts, g. II, Bennettites seed in longitudinal section, showing the dicotyledonous embryo; p, cotyledons; r, radicle; s, testa. (1-3, after Carruthers; 5, 8, 9 and to, after Wieland; 7, after Scott; I1, after Solms-Laubach.) and possess mature seeds. It is probable that the flowers of Bennettites were normally hermaphrodite, and they may have been markedly protandrous. We cannot decide at present whether the gynoecium in a flower, such as that represented in fig. 15, 7, has partially aborted or whether it would have matured later after the fall of the male organs. It is clear that Bennettites differed in many essential respects from the few modern survivors of the Cycadophyta. Fossil flowers of a type more like that of modern Cycads are few in number, and it is not by any means certain that all of those described as Cycadean flowers and seeds were borne by plants which should be included in the Cycadophyta; a few female flowers have been described from Rhaetic rocks of Scania and elsewhere under the name Zamiostrobus—these consist of an axis with slender pedicels or carpophylls given off at a wide angle and bearing two ovules at the distal end ; the structure is in fact similar to that of a Zamia female flower, in which the internodes of the peduncle have been elongated so as to give a looser arrangement to the carpels. It has been suggestedthat one at least of the flowers, that originally described by Mr Carruthers from the Inferior Oolite of Yorkshire as Beania gracilis, may have been borne by a member of the Ginkgoales. From Jurassic rocks of France and Italy a few imperfect specimens have been described as carpels of Cycads, like those of the recent genus Cycas (see GYMNOSPERMS); while a few of these may have been correctly identified, an inspection of some of the original examples in the Paris collections leads one to express the opinion that others are too imperfect to determine. Pinnate fronds of the Cycas type, characterized by the presence of a midrib and no lateral veins in the linear pinnae, are recorded from Rhaetic rocks of Germany, from Wealden strata in England (fig. II) and Portugal, and from Liassic beds in Dorsetshire. One large specimen is figured by Heer from Lower Cretaceous rocks of Greenland, and by the side of the frond is shown a carpel with lateral ovules, as in the female flower of Cycas; but an examination of the type-specimen in the Copenhagen Museum led the present writer to regard this supposed carpel as valueless. Professor Nathorst, as the result of a more recent examination of Heer's specimen, found that the segments of the frond are characterized by the presence of two parallel veins instead of a single midrib, with a row of stomata between them; for this type of Cycadean leaf he proposed the generic name Pseudocycas. Another well-known Cycadean genus is Williamsonia, so named by Mr Carruthers in 187o, and now applied to certain pinnate fronds — e.g. those pre- viously described as Zamites gigas (fig. 16), and others known under such names as Pterophyllum or Ptilophyllum pecten, &c., both common Jurassic species — as well as to stems bearing peduncles with terminal oval flowers, similar in form to those of Bennettites. There is good evidence for supporting Professor Williamson's conclusions as to the organic connexion between the flowers, originally de-scribed from Inferior Oolite rocks of Yorkshire and subsequently named Williamsonia (fig. 15, 4), and the fronds of Zamites gigas, now known as Williamsonia gigas (fig. 16). There can be little doubt that the majority of the Cycadean fronds of Jurassic and Wealden age, which are nearly always found detached from the rest of the plant, were borne on stems of the Bennettites type. Williamson was the first to express the opinion that the Bennettitean flowers known as Williamsonia were borne on the trunks which terminated in a crown of pinnate fronds FIG. 16.—Frond of Williamsonia gigas. of the type long known as Inferior Oolite, England. Zamites gigas; this view was regarded by Saporta and others as incorrect, and the nature of the Bennettitean foliage was left an open question. A re-examination of the English material in the museums of Paris and else-where has confirmed Williamson's conclusions. Mr Wieland has also described young bipinnate fronds, very like those of recent species of Zamia and Encephalartos, attached to a Bennettites stem, and exhibiting the vernation characters of many recent Cycads (fig. 15, 5). In Williamsonia the stem bore comparatively long fertile shoots, which, in contrast to those of Bennettites, projected several inches beyond the surface of the main trunk, and terminated in a flower which appears to have resembled those of the true Bennettites. Nathorst has recently described specimens of Williamsonia from the Jurassic rocks of Whitby with micro-Sporophylls like those of Wieland's species. Williamsonia occurs in the Upper Gondwana rocks of India; it is recorded also from strata ranging from the Rhaetic to the Lower Cretaceous period in England, Portugal, Sweden, Bornholm, Greenland, Italy and North America. Professor Nathorst has described another type of stem from the Rhaetic beds of Scania. It consists of a comparatively small and repeatedly forked axis bearing in each fork a flower; the flowers, which are regarded as male and female, appear to be similar to those of Bennettites. The leaves, borne on the regions between the false dichotomies, are those of Anomozaanites minor, a type of Cycadean frond originally determined by Brongniart. The flowers, or some of them, were originally described by Nathorst as Williamsonia angustifolia. This form of stem, of a habit entirely different from that of recent Cycads and extinct Bennettites, points to the existence in the Mesozoic era of , in addition to several well-preserved female flowers. C. A. Hollick another type of Gymnosperm allied to the Bennettitales of the Jurassic and Cretaceous periods by its flowers, but possessing a distinctive character in its vegetative organs. There is no doubt that the Cycadophyta, using the term suggested by Nathorst in 1902, was represented in the Mesozoic period by several distinct families or classes which played a dominant part in the floras of the world before the advent of the Angiosperms. In addition to the bisporangiate reproductive shoots of Bennettites, distinguished by many important features from the flowers of recent Cycads, a few specimens of flowers have been discovered exhibiting a much closer resemblance to those of existing Cycads, e.g. Androstrobus Balduini from Bathonian rocks of France; Zamites familiaris, described many years ago by Corda, from Lower Cretaceous rocks of Bohemia, and Androstrobus Nathorsti, from Wealden beds in Sussex. The majority of the species were, however, characterized by flowers of a different type known as Bennettites and Williamsonia. The living Maidenhair-tree (Ginkgo biloba) (see GvMNOSPERMS) remains, like Matonia and Dipteris, among the ferns, as an isolated relic in the midst ainkgoales. of recent vegetation. In Rhaetic, Jurassic and Wealden floras, the Ginkgoales were FIG. 18.—Leaves of Ginkgoales. exceedingly abundant (Map B, GI–Gl,) ; in addition to leaves agreeing almost exactly with those of the recent A, Ginkgodiunx, Japan (Jurassic). species (fig. 18), there are others separated as a distinct B, C, D, E, F, H, Ginkgo leaves.—B, from Franz Josef Land (Jurassic); genus, Baiera (fig. 18, G), characterized by the greater C, Greenland (Lower Cretaceous) ; D, Siberia (Jurassic) ; E, Germany number and narrower form of the segments, which may (Wealden); F, England (Jurassic); H, China (Rhaetic). be best compared with such leaves as those of the G, Baiera leaf, Inferior Oolite, England. recent fern Actiniopteris and of certain species of Schizaea. (A, after Yokoyama ; B, after Nalhorst; C, D, after Heer; E. after Schenk ; Male flowers, like those of Ginkgo biloba, but usually H, after Krasser. All the figures z nat. size.) characterized by a rather larger number of oval pollen-sacs on the stamens, have been found in England, Germany, Siberia and elsewhere in association with Ginkgo and Baiera foliage. The occasional occurrence of three or even four pollen-sacs on the stamens of the recent species affords a still closer agreement between the extinct and living types. Seeds like those of Ginkgo biloba have also been recorded as fossils in Jurassic rocks, and it is possible that the type of flower known as Beania, from the Inferior Oolite rocks of Yorkshire, may have been borne by Ginkgo or Baiera. The regions from which satisfactory examples of Ginkgoales (Baiera or Ginkgo) have been recorded are shown in Map B (G1-G,). Both Tertiary and Mesozoic localities are indicated in the map. An adequate account of fossil Mesozoic Conifers is impossible within the limits of this article. Coniferous twigs are very common in Mesozoic strata, but in most cases we are compelled Coniferales. to refer them to provisional genera as the evidence of vegetative shoots alone is not sufficient to enable us to determine their position within the Coniferae. There are, however, several forms which it is reasonable to include in the Araucarieae; that this family was to the fore in the vegetation of the Jurassic period is unquestionable. We have not merely the striking resemblance of vegetative shoots to those of recent species of Araucaria and Agathis, e. . species of Nageiopsis, abundantly represented in the Upper Jurassic beds of the Potomac area in North America, species of Pagiophyllum and other genera of Jurassicand Wealden age, but an abundance of fossil wood (Araucarioxylon) from Jurassic and Cretaceous strata in Europe, North America, Madagascar and elsewhere agreeing with that of recent Araucarieae, and E. C. Jeffrey have recently shown that some Lower Cretaceous specimens of the well-known genus Brachyphyllum obtained from Staten Island, N.Y., possess wood of the Araucarian type. This genus has long been known as a common and widely spread Jurassic and Cretaceous conifer, but owing to the absence of petrified specimens and of well-preserved cones, it has been impossible to refer it to a definite position in the Coniferales. It is now clear that some at least of the species of Brachyphyllum must be referred to the Araucarieae. In a recently published paper Seward and Ford have given a general account of the Araucarieae, recent and extinct, to which reference may be made for further details as to the geological history of this ancient section of the Coniferales. Some of the fossils referred to the genus Kaidocarpon, and originally described as monocotyledonous inflorescences, are undoubted Araucarian cones; other cones of the same type have been placed in the genus Cycadeostrobus and referred to Cycads. Araucarites Hudlestoni, described by Mr Carruthers from the Coralline Oolite rocks of Malton in Yorkshire; Araucarites sphaerocarpa from the Inferior Oolite of Somerset; also another cone found in the Northampton Sands, which is probably specifically identical with A. Hudlestoni, and named by Carruthers Kaidocarpon ooliticum, afford good illustrations of British Araucarian flowers. A flower of a rather different type, Pseudaraucaria major, exhibiting in the occurrence of two seeds in each scale an approach to the cones of Abietineae, has been described by Professor Fliche from Lower Cretaceous rocks of Argonne. The well-known Whitby jet of Upper Liassic age appears to have been formed to a large extent from Araucarian wood. Among the more abundant Conifers of Jurassic age may be mentioned such genera as Thuytes and Cupressites, which agree in their vegetative characters with members of the Cupressineae, but our knowledge of the cones is far from satisfactory. Many of the small female flowers borne on shoots with foliage of the Cupressus type consist of spirally disposed and not verticillate scales, e.g. Thuytes expansus, a common Jurassic species. Fossil wood, described under the name Cupressinoxylon, has been recorded from several Mesozoic horizons in Europe and elsewhere, but this term has been employed in a wide sense as a designation for a type of structure met with not only in the Cupressineae, but in members of other families of Coniferae. The Abietineae do not appear to have played a prominent part before the Wealden period; various older species, e.g. Rhaetic specimens from Scania, are recorded, but it is not until we come to the Upper Jurassic and Wealden periods that this modern family was abundantly represented. Fossil wood of the Pinites type (Pityoxylon) has been described from England, France, Germany, Sweden, Spitsbergen, North America and elsewhere; some of the best British examples have been obtained from the so-called Pine-raft, the remains of water-logged and petrified wood of Lower Greensand age, seen at low water near Brook Point in the Isle of Wight. Well-preserved Abietineous female flowers have been obtained from the Wealden rocks of England and Belgium, e.g. Pinites. Dunkeri, P. Solmsi, &c.; specimens of seeds and vegetative shoots are recorded also from Spitsbergen and other regions. Hollick and Jeffrey have recently added to our knowledge of the anatomy of Cretaceous species of Pinus, and Miss Stopes and Dr Fujii have made imortant contributions on the structure of Cretaceous plants from Japan. Cones of Lower Cretaceous age have been described by Fliche from Argonne, which bear a close resemblance to the female flowers of recent species of Cedrus. The two surviving species of Sequoia afford an illustration of the persistence of an old type, but unfortunately most of the Mesozoic species referred to this genus do not possess sufficiently perfect cones to confirm their identification as examples of Sequoia. Some of the best examples of cones and twigs referred to Sequoia are those described by Heer from Cretaceous rocks of Greenland, and Professor D. P. Penhallow of Montreal has described the anatomical structure of the stem of Sequoia Langsdorfii, a Tertiary species occurring in Europe and North America. There are a few points suggested by a general survey of the Mesozoic floras, which may be briefly touched on in conclusion. In following the progress of plant-life through those periods in the history of the earth of which records are left in ancient sediments, seams of coal or old land-surfaces, we recognize at certain stages a want of continuity between the floras of successive ages. The imperfection of the geological record, considered from the point of view of evolution, has been rendered familiar by Darwin's remarkable chapter in the Origin of Species. Breaks in the chain of life, as represented by gaps in the blurred and incomplete documents afforded by fragmentary fossils, are a necessary consequence of the general plan of geological evolution; they mark missing chapters rather than sudden breaks in an evolutionary series. On the other hand, a study of the plant-life of past ages tends to the conviction that too much stress may be laid on the imperfection of the geological record as a factor in the interpretation of palaeontological data. The doctrine of Uniformitarianism, as propounded by Lyell, served to establish geology on a firmer and more rational basis than it had previously possessed; but latterly the tendency has been to modify the Lyellian view by an admission of the probability of a more intense action of groups of forces at certain stages of the earth's history. As a definite instance a short review may be given of the evidence of palaeobotanical records as regards their bearing on plant-evolution. Starting with the Permo-Carboniferous vegetation, and omitting for the moment the Glossopteris flora, we find a comparatively homogeneous flora of wide geographical range, consisting to a large extent of arborescent lycopods, calamites, and other vascular cryptogams, plants which occupied a place comparable with that of Gymnosperms and Angiosperms in our modern forests; with these were other types of the greatest phylogenetic importance, which serve as finger-posts pointing to lines of evolution of which we have but the faintest signs among existing plants. Other types, again, which may be referred to the Gymnosperms, played a not unimportant part in the Palaeozoic vegetation. No conclusive proof has so far been adduced of the existence in those days of the Cycads, nor is there more than partial evidence of the occurrence of genera which can be placed with confidence in any of the existing families of Conifers. There are, moreover, no facts furnished by fossil plants in support of the view that Angiosperms were represented either in the low-lying forests or on the slopes of the mountains of the Coal period. Passing higher up the geological series, we find but scanty records of the vegetation that existed during the closing ages of the Permian period, and of the plants which witnessed the beginning of the Triassic period we have to be content with the most fragmentary relics. It is in rocks of Upper Triassic and Rhaetic age that abundant remains of rich floras are met with, and an examination of the general features of the vegetation reveals a striking contrast between the Lower Mesozoic plants and those of the Palaeozoic period. Arborescent Pteridophytes are barely represented, and such dominant types as Lepidodendron, Sigillaria, Calamites and Sphenophyllum have practically ceased to exist; Cycads and Conifers have assumed the leading role, and the still luxuriant fern vegetation has put on a different aspect. This description applies almost equally to the floras of the succeeding Jurassic and Wealden periods. The change to this newer type of vegetation was no doubt less sudden than it appears as read from palaeobotanical records, but the transition period between the Palaeozoic type of vegetation and that which flourished in the Lower Mesozoicera, and continued to the close of the Wealden age, was probably characterized by rapid or almost sudden changes. In the southern hemisphere the Glossopteris flora succeeded a Lower Carboniferous vegetation with a rapidity similar to that which marked the passage in the north from Palaeozoic to Mesozoic floras. This apparently rapid alteration ih the character of the southern vegetation took place at an earlier period than that which witnessed the transformation in the northern hemisphere. The appearance of a new type of vegetation in India and the southern hemisphere was probably connected with a widespread lowering of temperature, to which reference has already been made. It was from this Glossopteris flora that several types gradually migrated across the equator, where they formed part of the vegetation of more northern regions. The difference between the Glossopteris flora and those which have left traces in the Upper Gondwana rocks of India, in the Wianamatta and Hawkesbury beds of Australia, and in the Stormberg series of South Africa is much less marked than that between the Permo-Carboniferous flora of the northern hemisphere and the succeeding Mesozoic vegetation. In other words, the change took place at an earlier period in the south than in the north. To return to the northern hemisphere, it is clear that the Wealden flora, as represented by plants recorded from England, France, Belgium, Portugal, Russia, Germany and other European regions, as also from Japan and elsewhere, carries on, with minor differences, the facies of the older Jurassic floras. It was at the close of the Wealden period that a second evolutionary wave swept over the vegetation of the world. This change is most strikingly illustrated by the inrush of Angiosperms, in the equally marked decrease in the Cycads, and in the altered character of the ferns. It would appear that in this case the new influence, supplied by the advent of Angiosperms, had its origin in the north. Unfortunately, our knowledge of the later floras in the southern hemisphere is very incomplete, but a similar transformation appears to have characterized the vegetation south of the equator. As to the nature of the chief factors concerned in the two revolutions in the vegetable kingdom, if it is admissible to use so strong a term, only a guess can be hazarded. Physical conditions no doubt played an important part, but whatever cause may have had the greatest share in disturbing the equilibrium of evolutionary forces, it would seem that the apparently sudden appearance of Cycads and other types at the close of the Palaeozoic period made a widespread and sudden impression on the whole character of the vegetation. At a later stage—in post-Wealden days—it was the appearance of Angiosperms, probably in northern latitudes, that formed the chief motive power in accelerating the transition in the facies of plant-life from that which marked what we have called the Mesozoic floras, to the vegetation of the Upper Cretaceous and Tertiary periods. With the advent of Angiosperms began, as the late marquis of Saporta expressed it, " Une revolution, ainsi rapide dans sa marche qu'universelle dans ses effets." From the floras of the Tertiary age we pass by gradual stages to those which characterize the present phase of evolutionary progress. Among modern floras we find here and there isolated types, such as Ginkgo, Sequoia, Matonia, Dipteris and the Cycads, persisting as more successful survivals which have held their own through the course of ages; these plants remain as vestiges from a remote past, and as links connecting the vegetation of to-day with that of the Mesozoic era. AuTHoruTlEs.—Glossopteris Flora: Blanford, H. F., " On the age and correlation of the Plant-bearing Series of India, &c.," Quarterly Journal Geol. Soc. xxxi. (1875); Feistmantel, " Fossil Flora of the Gondwana System," Mem. Geol. Sure. India, vols. iii., &c. (1899, &c.) ; Seward, Fossil Plants as Tests of Climate (Cambridge, 1892), with bibliography; " The Glossopteris Flora," Science Progress, with bibliography; " On the Association of Sigillaria and Glossopteris in South Africa," Q.J.G.S., vol. H. (1897); E. A. N. Arber, Catalogue of the Fossil Plants of the Glossopteris Flora in the Department of Geology (British Museum, Nat. Hist., Brit. Mus. Catalogue (London, r9o5), with full bibliography; Medlicott and Blanford, Manual of the Geology of India (2nd ed., Oldham, R. D., Calcutta, 1893) ; David, " Evidences of Glacial Action in Australia in Permo-Carboniferous time," Q.J.G.S., vol. Iii. (1896); Zeiller, Elements de paleubotanique (Paris, 19:>0) ; Potonie, " Fossile Pflanzen 550 aus deutsch and portugiesisch Ostafrika," Deutsch-Ostafrika, vii. (Berlin, 1900), with bibliography. General : Potonie, Lehrbuch der Pflanzenpalaeontologie (Berlin, 1899) ; Scott, Studies in Fossil Botany (1900) ; Seward, Fossil Plants (Cambridge: vol. i., 1898) ; vol. ii. 1910, with bibliography; Zeiller, " Revue des travaux de paleontologie vegetate," Rev. gen. bot. (1903) et seq. Catalogue of the Mesozoic Plants in the British Museum, (a) " Wealden Flora," pts. i. and ii.; (b) " Jurassic Flora," pt. i. (1894–1901), pt. ii. (1904), with bibliography; " On the Structure and Affinities of Matonia pectinata, with Notes on the Geological History of the Matonineae," Phil. Trans. cxci. (1899) ; " On the Structure. &c., of Dipteris," ibid. cxciv. (1901, with bibliography; Seward and Ford, " The Araucarieae, recent and extinct," Phil. Trans. R. Soc. (London, 1906) ; G. R. Wieland, "American Fossil Cycads," Publication Carnegie Instil. (Washington, 1906) ; Nathorst, " Palaobotanische Mittel," K. Svensk. Vetenskaps. Akad. Hand. xlii., No. 5 (1907) ; The Norwegian North-Polar Expedition, iii. (1893–1896); " Fossil Plants from Franz Josef Land;" L. F. Ward, " Status of the Mesozoic Floras of the United States," Twentieth Ann. Rep. Geol. Survey (Washington, 1900) ; Solms-Laubach, " Ueber das Genus Pleuromeia," Bot. Zeit. (1899) ; Newton and T'eall, " Notes on a Collection of Rocks and Fossils from Franz Josef Land," Q.J.G.S. liii. (1897) ; Hollick and Jeffrey, " Studies of Cretaceous Coniferous remains, Mem. New York Botanical Garden, vol. iii. (1909); Stopes and Fujii, " Structure and Affinities of Cretaceous Plants," Phil. Trans. R. Soc. (1910). References to important papers on Mesozoic botany will be found in the bibliographies mentioned in the above list. (A. C. SE.) After the Wealden period, and before the deposition of the lowest strata of the Chalk, so remarkable a change takes place in the character of the vegetation that this break Lower must be taken as, botanically, the transition point Cretaceous. from a Secondary to a Tertiary flora. A flora consisting entirely, with a single doubtful exception, of Gymnosperms and Cryptogams gives place to one containing many flowering plants; and these increase so rapidly that before long they seem to have crowded out many of the earlier types, and to have themselves become the dominant forms. Not only do Angiosperms suddenly become dominant in all known plant-bearing deposits of Upper Cretaceous age, but strangely enough the earliest found seem to belong to living orders, and commonly have been referred to existing genera. From Cretaceous times onwards local distribution may change; yet the successive floras can be analysed in the same way as, and compared with, the living floras of different regions. World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones. This being the case, it will be most convenient to discuss the Tertiary floras in successive order of appearance, since the main interest no longer lies in the occurrence of strange extinct plants or of transitional forms connecting orders now completely isolated. The accurate correlation in time of the various scattered plant-bearing deposits is a matter of considerable difficulty, for plant-remains are preserved principally in lacustrine strata laid down in separate basins of small' extent. This it is obvious must commonly be the case, as most leaves and fruits are not calculated to drift far in the sea without injury or in abundance; nor are they likely as a rule to be associated with marine organisms. Deposits containing marine fossils can be compared even when widely separated, for the ocean is continuous and many marine species are world-wide. Plants, on the other hand, like land and fresh-water animals, occupied areas which may or may not have been continuous. Therefore, without a knowledge of the physical geography of any particular period, we cannot know whether like or unlike floras might be expected in neighbouring areas during that period. If, however, we discover plant-bearing strata interstratified with deposits containing marine fossils, we can fix the period to which the plants belong, and may be able to correlate them in distinct areas, even though the floras be unlike. This clear stratigraphical evidence is, however, so rarely found that much uncertainty still remains as to the true age of several of the floras now to be described. In rocks approximately equivalent to the Lower Greensand of England, or slightly earlier, Angiosperms make their first appearance; but as the only strata of this age in Britain are of marine origin, we have to turn to other countries for the evidence. [TERTIARY The earliest Angiosperm yet found in Europe is a single mono- cotyledonous leaf of doubtful affinities, named by Saporta Alismacites primaevus (fig. I), and found in the Valenginian strata of Portugal. These deposits seem to be equivalent to British Wealden rocks, though in the latter, even in their upper part, no trace of Angiosperms has been discovered. No other undoubted Angiosperm has yet been discovered in Europe in strata of this age, but Heer records a poplar-like leaf from Urgonian strata, a stage newer than the Valenginian, in Greenland, and Saporta has described from strata of the same date in Portugal a Euphorbiaceous plant apparently closely allied to the living Phyllanthus and named by him Choffatia Francheti (fig. 2). We FIG. 1. Alismacites must turn to North America for a fuller primaevus. knowledge of the earlist flowering-plants. In S. Dakota a remarkable series has been discovered, lying unmistakably between marine Upper Jurassic rocks below and Upper Cretaceous above. There has been a certain amount of confusion as to the exact strata in which CreAmerican taceous. the plants occur, but this has now been cleared up by the researches of Lester F. Ward, who has shown how the Secondary flora gives place to one of Tertiary character. The lower strata—i.e. those most allied to the Jurassic—contain only Gymnosperms and Cryptogams. The next division (Dakota fined mainly to the Older Potomac, FIG. 2.—Choffatia Francheti. while the Dicotyledons are principally represented in the Newer Potomac, though occurring more rarely even down to the base of the series. Six successive stages have been defined in the Potomac formation. The Mount Vernon beds, which occur about the middle of the series, have as yet yielded only a small number of species, though these include the most interesting early Angiosperms. Among them are recorded a Casuarina, a leaf of Sagittaria (which however, as observed by Zeiller, may belong to Smilax), two species of poplar-like leaves with remarkably cordate bases, Menispermites (possibly a water-lily) and Celastrophyllum (perhaps allied to Celastrus). Proteophyllum, found in the same bed, and also in the Infra-Cretaceous of Portugal, seems to have belonged to a Proteaceous plant, though only leaves without fruits have yet been discovered in deposits of this early date. Whatever doubt may be left as to the exact botanical position of these early Lower Cretaceous Angiosperms, it is clear that both Monocotyledons and Dicotyledons are represented by several types of leaves, and that the flora ex-tended over wide areas in North America and Greenland, and is found again at a few points in Europe. There is yet no clear evidence either of climatic zones or of the existence of geographical provinces during this period. The next strata, the Aquila Creek series, contain a well-marked dicotyledonous flora, in which both the form and nervation of the leaves begin to approximate to those of recent times. The leading' characteristic of this Middle Potomac flora is the proportion of Dicotyledons. Notwithstanding this apparent passage-bed, there is a marked difference between the Older and the Newer Potomac floras, very few species passing from the one to the other. Only 15 out of 405 plants in the older series occur in the beds above No. 2 of Meek and Hayden) contains Gymnosperms and Ferns of Neocomian types, or even of Neocomian species; but mingled with these occur a few dicotyledonous leaves belonging to four genera. The specimens are very fragmentary, and all that can be said is that one of the forms may be allied to oak, another to fig, a third to Sapindus, and the fourth may perhaps be near to elm. The " Potomac Formation " of Virginia and Maryland is doubtless also mainly of Neocomian age, for though it rests unconformably on much older strata, the successive floras found in it are so allied to those of S. Dakota as to leave little doubt as to the general homotaxis of the series. Lester Ward re-cords no fewer than 737 distinct forms, consisting chiefly of Ferns, Cycads, Conifers and Dicotyledons, t r and cads bei con-e s g though already more than 35o species have been determined from this newer series. The plants from the Amboy Clays, which form the most important division of the Newer Potomac series and were monographed in 1895 by J. S. Newberry, seem to belong to the commencement of the Upper Cretaceous period. It is remarkable that nearly 8o% of the species are Dicotyledons, and that no Monocotyledons have been found. The mere enumeration of the genera will indicate how close the flowering plants are to living forms. Newberry records Juglans, Myrica (7 species), Populus, Salix (5 species), Quercus, Planera, Ficus (3 species), Persoonia and another extinct Proteaceous genus named Proteoides, Magnolia (7 species), Liriodendron (4 species), Menispermites, Laurus and allied plants, Sassafras (3 species), Cinnamomum, Prunus, Hymenaea, Dalbergia, Bauhinia, Caesalpinia, Fontainea, Colutea and other Leguminosae, Ilex, Celastrus, Celastrophyllum (to species), Acer, Rhamnites, Paliurus, Cissites, Tiliaephyllum, Passiflora, Eucalyptus (5 species), Hedera, Aralia (8 species), Cornophyllum, Andromeda (4 species), Myrsine, Sapotacites, Diospyros, Acerates, Viburnum and various genera of uncertain affinities. The points that suggest themselves with regard to this flora are, that it includes a fair representation of the existing orders of warm-temperate deciduous trees; that the more primitive types—such as the Amentaceae—do not appear to preponderate to a greater extent than they do in the existing temperate flora; that the assemblage somewhat suggests American affinities; and that when we take into account deficient collecting, local conditions, and the non-preservation of succulent plants, there is no reason for saying that certain other orders must have been absent. The great rarity of Monocotyledons is a common characteristic of fossil floras known only, as this one is, from leaves principally belonging to deciduous trees. With regard to suggested American affinities, it must be borne in mind that the Neocomian Angiosperms are little known except in America and in Greenland, and that we therefore cannot yet say whether families now mainly American were not formerly of world-wide distribution. We know that this was the case with some, such as Liriodendron; and in Eucalyptus we see the converse, where a genus formerly American is now confined to a far distant region. The Neocomian flora has been collected from an area extending over about 3o° of latitude; but there is little evidence of any corresponding climatic change. We cannot yet say, however, that the deposits are exactly contemporaneous, and the great climatic variations that have taken place in the northern hemisphere during the existence of our living flora should make us hesitate to correlate too minutely from the evidence of plants alone. The highest division of the Dakota series (known as Dakota No. I) which lies immediately beneath Upper Cretaceous strata with marine fossils, contains a flora so like that of the Tertiary deposits that only the clearest geological evidence has been considered sufficient to prove that Heer was wrong when he spoke of the plants as Miocene. These highest plant-bearing strata rest, according to Lester Ward, somewhat unconformably on the Dakota No. 2; they show also a marked difference in the included plants. The genera of Dicotyledons represented are Quercus, Sassafras, Platanus, Celastrophyllum, Cissites, Viburnites. In the central parts of North America the lacustrine plant-bearing deposits are of enormous thickness, the Dakota series being followed by marine Cretaceous strata known as the Colorado and Montana groups, and these being succeeded conformably by a thousand feet or more of lacustrine shales, sandstones and coal-seams, belonging to the Laramie series. This also contains occasional marine Upper Cretaceous fossils, as well as reptiles of Cretaceous types. An extensive literature has grown up relating to these Laramie strata, for owing to the Tertiary aspect of the contained plants, geologists were slow to recognize that they could be truly contemporaneous and interbedded with others yielding Cretaceous animals. In addition to this, the earlier writers included in the Laramie series many deposits now known to be of later date and truly Tertiary, and the process of separation is even now only partially completed. It will be safest in these circumstances to accept as our guide to the true Laramie flora the carefully compiled " Catalogue " of F. H. Knowlton. According to this catalogue, the true Laramie flora includes about 250 species, more than half of which are deciduous forest trees, herbaceous Dicotyledons, Monocotyledons and Cryptogams, all being but poorly represented. Among the few Monocotyledons are leaves and fruits of palms, and traces of grasses and sedges. The Dicotyledons include several water-lilies, a somewhat doubtful Trapa, and many genera of forest trees still common in America. The genera best represented are Ficus (21 species), Quercus (16 species), Populus (II species), Rhamnus 9 species), Platanus (8 species), Viburnum (7 species), Magnolia (6 species), Cornus (5 species), Cinnamomum (5 species), Juglans (4 species), Acer (4 species), Salix (4 species), Aralia (3 species), Rhus (3 species), Sequoia (3 species). Of trees now extinct in America, Eucalyptus and Ginkgo are perhaps the most noticeable. So large a proportion of the trees still belongs to the flora of North America that one is apt to overlook the fact that among the more specialized plants some of the largest American orders, such as the Compositae, are still missing from strata belonging to the Cretaceous period. The imperfection and want of continuity of the records in Europe have made it necessary in dealing with the Cretaceous floras for us to give the first place to America. But European it is now advisable to return to Europe, where cretaceous. Upper Cretaceous plants are not uncommon, and the position of the deposits in the Cretaceous series can often be fixed accurately by their close association with marine strata belonging to definite subdivisions. As these divisions of Cretaceous time will have to be referred to more than once, it will be useful to tabulate them, thus showing which plant-beds seem to be referable to each, and what are the British strata of like age. It has not yet been found possible so closely to correlate the strata of Europe with those of America, where distance has allowed geographical differences in both fauna and flora to come into play; therefore, beyond the references to Lower or Upper Cretaceous, no classification of the American Cretaceous strata has here been given. In Europe the most commonly accepted divisions of the Cretaceous period are as follows:
End of Article: MAP B

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