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J.L.S.B. LVI] FOSSIL PLANTS AND EVOLUTION 123 FOSSIL PLANTS AND EVOLUTION By H. HAMSHAW THOMAS, F.R.S. Introduction. The geological record of plant life on the earth as known t o Darwin and Wallace provided no support to the theory of Evolution. It showed plant remains in the Tertiary and Upper Cretaceous rocks which could be closely matched with living genera of flowering plants and conifers: the fossil vegetation of the Mesozoic period appeared to consist only of ferns, cycads and conifers ; while in the Carboniferous period the earth was thought to have been clothed with plants referable to the vascular cryptogams. Very little was known of the structure and reproduction of any fossil forms, most of which were referred to living groups on some similarity of external appearance. The publication of the Origin of Species, and the discovery in certain coal seams of well-preserved petrified plant remains, gave a great impetus to palaeobotany in Britain and its study was taken up by several eminent botanists led by W. C. Williamson. The work of D. H. Scott, R. Kidston, A. C. Seward and F. W. Oliver provided a mass of information which was much more reliable than that previously available. At the opening of this century Oliver and Scott showed that many of the fossils previously regarded as the remains of ferns were in fact gymnosperms with complex ovules and tassels of pollen-bearing sporangia. In 1909 D. H. Scott, then President of the Linnean Society, surveyed the bearing of contemporary knowledge on the doctrine of Evolution. He expressed the view that the records of plant history suggested a connection between some of the main groups of living forms, but that they provided little evidence of the derivation of one species or genus from another. Since then our knowledge of the plants of past ages has increased enormously, both by the discovery of forms not previously known and by the investigation of the structure and reproduction of types only known previously from leaf impressions. A successful search for plant remains in the Upper Silurian and Devonian rocks produced examples of great evolutionary significance. The discovery of petrified remains in the Coal Measures of North America has enriched our knowledge by some 150 species (Andrews, 1951). Large collections have been made of Mesozoic plants in various parts of the world, which have given a new view of the vegetation of this period, and the study of Tertiary and Quaternary remains have shown the changes which took place in plant life at a comparatively recent date. Research has been greatly assisted by the introduction of new technical methods for the microscopical examination of fossil plants. Nathorst and his pupils improved the old method of macerating carbonized remains, and showed their great value in the study of Mesozoic plants. The discovery and perfection by Prof. Walton of a way of preparing thin sections of petrified material by the peel method gave considerable impetus to the study of such remains. It placed the production of series of thin sections within the easy reach of almost any worker. We know so much more about the plants of the past than our predecessors that it may be useful t o consider the bearing of our present knowledge on the theory of evolution. Changes in the World’s Vegetation. A n important argument for t h e theory of evolution was based on the evidence from the geological record which showed that many species formerly living in the world had become extinct while other new species, genera or classes had made their appearance. It was held by some that the plant record did not show this with any certainty. Many genera of flowering plants can be traced back t o the Upper Cretaceous, while the living species of Ginkgo, Taxus and Marattia are very similar to forms 124 H. HAMSRAW THOMAS: [J.L.s.z. XLIV, of these genera found in Mesozoic rocks. But there can now be no doubt that since Devonian times there have been a series of complete changes in the plant covering of the earth. Darwin (1888) thought that " the geological record, a t all times imperfect, does not extend far enough back to show with unmistakable clearness that within the known history of the world organisation has largely advanced ". Fifty years ago Scott (1909) considered that this statement was still true. But the discovery of fossil plants in the Silurian and Lower Devonian rocks has greatly changed the outlook. I n these forms we have unmistakable evidence of the existence of plants with little external differentiation and with a simple anatomical structure ; there were no fully differentiated leaves, and the simple sporangia terminated the main axes or side branches. It has been suggested that the simplicity of some of these forms was due to reduction, but in view of the number of different genera now known, this is unlikely. After Kidston & Lang published in 1917 their remarkable description of the Rhynie plants it seemed that the Psilophytales might well be the ancestors of all the higher plants, especially when this was followed by further discoveries of somewhat similar simple types. But these forms are probably not members of a single group of closely related types ; they should be regarded (Leclercq, 1954) as representatives of several phyletic lines from which later groups of the Pteridophyta developed. Although showing differences in form and anatomical structure all these types display an unmistakably primitive organization which suggests their origin from algal ancestors. It is worth noting that these early land plants are accompanied by the remains of thallophytic forms, which exhibit considerable size a,nd complexity but show no trace of the differentiation of xylem, the characteristic feature of land plants. The long known trunks composed of tubular elements and called by the inappropriate name of Prototazites have now been found in several parts of the world and in some places appear to have flabellate branches. They have been referred to the Phaeophyceae, but on inadequate grounds. Another interesting type is Lang's Nematothallus (Lang, 1937) which had a flat frond-like expanse of tissue composed of interlacing tubes, often of two distinct orders of size, and covered by a cuticle with a pseudocellular pattern. Within the cuticle and among the tubes were firm walled (cuticularized) spores of various sizes. Another curious form, described by Andrews & Alt (1956) as C'rocalophyton, had a unique structure with radially elongated strands. This also shows that thallophytic forms had developed considerable size and complexity by Silurian times. We now know that plants resembling Algae had long been in existence for Tyler & Barghoorn (1954) have described indubitable remains of filamentous plants from the Pre-Cambrian of Southern Ontario which are perhaps 1300 million years old. These and the varied remains of thallophytic plants from the Cambrian rocks show that the pteridophytic plants may well have evolved from thallophytic forms long before their appearance in Silurian times. Interrelationships of the Pteridophyta. The general similarities in life-history and in the reproductive structures of the different groups of the Pteridophyta provided grounds for considering that the Ferns, Lycopods, Equisetums, Sphenophylls and Psilotum were probably derived from a common ancestor. I n the early part of this century much thought and discussion was devoted t o this matter. It was rendered more complicated by the morphological dogma that all sporangia were borne on sporophylls of a more or less leaflike nature.' Our present evidence shows however that these groups have been quite distinct from Devonian times and that each has evolved independently. The Lycopods have shown relatively little change in their sporophytes, apart from variations in size and minor changes in anatomical structure. The ferns, on the other hand, have developed in a great number of ways, in form, size, anatomical structure and sporangia. The Equisetales and Sphenophyllales are perhaps interrelated, both having werticillate J.L.S.B. LVI] FOSSIL PLANTS AND EVOLUTION 125 construction and similarities in their fertile shoots and sporangia, but they were already distinct in Devonian times. The discovery of early examples of the fertile shoots of these forms, in which the sporangia were borne a t the tips of naked branches and not on leafy sporophylls, indicates the futility of the older discussions about the morphological nature of their Carboniferous cones. It was almost impossible to interpret the many known forms of these structures on the sporophyll theory. The historical record now seems to show that their bracts were not sporophylls but arose on the fertile axes between the sporangiophores. Mesozoic and modern members of the Equisetales lack any such bracts and retain the primitive structure. The members of the Pteridophyta have shown some parallel evolution in the development of their spores. Each group has a t some time developed heterospory, though most of the forms with this character have become extinct. Origin of the Xeed-bearing Plants. Before the coming of evolutionary ideas taxonomists regarded the possession of seeds as an indication of affinity, and this view persisted well into the present century, when affinity had come to mean genetic relationship. Now, however, the view is widely held that the seed habit originated independently in different groups of plants. D. H. Scott pointed out that the integumented megasporangia which were formed in a t least two genera of Palaeozoic Lycopods, were seeds in a biological sense. From the same period we find remains of two other great groups of plants which produced seeds, these were the Cordaitales and the Pteridospermae. They were totally different from one another in general form, in anatomical structure, and in the the arrangement of their fertile structures, they cannot be considered as closely related. Both had ovules of a primitive type, but in the Cordaitales the integument was derived from a pair of opposite members (bracts or branches), while the pteridosperm integument was formed from the members of a whorl, five or six in number. These structures grew just below the megasporangium on the fertile axis, and in some forms were concentric in anatomical structure and free from each other a t their tips. I consider that we should not regard these structures as modifications of some pre-existing organs but new formations on the fertile apex. It seems likely that the later pteridosperms, the Bennettitales, the Cycads, and probably the angiosperms, were derived from plants having seeds of this type, while the Coniferales, Taxales and perhaps the Ginkgoales, sprang from plants with the Cordaitean type of seed. Emberger (1949) has drawn attention to the absence of any embryos in the petrified ovular structures from the Upper Palaeozoic rocks. He therefore considers that, contrary to past usage, they should not be described as seeds ; he proposes a new class, the Prephanerogams, for the plants having these ovules and includes in it the Ginkgoales and Cycadales. It may well be that from an evolutionary point of view a useful distinction may be drawn between plants in which the embryo is only developed after the dispersal of the ovule (seed), and those in which full development occurs in attachment to the fertile branch. But differences of this kind cannot be held to indicate a genetic connection between such diverse groups as the Cordaitales and pteridosperms, or a separation of the Bennettitales from Cycads and pteridosperms. It may be that none of the earlier seed plants, including the conifers, completed the formation of embryos on the parent plant, but it is unlikely that we yet know all the facts relating to this problem. Mesozoic Gymnosperms. The discovery during the last half century of six or more types of plant which flourished in Mesozoic times and bore seeds (ovules) has been one of the most important of the recent contributions of palaeobotany to our knowledge of plant evolution. Formerly only the conifers, ginkgos, and Bennettitales were known from this period. The existence of the Cycads was inferred from the remains ,of leaves, many of which 126 H. HAMSHAW THOMAS: [J.L.s.z. XLIV, have since been shown to belong to the Bennettitales, but their reproductive structures were virtually unknown. The work on the Mesozoic gymnosperms has given quite a new conception of the evolutionary changes which seem to have taken place among many of the megaphyllous seed plants about the end of the Palaeozoic period. Some of these changes may be significant when the origin of the angiosperms is being considered. The well-known work of Prof. Florin (193845) has greatly elucidated the history of the Coniferales which were widespread a t this time, with Araucarian elements especially abundant. Various genera of the Ginkgoales were plentiful during Triassic and Jurassic times, as shown by their characteristic leaves, but we have as yet little certain knowledge of their reproductive structures. Prof. Harris (1951) has furnished good evidence for thinking that the simple or forking linear leaves known as Solenites or Czekanowskia, and previously thought t o be allied t o the Ginkgoales, had fructifications of a unique type which he referred to the genus Leptostrobus. They show a slender axis bearing fertile appendages in a loose spiral ; each fertile structure formed a flattened capsule composed of two valves joined a t their base and containing about five seeds. The micropyles of the seeds faced the axis, and pollen was also found within the valves. The problem of the evolution of the Cycadales has been little advanced by the discovery of the male and female cones of these plants. Both were very similar to those of living types, though in Beania the seed-bearing structures did not form a compact cone. The Bennettitales have proved to be more varied in form and structure than had been suspected. The earlier types had elongated stems with widely separated leaf bases, unlike those of the Lower Cretaceous Cycadeoideas. The leaves were pinnate or entire. The ‘‘ flowers ” were terminal structures, sometimes on a naked peduncle they frequently had a spiral group of bracts just below the fertile organs. Some of the flowers were unisexual, others bisexual. The male sporangiophores show considerable variation in form but the ovulate parts are remarkably uniform. Some evidence suggests that the characteristic intersemuial scales originated from sterile ovules, nothing has been found to suggest that they ever had the nature of bracts or sporophylls. One of the new groups is the Pentoxylae, an assemblage of plant remains from the Jurassic rocks of India preserved in a petrified condition. Our knowledge of these peculiar types is due to Sahni (1949) and his pupils (Vishnu Mittre, 1953). The stems had an unusual structure with five or more steles and closely-packed leaves, which were probably entire elliptical structures reaching a length of 7 cm. The female infructescences were branched and had compact cone-like structures a t the ends of the branches ; each cone consisted of a number of closely-packed seeds with thick integuments and the remains of micropyles projecting to the outside. The pollen was produced in rounded sporangia terminating the branches of slender elongated structures, which arose from the apex of a stem and may have been fused a t their bases. These plants seem morphologically distinct from any other known type, though a comparison with the Bennettitales is suggested. Another new group from the Triassic of the Southern Hemisphere has been named the Corystospermaceae (Thomas, 1933). These plants were essentially similar to the Palaeozoic pteridosperms. Their leaves varied considerably in form, from bipinnate t o simple, and they had separate male and female inflorescences. The latter were branched, the branches being either in one plane or spirally arranged. The female structures bore bracts and bracteoles, their branches terminated in recurved cupulate ovules with projecting micropyles. The branches of the male inflorescences were expanded a t their tips and produced a number of elongated sporangia on their lower surfaces. The pollen grains were winged, probably an adaptation for the fertilization of the pendulous ovules by a drop mechanism. When ripe the seeds had a stout testa. The Peltaspermaceae have been found in both the Northern and Southern Henii- J.L.S.B. LVI] FOSSIL PLANTS AND EVOLUTION 127 spheres ; they had well-cutinized bipinnate leaves, known as Lepidopteris (Harris, 1932). Their pollen was borne in short sporangia formed a t the tips of a branched structure and sometimes showing a radial arrangement. The ovules were produced on peltate structures a t the ends of branches which arose spirally from a central axis. The Caytoniales (Thomas, 1925 ; Harris, 1940) were a somewhat later group in the Northern Hemisphere. Their leaves were compound, having four or fewer leaflets with closed reticulate venation. The male and female inflorescences were branched in one plane and fertilization was gymnospermous. The pollen was produced in elongate loculate sporangia like the anthers of some flowering plants. The ovulate branches ended in small cupular structures containing a number of ovules and having a small opening a t their base near the stalk. After fertilization this opening became completely closed and the structure developed into a fleshy berry-like fruit containing many seeds with stony testas. This group is interesting because it shows several structural and biological features seen in the angiosperms. The present writer has been widely credited with the view that the Caytoniales may be the ancestors of the flowering plants, in spite of his clear statement on the subject (Thomas, 1931). He considers the Caytoniales to be descended from the Pteridosperms, like the Cycadales and Bennettitales ; the angiosperms probably arose independently from the Palaeozoic pteridosperms, and the features that they show in common with the Caytoniales are due to parallel development. A quite different and highly problematical form, which may well have been a seed plant, was abundant in Permian and Triassic times in the Southern Hemisphere and India. I t s leaves have long been known as Glossopteris, but this is a form genus, and two or more types of plant had leaves of this character. The reproductive organs of several species have been described by M?s. Plumstead (1956)from impressions in which almost all the remains of plant tissue have disappeared. I n these specimens a short fertile axis sprang from the midrib of the leaf and terminated in a close conical group of contiguous seed-like bodies enclosed in a bivalved envelope. Mrs. Plumstead thinks that the seed-like structures grew on the valve nearest to the lamina and that the other valve gave rise to large pollen-bearing structures. The specimens seem to justify this interpretation, but as no other plant is known to have a similar morphological construction the discovery of examples showing remains of the plant tissue is most desirable. I n any event i t seems likely that the material with the Scutum or Ottokaria type of reproductive organ, represents another distinct type of seed-bearing plant, previously unknown. Other, more scanty, remains have been found in South Africa and elsewhere which show that seed plants had appeared in considerable variety by the end of the Triassic period. It is not improbable that the real ancestors of the angiosperms may yet be found to have occurred a t the same period, for many hundreds of square miles of plant-bearing strata in southern Africa still remain to be examined. The Problems of Plant Phylogeny. If phylogeny is defined as the racial history of a species or genus, the historical record of plant life provides almost no evidence for its study. This is because we are very seldom able to give a complete description of a fossil species. Our material consists mainly of isolated organs, separate from one another, and we can rarely be sure of linking up the stems, leaves and reproductive structures t o show what the complete organism was like. If a species cannot be defined in the same way as a living plant, the form of its ancestors or progeny is always a speculation. In this situation two different methods have been followed by those interested in phylogenetic studies. Both of these aspects have been fully discussed by Zimmermann, whose writings have been largely responsible for the interest shown by a n increasing number of botanists in the bearing of palaeobotanical knowledge on our understanding of plant evolution. One mode of phylogenetic study is the examhation of the prob5 JOURN. LINN. S0C.-ZOOLOGY, VOL. XLIV-BOTANY,, VOL. LVI. 128 H. HAMSHAW THOMAS: [J.L.S.Z. XLIV, able relationships of the major groups of land plants, a second is concerned with the historical study of the forms and characters of organs of the same kind in one or a few closely related taxonomic groups. The problems of the interrelationships of the larger groups of extinct and recent plants have given rise t o a considerable literature. Phylogenetic schemes have been put forward by Zimmermann (1930),Lam (1955),Emberger (1944), Magdefrau (1942), Daniker (1954), Greguss (1955) and others. In spite of the paucity of our knowledge of fossil species, we know much about the various forms of stems, leaves and the reproductive structures in the principal groups ; we have some knowledge of the periods in the past when they lived, and of their relative abundance. Our information is doubtless incomplete, but it provides much material for critical consideration, and the attempt to link up the data into a scheme showing the evolution of plant structure is a valid scientific discipline. While the suggestions of some authors seem speculative, the subject is worthy of close attention for it has led t o the appreciation of some aspects of plant structure that may have been previously overlooked. The details of the various schemes that have been proposed cannot be given here, but reference must be made to the papers of Prof. Lam, who has kept in touch with recent palaeobotanical discoveries and has considered the views of other writers on the subject. He has suggested a revised way of classifying the plant world, based on extinct as well as on living forms. A certain amount of agreement is to be found among the various writers on the general evolutionary trends in several groups, but, as may be expected, diverse views are held about the ancestry of the angiosperms. Character Phylogeny. Many botanists have made comparisons of the structures found in taxonomic groups of modern plants, with the object of discovering the probable evolutionary development of those structures ; such work generally depends on suppositions as to the form of the more primitive types. Character phylogeny has the same aim, but is based on the examination of stems, leaves or reproductive structures in groups of apparently related plants, taken in the historical sequence of their appearance in the fossil record. When all the known forms of a particular organ from successive horizons are taken into account, the general pattern of the changes that have taken place can generally be discerned. The types of change and their rates vary independently, and examples of the more primitive forms may persist for some time among more advanced types. The photosynthetic organs of megaphyllous plants, being the parts most commonly preserved, present much material for this kind of study. The general sequence of changes is found to be, from a highly branched lateral shoot to a flat compound frond with small pinnules, then to structures in which pinnules and pinnae become more or less united by parenchymatous tissue forming the mesophyll, with the original branches, pinnae axes, etc. represented by the veins. Comparable changes appear to have taken place a t different times in the ferns and the seed plants. Early in this century Lignier (1903) brought forward the hypothesis, based on the succession of the fossil forms that the fronds of ferns were derived from branch systems. More recently Zimmermmn (1952), Emberger (1953), and others have dealt with similar series. It seems possible t o regard most, if not all, of the changes in the form of fronds and leaves as due to mutations producing changes in the relative growth rates of the different parts of the organ. Zimmermann, who has discussed the question in several publications, considers that the changes have arisen by the action of several " elementary processes " among which he distinguishes ( a ) Overtopping, a process leading to a distinction between rachis and leaflets, pinnae and pinnules, etc. ( b )Plantation, when the telomes and mesomes became arranged in one plane. (c) Syngenesis, where the telomes and mesomes became connected by parenchyma (forming mesophyll), or by their steles. ( d ) Reduction. (e) Incurvation, unequal growth on two oppgsite sides J.L.S.B. LVI] FOSSIL PLANTS AND EVOLUTION 129 of the organ. (f)Longitudinal differentiation, usually affecting the extent of the other processes a t different parts of a leafy structure. The different types of change are regarded as independent of one another. There seems t o be abundant evidence for such a theory from the fossils. It also obtains support from recent observations on the development of leaves in living dicotyledons (cf. Slade, 1957),and on the influence of chemical substances on leaf form. It is also probable that environmental conditions have played some part in causing, or selecting, changes in leaf form, which may also have been influenced by evolutionary changes in the stems and roots of the plants. It is of some general interest to notice that the fossils seem to show a high probability that compound leaves are more primitive than simple leaves, and this may be applicable to the flowering plants. But large simple leaves with a reticulate venation appear in the fossil record towards the end of the Palaeozoic period. Reproductive structures of successive periods may be usefully compared in a similar way to the comparison of leaves above mentioned. A n outstanding example of such work has been provided by the researches of Prof. Florin (193845) on the Cordaitales and Coniferales. These have cleared up the long debated problem of the morphological interpretation of the female cones of the living conifers. Similar studies in other groups may well lead t o results of comparable interest and value. The Origin qf the Bngiosperms. From the early days of the theory of Evolution the origin of the flowering plants has been one of its major problems. This group, now the largest, the most widespread, and the most diversified class of the higher land plants, is greatly different in structure and reproduction from all other living forms. Dicotyledons appeared in considerable numbers and variety in Upper Cretaceous times, and their evolution may well have continued a t a rapid rate since that period. It is widely held that the fossil record throws no light on their Pre-cretaceous ancestors, but it is difficult to believe that no evidence bearing on their origin exists among the fossil floras that have been discovered. It is quite possible that being essentially plants of dry lands (Axelrod, 1952) they did not grow in places where their remains had any chance of being preserved, or that the areas in which they originated have not yet been thoroughly searched. But we may have found the remains of their ancestors, or of allied types, without recognizing them. The writer believes that this may be due in part to the persistence of the old ideas on original plant form based on the classical morphology of Goethe, and in which the parts of the flower had to be either leaves or stems. These concepts, which cannot be applied to evolutionary studies, made botanists think that they knew what was the form of the early flowering plants, and they made a completely unsuccessful search for fossils which realized their ideas. The leaves of the dicotyledons have a characteristic form, but apart from two isolated specimens from the Jurassic rocks, the only species with leaves of this type which he called Purczcla granulifera was found by Prof. Harris (1932) in East Greenland in Rhaetic rocks. We now know, however, that the veins in many living forms show a simple closed reticulum a t an early stage in their development ; a type of venation which was present in many earlier forms, such as Glossopteris, Anthophyopsis, and Caytonia. The Mesozoic and some Palaeozoic gymnosperms had inflorescences with groups of reproductive organs terminating their branches. It is easy to interpret the flowers of the angiosperms by reference t o these, and it seems more logical to do so than to regard the floral organs as a series of modified leaves, a type of structure which is quite unknown among the plants of the past. The fossil record of reproductive branches gives strong support to the view that bracts and bracteoles, sepals and petals are new structures due to changes in the growth of the reproductive axes and not modifications of pre-existing leaves. These ideas may form the basis of a re-assessment of the living angiosperms, and a new search for the characters correlated with strucJOURN. LINN. S0C.-ZOOLOGY, VOL. XL1V.-BOTANY, VOL. LVI. 5s 130 H. HAMSHAW THOMAS: [J.L.s.z. XLIV, tures known to be primitive. The work of Sporne (1949) on these lines may well show that the origin of the flowering plants is not so niysterious as was previously thought. Apart from the search for primitive angiosperms, palaeobotanical work of great importance has been carried out on the angiosperm remains from the Upper Cretaceous and Tertiary rocks. I n Darwin’s time the identification of genera was mainly based on the identification of leaf impressions, and many of the names given were open t o criticism. Now, by the patient labours of many investigators, we have evidence from fruits, seeds, pollen grains and some remains of flowers, as well as from wellpreserved leaf remains. The pioneer work of Clement and Eleanor Reid on ancient fruits and seeds, has been well followed up in Europe, while the studies of E. W. Berry and Prof. Chaney and others on leaf remains in America have been very valuable in disclosing much of the later history of the angiosperms. This work, which is still going on, will tell us much about the evolution of the group. Palaeobotany and Natural Xelection. The study of the past history of plant life might be expected to furnish some evidence of the mechanism of evolution and especially of the role of natural selection in the origin of species. It must, however, be remembered that plants differ greatly from animals in their structure and their mode of life. Anatomically they are very simple when compared with most animals ; apart from the fungi, their nutrition is very uniform in character, they are fixed in the soil from which they draw water and mineral salts, all of them need suitable illumination and temperature for growth. The competition for existence is different from that in the animal world, where food chains of a complicated nature may determine the perpetuation or extinction of a race. I n the past many botanists have tended to regard structural characters as the all important factors determining the viability of a plant species, and have looked on all variation in form as due t o natural selection. There were so many varieties in floral structure which seemed like adaptations for pollination by insects or birds ; there were different types of fruit affecting their seed dispersal, and so evolution was considered as mainly a question of structural change. But few of those who discussed evolutionary problems were acquainted with the physiology of plant growth, or had studied the ways in which it is affected by environmental conditions, as shown by observations on species in the field or under cultivation. Thus the importance of the internal biochemical constitution of the plant was overlooked. I n nature competition between individuals and species undoubtedly exists. In many places species are seen with structural characters apparently related t o their environment, such as succulence, but they often grow alongside plants without any such modifications of structure, moreover succulence may have originated from internal biochemical changes. More important than features of shape and structure is the water balance of a species, the optimum, maximum and minimum temperatures for growth and tlowering, its illumination needs €or vegetative growth and for flowering, the rate ofitsreproductive cycle. Thus it is not unlikely that biochemical changes arising in the nucleus, affecting the enzyme systems in the plant and possibly produced by cosmic ray bombardment, may be the real cause ofthe appearance of new varieties or species, and that visible morphological change may be often a secondary result. Physiological changes may become fixed by natural selection, especially when the species is subjected to climatic changes affecting its habitat. If this view has any validity we could scarcely expect to obtain any information about the origin of variations or the process of natural selection from the study of fossil plants. Rut this study shows t,hat considerable changes in the vegetation of all parts of the earth often took place in the past, and these were associated with climatic fluctuations. A n interesting example of the relation between climate and structure is furnished by the Upper Palaeozoic Lepidodendrales. These plants were very abundant in the swamps of the Coal Measure period and many species referable to sqveral genera had J.L.S.B. LVI] FOSSIL PLANTS A N D EVOLUTION 131 appeared. Their anatomical structure, which is very fully known, had become highly specialized for their mode of life. They formed trees of considerable size, with many leafy branches, but the stems and leaves contained very little water-conducting tissue. On the other hand they had a very large bulk of aerating tissue, essential for the growth of their Stigmarian axes and rootlets, which stood in water or anaerobic mud. This tissue filled a large part of the stems, it communicated with the rootlets below, and with both the mesophyll of the leaves and with the outside air above. When, a t the close of the Carboniferous period, the swamps and lakes of the coal basins dried up, plants with such structure had little chance of survival and they became almost extinct. Only one small unbranched type, Pleuromeia, seems to have survived into Triassic times as a last and rare remnant of a large group. There is geological evidence of frequent climatic variations and of changes in the distribution of land and water in past times. When these changes occurred some land plants were doubtless able t o migrate t o habitats where the soil, rainfall and temperature were suitable, although their light requirements may have limited their movements towards or away from the equator of the period. The information we now have about the effects of the climatic fluctuations in Quaternary times on the survival and distribution of species, provides an example of the type of change which has taken place constantly over the many millions of years of plant growth on the earth. Thus the great glacial period in the Southern Hemisphere at the close of Palaeozoic times must have had wide effects on the flora. After its maximum very large tracts of land were probably left as open habitats for plant growth, and for a long time the vegetation seems to have been sparse, although conditions were probably favourable for the establishment and preservation of species. It may be that this area provided the conditions favouring the rapid evolution and spread of the flowering plants. I n fact a few remains of structures resembling the flowers of angiosperms have been found in the Triassic rocks of South Africa. It is, of course, possible t o point to different structures found among fossil floras and to claim with some degree of possibility that they originated through natural selection. The tightly closed cones found in Cretaceous times may have been responsible for the development and spread of the Abietineae. The fleshy, berry-like fruits of the Caytoniales with their many small stony seeds may have favoured the spread of these plants during the Jurassic period, when birds and small mammals were present to feed upon them. The Bennettitales in Lower Cretaceous times became much dwarfed, with their “flower ” buds almost buried amid an armour of leaf bases, probably related to a gradual climatic dessication. But neither the Caytonias nor the Cycadeoideas seem to have survived beyond Lower Cretaceous times. The changes in frond form in the ferns and pteridosperms that have been mentioned in a n earlier section, might also be ascribed to the natural selection of variations tending t o produce a more efficient photosynthetic organ. Although selection may account for the disappearance of some of the earlier frond forms, the idea can scarcely be applied to the fully expanded bi- and tripinnate types, such as are seen to-day in the ubiquitous Pteridium aquilinum. The formation and increase of auxins influencing the development of the growing points of the stems and fronds provides a more likely reason for the changes from a highly compound to a more simple form of leaf, than the selection of a less dissected form that had arisen by a chance variation. Fossil Plants and Geographical Distribution. The theory of Evolution involved the view that each species was first produced within a single region (Darwin, 1888, p. 320). It was therefore necessary for Darwin t o consider the present discontinuous distribution of species, and to extend this t o the cases where different species of one genus occupied widely separated areas. Two chapters of the Origin of Species were devoted to the discussion of Geographical Distribution, in which special attention was paid to plants. The theory was put 132 H . HAMSHAW THOMAS: [J.L.S.Z. XLIV, forward that discontinuous distribution was mainly due t o the migration of plants in former periods when climatic conditions changed and were different from those of the present day. The possibility of changes occurring in specific characters during such migrations was envisaged. The argument was developed (Origin, p. 330) from the explanation given by Edward Forbes of " the former influence of the glacial climate on the distribution of the inhabitants of Europe ". At the period of maximum glaciation much of Europe and North America, now having a temperate climate, were thought to be inhabited by an arctic flora and fauna, some of whose members ascended the higher mountains and remained there when the climate was ameliorated. This view has long been accepted as the most probable explanation of the discontinuous distribution of certain arctic-alpine species, but it is only in recent years that unquestionable evidence has been found to prove that such migration is an historical fact. The researches of Clement Reid published in The Origin of the British Flora (1899) began the investigation of the remains of Quaternary plants, which in the last thirty years has been actively pursued by many workers in different countries. Dr. Godwin, one of the leaders in this work, has shown in his History of the British Flora (1956) that we have ample evidence from pollen, seeds and other macroscopic remains of the widespread occurrence of arctic species in England after the retreat of the ice. Much is known of the changes in climate, from the last glaciation to to-day, and of the succession of species in the vegetation ; we also have some evidence of the composition of the flora of the previous interglacial period. After giving an account of the theory of Forbes on arctic-alpine species, Darwin went on to consider the possibility that early in the Pliocene period, when the climate was warmer, plants now growing in the temperate regions of the Old and New World extended northwards and were perhaps circumpolar. He thought that towards the end of the Pliocene, when the climate became colder, these plants would probably have retreated southwards, and so become separated in two distinct areas. He was strongly inclined t o extend this view to some still earlier and still warmer period, when other plants with a circumpolar distribution may have extended southwards to those regions where we now see their descendants, mostly in a modified condition. Recent research on the fossil plants from the Tertiary rocks has shown that these suppositions also are in the main historically justified. I n 1920 Mrs. Reid provided evidence of a southward migration of the floras in Pliocene times resulting in floristic differences in different regions. Her conclusions were drawn from the collections of fossil seeds from different horizons in Western Europe. A comparison of the species identified showed that early in the period the flora of Western Europe contained a considerable number of species (64 per cent of the total) which are now extinct in Europe but are found in mountainous regions in North America and China. As time went on the percentage of such species diminished, suggesting a retreat with the onset of colder conditions. I n Europe this retreat was blocked by the east-west mountain ranges and the Mediterranean Sea and many species perished, while, in North America and China, passages to warmer regions were open, and a t a later date many forms were able to return northwards to the areas in which they still grow. Further research has provided evidence of an earlier southward migration in Oligocene or Miocene times. The Eocene floras of Southern England have been shown by Mrs. Reid & Miss ChandIer (1933) to contain many forms closely allied t o species now growing in the tropical rain forests of the Malayan region. Similar vegetation was found in North America, while further to the north there was temperate woodland extending to Greenland and Spitzbergen, and probably circumpolar. The evidence, especially that found in North America by Prof. Chaney (1940) and his collaborators, indicates that as the climate became cooler, and probably drier, the temperate woodland plants moved southwards, and probabIy xerophyllous woodland, savanna and grassland became extensive. Axelrod (19526) has summarized the probabilities of dispersal during this period. Further evidence of migrations in North America has J.L.S.B. LVI] FOSSIL PLANTS AND EVOLUTION 133 been provided by a statistical analysis of the floristic composition of the fossil remains from Upper Cretaceous to Pliocene times by Dr. Barghoorn (1951). This shows a steady change during the period in the number of genera with representatives in the existing flora of the geographic region of the deposit, and in the number of exotic genera. He also provided data showing the relative numbers of unidentified, " extinct", or form-genera present a t each stage ; this may perhaps give some indication of the morphological changes that occurred during the period, for i t seems unlikely that the high percentage of unidentified forms in the earlier floras is entirely due to the extinction of genera. Apart from the problems which Darwin considered, palaeobotanical research has disclosed many other difficult questions relating to past geographical distribution, on which valuable data were collected and presented by Seward in his Plant Life through the Ages (1931). Some of these have given rise to considerable discussion, such as the problem of the discontinuous distribution of the Gondwana flora of Late Palaeozoic and Early Mesozoic times. The remarkable similarities between the representatives of this flora in India, South Africa, Australia and South America seem inexplicable if the continental masses and ocean basins had in Late Palaeozoic times the positions which they occupy to-day. Our fossil floras also supply evidence of many wide ranging forms in past times, such as the occurrence of Lepidopteris in the Triassic rocks of East Greenland, Sweden and South Africa. We may thus conclude that at all periods land plants have migrated over long distances, and that Darwin's suggestions, long regarded as providing a satisfactory theory, now have the status of proved historical fact. Summary and Conclusion. The study of fossil plants during recent years has given a much wider and more comprehensive view of the vegetation of the past, extending back to the Silurian period. It has shown that many groups ofthe higher plants formerly existed whose reproductive features were very different from those of any living types, though in their basic units they are often somewhat similar. The evidence suggests the derivation of the higher land-plants from five or six ancient types, although nothing is really known about the possible connections of these ancestral forms. Modern technical methods of treating the fossil remains, and systematic collecting by trained botanists, have rendered our knowledge more precise and reliable. Owing to the comparative rarity of conditions favouring the preservation of plants as fossils, no long series of forms have been found from which the evolutionary history of species or genera can be determined. The general historical trends in certain large groups, and in the characters of some ofthe organs within those groups may be inferred from the fossil evidence. The reproductive branches of most of the higher plants show increasing complexity with the passage of time. The fossil record suggests that this was often due to the formation of new structures in the region between the site of reproductive activity and that of purely vegetative growth. These new structures (cupules, bracts, etc.) do not seem t o be, in the historical sense, modifications of pre-existing organs. No evidence has been obtained as to the way in which species have originated. Darwin's views on the migration of plants in the Tertiary and Quaternary periods have been shown by direct evidence to be fully justified. It seems certain that similar migrations have take place repeatedly in the distant past. The idealistic concepts of plant form laid down by botanists under the influence of the belief in special creation appear t o have no historical foundations. Many early plants had no real leaves or roots, and there is good evidence that fronds and large leaves have evolved from branch systems. Hence the distinction between branches and leaves, which has played in the past such an important part in the discussion of evolutionary problems, has no fundamental significance. The belief that the JOURN. LINN. S0C.-ZOOLOGY, VOL. XLTv.-BOTANY, VOL. LVI. 50s 134 H . HAMSHAW THOMAS: [J.L.s.z. XLIV, sporangia of all the higher plants were originally borne on fertile leaves, or sporophylls, is without material foundation. 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