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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
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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
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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
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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-
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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
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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
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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
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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
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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,
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50s
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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.
The fossil record now available provides the groundwork for a new and materialistic
philosophy of plant structure. It shows that the idealistic view of plant form, derived
from the ancients, is not applicable to the evolutionary point of view. It indicates,
as Dr. Agnes Arber (1950) has ably shown, that a philosophical distinction must be
made between idealistic morphology and materialistic (evolutionary) morphology,
although both are valid modes of approaching the study of plant form. But during
the past century the two view points have been confused, and this has resulted
in much fallacious reasoning.
It seems likely that the investigation of fossil plants will influence the study of
plant evohtion most strongly by showing that a completely new approach to the
problems of plant form must be adopted. I n this almost all aspects of botanical
study should be integrated with the object of discovering how plants have come to be
organisms such as we now see them.
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