THE NATURE OF DIVERSITY
THE NATURE OF DIVERSITY
EVOLUTION
THE EARLIEST LAND PLANTS
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THE NATURE OF DIVERSITY The plant world exhibits a tremendous diversity. The nature of this diversity can be described in a number of ways.
First, there is variety in the types of organisms: bacteria and blue-green algae and fungi, algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Many biologists now consider that bacteria, blue-green algae and fungi are not part of the plant kingdom. Algae are found largely in aquatic environments. We are mainly concerned with land plants - dealing only with the bryophytes onwards.
Second aspect of diversity is the variation in structure. Plants range from being microscopic, just one cell in the case of some algae, to immense trees, e.g. Redwoods (Sequoias) 350' in height.
Third, there is huge variation in the types of environment in which we find plants - in fact, there are very few environments in which plants cannot survive. Algae can survive in sulphurous hot springs and in snow banks. The environment where a plant lives is very important as it is likely to affect the plant's shape and size, rate of growth and sometimes how it functions physiologically. Particular types of plants will do well in different environments and it is for this reason that we can identify habitats. In damp climates trees proliferate and we see rainforests in the tropics and boreal forests at temperate latitudes. In drier conditions, grasses are more common providing savanna grasslands and the grasslands of the great plains. Habitats are diverse also diverse and essentially a function of the plants response to the environment.
The next question is numbers - how many different species are there? We don't really know, as there are thousands which are not yet described. Even estimates of numbers of plants which have been described vary greatly. However a rough indication (including Protista and Fungi for comparison) -
Bacteria & Blue-Green Algae - 3,800 species
Fungi - 93,000 species
Algae - 22,200 species
Bryophytes - 22,850 species
Pteridophytes - 10,715 species
Gymnosperms - 760 species
Angiosperms - 260,000 species
Can see that angiosperms are the most numerous. The second aspect to numbers is also important - we should not only be looking at the number of species within each group, we should also consider the number of individuals within each species. For example, some species of flowering plant only have a very few individuals located in one small area of the world. Gymnosperms appear an insignificant group with only 750 species and yet they are usually large in size and some species contain enormous numbers of individuals making them a very important group. In fact, in terms of numbers of species, Gymnosperms make up 0.2% of the total number of species in the plant kingdom and yet a third of all forested areas are populated by conifers.
Easy answer is evolution. The mechanics of evolution are covered in other modules. The sort of basic questions that evolutionary biologists might ask are what is a species, what are the mechanisms that allow the formation of new species and can natural populations be defined which represent the basic unit of evolution (tend to see a continuum of changing features in populations of species). Could spend a module discussing this. But it is important to recognize that all species exist because they are adapted to a particular niche. The patchiness of the environment means there are many niches to fill, and this is one of the reasons we see such huge numbers of species. The concept of adaptation is the motor driving evolution and needs defining as an introduction to the evolution lectures to follow.
Adaptations are changes in the structure or physiology of a plant in response to a changed environment. The term adaptation refers to a particular feature of an organism e.g. spines, tendrils, sunken stomata. Adaptations may be separated into two categories: general and special. General adaptations fundamentally reorganize the way of life of plants and tend to be characteristic of groups of organisms. If a group of organisms show a particular general adaptation it is likely that they will have had a common ancestor, so these features are useful for indicating evolutionary relationships between plants. Special adaptations fit a particular organism or population of organisms closely to their immediate environment. These adaptations are usually associated with structural or ecological detail. Of course all adaptations are restricted by the gene pool available to the plant. This means that plants cannot change radically, so it is possible to trace back lineages and identify related groups of plants.
In order to study plant diversity, we need to find a useful way of describing plants. We need some sort of classification to provide us with a sense of order. The history of botany is traced back to a Greek named Theophrastus. 2,300 years ago he wrote a book on plants in which he classified them into herbs, shrubs and trees. This was a classification based on the life form of the plant, a type of classification which is still used extensively by ecologists. Many other early classifications were based on the uses of plants. For example, medicinal classifications were common, with plants classified according to the ailments that they cured. There are quite a number of these artificial or utilitarian classifications still in use today. Many deal with physiological aspects, for example, classifying plants by their tolerance to water stress, e.g. hydrophytes, mesophytes and xerophytes. Other similar classification systems consider a plants reaction to the presence of calcium in the soil - calcicoles and calcifuges. Although useful for particular ecological or physiological studies, these types of classification split plants into so few categories that they are not sufficient if we are to study the diversity of the plant kingdom.
Clearly, these types of classifications are not sufficient when we are looking at 260,000 species of angiosperms alone. "Natural Classification" of plants is designed to follow the natural order that exists - a way of describing every plant with the aim being to reflect evolutionary relationships by taking into account as many features as possible when assigning plants to particular groups. This sort of comprehensive classification was first attempted by Linnaeus, a Swedish Biologist (18th Century). Although he lived a century before Darwin so never comprehended to concept of evolution, he classified plants by their physical features which in many cases mirrored evolutionary relationships. Linnaeus set up a binomial system (2 name) which is still in use today. The basic unit is the species and the family or generic name is termed the genus. Closely related species will have the same generic name, and every species within hat genus will have a different name. Thus, by using a 2 name system, every species can be identified separately. Plants were collected from all over the world for him to describe, and many of the Latin names he assigned to plants still remain.
The study of biological diversity and the placing of organisms into groups is taxonomy or systematics. The aim is to reflect the evolutionary relationships between plants. Botanists therefore look at all sorts of features which might indicate whether plants are related. These will include morphological, anatomical, ultrastructural, biochemical, genetic and fossil evidence. A group of organisms is known as a taxon and they are arranged in a hierarchy. The labels for the taxa lowest in the hierarchy are latinized and in typed text in italics or underlined.
The key ranks are indicated:
Kingdom Plantae
Division Magnoliophyta
Class Magnoliopsida
Order Asterales
Family Asteraceae
Genus Bellis
Species Bellis perennis L.
Division is equivalent to a phylum in animals. In scientific papers it is also usual to indicate the author of the specific name to show the classification used as many species have been renamed as new information further elucidates evolutionary relationships. Many species were originally named by Linnaeus, so his name is abbreviated to the initial L.
The study of plant diversity is more simple than the study of animal diversity in some respects. Firstly, the metabolic requirements of all plants are identical or very similar. The majority of plants require very similar resources to synthesize their primary and secondary metabolites. The biosynthetic pathways leading to these metabolites are also generally very similar. Hence the physiological diversity amongst algae up to flowering plants is much more limited than that of the vertebrates. The morphology (structure/form) of plants is extremely varied. The huge variation in size and structures appears confusing at first, however, all terrestrial plants must fulfill the basic requirements for survival growth and reproduction. They must intercept sunlight which is the energy source for photosynthesis. They must sustain the weight of portions elevated above ground. They must exchange gases with the external atmosphere. They must absorb and conduct liquids. They must reproduce. These basic requirements mean that plants will tend to have the same basic structures.
Consider the environment of a plant living on the land. A plant needs water and nutrients to survive. These are patchy within the environment. In an attempt to maximize its sunlight, a plant must overcome gravity. Generally, a plant has to find a plant of the same species to reproduce with (not easy when you can't move) Offspring must be dispersed and find their own space. Finally there are animals. As a plant you are stuck to one spot, so its quite important to defend yourself against mobile animals who want to eat you. Plants therefore generally have the following features: Root system: absorption of water/nutrients. Anchorage. Stem System: transport, support. Leaf system: photosynthesis. Reproduction. These features are typical of vascular plants, similar structures can be found in nonvascular plants.
Description of plant structure involves precise terminology: Leaves have an abaxial and adaxial surface and petiole. Stems have an internode and nodes, axillary buds and shoots, terminal bud, scars, inflorescence, flowers, bracts. Roots, root buds and adventitious roots. Proximal and distal (= near and far). Developmentally roots and shoots arise in different ways. A plant has a primary root and shoot system which are connected by a transitional region, the hypocotyl. The shoot and root develop from counterpart embryonic systems, called the plumule and radicle respectively. This provides a basic definition for roots and shoots, although it is possible for roots to give rise to shoots directly later in development and adventitious roots are produced by shoots. In many plants the shoot system can grow horizontally - either in the soil, as a rhizome, or at the soil surface as a stolon. Shoot systems can also be modified as a storage organ- forming a tuber, bulb or corm. Roots may also be modified in this way - forming tap roots or root tubers. Some roots are contractile - shorten concertina fashion, to pull the base of the plant under the soil.
EVOLUTIONWhat were the origins of the land plants? When did plants evolve and what were the major events in their evolution? What were the first plants like? Why did they evolve at this time? How did they evolve?
The oldest igneous and crystalline rocks indicate that the earth is about 4,700 million years old. As these rocks eroded, sediments were carried to new locations and deposited where they eventually formed sedimentary rocks. The sedimentary rocks formed layers which have been dated and these form the basis of our Geological Time. Geologists recognize five major layers in sedimentary rock formations - each is called an era and is named after the type of fossils contained: Archeozoic - Proterozoic - Paleozoic - Mesozoic - Cenozoic. These Eras are then subdivided into Periods, which are named after the locality where the strata are found or after significant feature of the strata. Thus - The Cambrian period was 570 Mya. Cambrian is the Roman name for Wales, and our Welsh mountains date back to this time. The Silurian Period was named after an ancient Celtic tribe and the Carboniferous deposits are rich in carbon or coal.
Having described the geological time scale I want to consider how the evolution of plants fits within this - In trying to put this into a meaningful time-frame imagine a journey from London to Aberystwyth - a distance of about 300 km. The first third of the journey passes slowly, but you eventually get to Chipping Norton (NW of Oxford). It is here that we see the first life - very simple prokaryotes, obtaining nutrition from organic chemicals. Another 50 km and you are halfway to Aberystwyth, you have just reached the M5 at Tewkesbury and the countryside is turning green. The development of photosynthesis was a major step in evolutionary terms. Organisms were still simple - just a single cell. The development of eukaryotes occurs as you reach the Welsh border South of Leominster. You have reached Craig Goch above Llangurig as plants finally colonize the land, only 25 km from your destination. If you look around today, most of the vegetation is made up of flowering plants. When did these evolve? You are walking down the hill into Capel Bangor before you see the first flowers. This is an indication of the length of time that passed before plants invaded the land and the rapidity of evolution since then.
Evidence can be of two types. Either we can use at the status of modern day flora to draw conclusions about what has happened in the past (analogues) or we can look for evidence of those first plants themselves in the form of fossils. We can use analogues to answer the first question on the list: What were the origins of the land flora? There are a number of common features between the Chlorophyta (green algae) which lead to the conclusion that they included the ancestors of the land plants: a. Pigmentation, the same major photosynthetic pigments - chlorophyll a & b, xanthophylls in plastids in more or less equivalent proportions. b. Starch storage. c. multicellular gametangia. d. cell wall chemistry very similar. e. alternation of generations in which sporophytes and gametophytes alternate.
In order to answer the other questions we need to look for more direct evidence in the form of fossils. Can find extractable carbon compounds such as DNA and amino acids in very ancient rocks. This evidence of organic compounds enables us to estimate when life on earth began. Other types of fossils are formed from the remains of organisms. The quality of fossils depends upon the conditions in which they were created. Clearly, the less they have decomposed the better, so if they are initially in anaerobic conditions and have sediments deposited on top immediately, the condition is likely to be good. Fine textured sediments provide the most detail, so the best fossils will have been deposited in flood plains, lakes, swamps, bogs and estuaries. In addition the more durable structures are more likely to be preserved, so spores, which have a waterproof, durable outer coating tend to be well preserved. Types of fossils are -
Mould/Cast. Material is deposited around the object, and as the plant decomposed the hollow left slowly fills up with a different sediment. This yields a cast showing the external morphology of the plant.
Compression/Impression. The mechanism is the same with deposition followed by decomposition of the plant material. However, the plant material is compressed by the weight of sediments being deposited above, so the external features of the plant are distorted, and more delicate structures, such as leaves tend to be lost. Splitting of the rock will leave a compression - the cast of the squashed plant, and an impression - the flat surface left behind.
Petrification. Petrification is the form of fossilization which yields the most information. Here the original cell shape or structure remain as minerals partially replace organic substances. This occurs as water rich in minerals circulates and infiltrates the plants. The preservation is so good that the fossil produced can be carefully sectioned and the cellular structure can still be seen through a microscope.
These fossils are very useful in determining the origins of the plants we see today. But there are a number of reasons why the fossil record is incomplete:
1. Lowland Plants. Land plant fossils probably mostly represent plants growing in lowlands where the land slowly increases in height next to a delta or river. This is not a problem when looking at the first land plants as it is likely they would all have evolved in this situation any way. But the evolution of flowering plants is thought to have occurred in mountainous areas, so we have no fossils of this.
2. Distribution. The distribution of plants which are about to be fossilized can be affected by the elements of a plant having different masses and sizes. For example, pollen is likely to be washed further down an estuary than the stem of a plant. It is also likely that pollen will be deposited in finer sediments. We therefore find accumulations of specific plant organs, all separated from the rest of the plant.
3. Plant Characteristics. Fossils only show certain characteristics of plants - such as leaf morphology or the anatomy of wood, they cannot show features such as the habit of the plant, the structure of a flower or the chromosome number. Add to this that we may only have found one of the organs of the plant - its leaves or its stem, and it is clear that the identification of what species a fossil belongs to and the relationships between the ancient species is extremely difficult.
With this in mind we can use the fossil evidence to answer our next questions: When did they evolve, what were they like? As well as the biochemical evidence microscopic fossil remains of prokaryotes have also been found . These existed primarily in shallow seas and intertidal coastal regions dating back over 3000 million years. Initially, these were organic fermenters in energy terms; they were anaerobes. Evidence for the existence of chlorophyll and thus the ability to photosynthesize dates back to the late Precambrian prokaryotes, perhaps appearing around 2300 Mya. Probably around 1400 Mya eukaryotes appeared some of which were able to photosynthesize possibly as a result of symbiotic relations with prokaryotes forming the membrane bound organelles that we recognized as plastids. The first land plants appeared about 420 Mya in the late Silurian. By the late Devonian examples of these plants were found in Ireland, South Wales, and as far afield as Russia, North Africa and North America.
The atmosphere was of volcanic origin and therefore devoid of oxygen. It seems that the increasing amount of photosynthetically produced oxygen in the atmosphere had a profound effect on evolving early life.
Initially, the oxygen produced by the photosynthesizing cyanobacteria and later green algae, perhaps over a period of several hundred million years went into the precipitation of iron dissolved in the oceans. Once this was all precipitated as ferric oxide, free oxygen began to accumulate in the atmosphere. The consequence was continued increases in the concentration of oxygen in the atmosphere. The earth became progressively an oxygen-rich planet. The evolution of land plants appears to be related to this build up of oxygen. But plants don't utilize oxygen in a way that oxygen levels would be critical - so it must be something else. Ozone absorbs harmful ultraviolet radiation which will cause damage to nucleic acids and proteins. Now plants could develop a pelagic lifestyle, whereas organisms only lived in deep waters previously where water had fulfilled the same function of protection, now they could colonize surface waters. In addition, they could get to the surface of the water and use aerobic respiration which is far superior to the anaerobic fermentation in that the efficiency of energy generation meant they could achieve more complex cellular organizations.
Quite unlike anything we are familiar with today, totally lacking in vegetation which would have a number of effects - a. temperatures high during the day with reduced cloud cover - very high surface temperatures, rapid cooling at night to very low temperatures. b. rainfall would either rapidly drain through sandy substrates or fail to infiltrate clayey "soils". Considerable erosion, rapid weathering of minerals but a great deal just washed away. c. frequent dust storms and large shifting dunes. d. no organic matter in "soils" to dampen the wild fluctuations in the soil environment.
The major problems encountered are associated with desiccation. It is likely that a number of evolutionary lines colonized the terrestrial environment with the following strategies -
a. The ability to move (disperse) over considerable distances - clearly vicariously in the form of desiccation tolerant spores. Those that we find in the early members of the land flora have a characteristic form - trilete - produced in tetrads. Green algae produce resistant spores but not trilete - an indicator of the land plants? They appear early in the fossil record but the plants that produced the resistant spores at the early stage are not known.
b. A cuticle; if desiccation is the major problem in the terrestrial environment then a waterproof jacket is a major consideration. A cuticle will also be resistant to chemical substances, microbial attack, abrasion and mechanical injury. Therefore there are many benefits to a cuticle. The main disadvantage is that it inhibits gaseous exchange, however, if its thin, gaseous diffusion may not become limiting. A true cuticle probably comes along late maybe even after invasion, for in a marine environment the fatty acids that would comprise cutin would saponify (be converted into soap). Isolated cuticles are found in the fossil record.
c. Stomata: One way of overcoming the limitation on gaseous exchange is to have holes which will let in the gases. Special regulatory structures are found in the form of stomata. They control the entry of gases and loss of water vapour.
d. If water and nutrients cannot be taken in through the surface of the whole plant, in water they can be, there is a need for a transport system. This is found in a whole range of land plants; the structure is also unique to land plants; but it may not be essential - we find non-vascularized plants among vascularized plants in the fossil record. This is only really essential when the plant has an upright habit.
e. Often these are in the form of tracheids - algae lack tracheids. There may have been a range of tracheid-like or prototracheid structures and indeed lignified tubes unconnected to any plant material have been found in the fossil record - what they belonged to is not clear, they occur in the Silurian and Devonian. Lignin appears to be a by-product of photosynthesis maybe even a detoxification product. Its advantages are that it increases the efficiency of transport in that water won't leak through tracheid walls impregnated with lignin, secondly it increases mechanical strength for plants with an upright habit. Aquatic plants did not need lignin because buoyancy supported the plant.
f. Mycorrhizae. Nutrients in the terrestrial environment are patchy often locked up in an available form, in contrast to aquatic environments where they are available (in solution) and more uniform (although local depletions do occur). It has been noticed that many of these early land plants appear to be infected with fungi, not pathogenic but symbiotic. There modern day equivalents are well known to fulfill an important role in plant nutrition.
In conclusion the evidence from fossils is scattered and fragmentary. In fact, we know far less about the evolution of plants relative to the evolution of the animal kingdom. Much is still a mystery which we are slowly piecing together - however there is enough evidence to allow us to identify the major evolutionary advances and it is this which will consider in the next lecture.
THE EARLIEST LAND PLANTSThree main Divisions appeared early on:
Rhyniophytes
Zosterophyllophytes
Trimerophytes
The fossils of Cooksonia are the oldest fossil land plants found to date. It appears in the Late Silurian (Wenlockian) and it survived through to the Lower Devonian for at least 35 My. We have built up knowledge of the nature of this plant from fossils found in Ireland, S. Wales, Scotland, Germany, Czechoslovakia, Russia, North Africa, North America. The axes were extremely slender - max 1.5 mm with single rounded sporangia at the ends of each stem. These contain the spores which are now resistant to desiccation (and trilete). From Late Silurian onwards, we see a rapid radiation in form, both vegetative and reproductive. Spores and sculpturing of spores tell the same story.
There is a village in Aberdeenshire - Rhynie - near which is an area of rock called Chert dating back to 380 Mya. This contains the best examples of the first land plants in the world. The plants grew in a sort of reed marsh close to an active volcano. When the volcano erupted the area was covered by boiling silicate rich water. This killed the plants which were also covered so preventing decomposition. The silicates invaded the plants and preserved them by the process of permineralization, so even the cellular detail was maintained in some cases. As the area was covered again by sand, plants regrew on the area until the volcano erupted again - happened several times. The main species discovered was named Rhynia, and other similar plants fall into the Division known as the Rhyniophyta. Rhynia is distinct from Cooksonia on the basis of size, spore size and shape of the terminal sporangia (plant height 1.3 to 6.5 cm). These were plants of simple structure - consisting of a photosynthetic stem, no leaves or roots, with fertile sporangia containing the spores at the tips of the stems (terminal). The axis was dichotomously branched. Rhizomatous portion (horizontal stem) was probably subterranean with rhizoids (single cells, outgrowths from the epidermis, which absorb water and nutrients).
The excellent preservation means we can even show the internal structure with a central strand of vascular tissue. We see, even in these earliest land plants that they have indeed followed some of the strategy with xylem vessels internally thickened by lignin. It is also found in cells other than transport cells - to give mechanical support to the tissues. The evolution of lignin - thought to be a byproduct of cellular activities- clearly, the advantages it endowed were selected for. It also seems that lignin may have been deposited in xylem vessels to provide mechanical strength and prevent them imploding - with the waterproof nature of lignin being a secondary advantage. This was a major evolutionary step and probably the first to allow them onto dry land. There must also have been some sort of cuticle or protective coat, as even in these very early plants we find stomata present - an adaptation to the need to take O2 through an otherwise impermeable layer.
In fact the Rhynie Chert fossils are particularly interesting because they show a community of early land plants growing together. As well as Rhynia there were 5 other species clearly identifiable, including Horneophyton. Horneophyton is similar to Rhynia in having dichotomously branched stems with no leaves. It is clearly different in having corm like structures at the base of each stem, with rhizoids. This basal corm poses a problem as it has no vascularization, so we don't know how water and minerals were effectively transported through it. The sporangium is again terminal, but is also unusual in being forked with a central column of sterile tissue running through each fork. We don't know why this species arose or where it sits in an evolutionary sense as the other Rhyniophyta are different.
From Quebec, Renalia, again a simple axis with terminal sporangia and with these we have evidence of how they split (11cm)
This subdivision appeared in the Lower Devonian and about 7 Genera have been identified. Zosterophyllum appears in the fossil record in the Gedinnian Period in the Lower Devonian of Europe. The variety of species is more diverse in this group.
a. Zosterophyllum was a plant with similar features to Rhyniophyta but was more specialized had a horizontal axis from which arose two types of branch. One going vertically up with sporangia spiralled aggregations around the stem towards the top, the other vertically down - suggested that this was a plant of marshy environments. Occasional stomata are also seen on the stems which grew up to 15 cm.
b. Gosslingia - implies a sprawling habit growing up to 50 cm in height. The epidermis of the stem was cutinized - a true cuticle, unable to be produced before due to the saponification of cutin in presence of seawater. The stems were thicker and the differentiation relatively great.
c. Sawdonia had sporangia scattered all along the shoot - its morphology suggests a humid environment with spirally arranged short spines that were perhaps glandular in function.
In the Zosterophyllophyta the sporangia are lateral. Branching is pseudomonopodial. The species in the group are more specialized in terms of their diversity of form and the positions of their sporangia.
Even more specialized, both in their branching systems and the nature of their sporangia. The group also appeared in the Lower Devonian and survived 20-30 My - disappearing again in the Mid Devonian. Differs from the other Divisions in terms of the branching of the stem, where a main vertical stem has smaller lateral branches breaking off. At the tips of the lateral branches find clusters of terminal sporangia. The main genus is Psilophyton, with a number of species, some of which were showed much wider branches of 1.5 cm and up to 1 m in height. Some species showed a separation of functions between stems, with some being vegetative and others reproductive. Some species also have enations which may be precursors to leaves.
Again characterized as a group by the monopodial branching with more or less indeterminate growth of the main axis with limited development of lateral branches. The growth form gave these plants considerable potential. Small shrubby or bushy growth forms become possible. Some had a relatively solid cylinder of vascular tissue which gave added strength and plants grew up to 3 m in height.
That describes the main features of the earliest land plants; we know about their growth form. Many of them are weak in terms of their supporting structures; they seem to grow together with horizontal vegetative branches giving rise to upright shoots growing together for mutual support. Unfortunately, their reproduction - alternation of generations poses real problems - we have no idea what the sexual reproduction stage was like. No sexual reproductive structures have been demonstrated convincingly. More frustratingly, what hasn't been determined is the evolutionary positions of these groups. Were they a sequence of evolution or was it coevolution? Which were the stock for the later development of the plants we see today? These are fundamental questions that we may never be able to answer. What we know is that we have mixtures occurring in the same deposits, they were growing together in communities. For example, the Gaspe flora of Quebec contains Renalia (Rhyniophyta) with Sawdonia (Zosterophyllophyta) and Trimerophyton (Trimerophytophyta) all growing together. It seems that these are families of species - probably independent lines evolving rapidly.
Why should they be evolving rapidly at this time? Plenty of opportunity - no other plants competing so any mutation unless its lethal is likely to survive. Genetically they are simple so mutation is likely to be meaningful, no polyploidy to suppress mutant expression. Polyploidy represents another opportunity for evolutionary routes. Most of these early species disappeared in the Devonian. It seems likely that the Zosterophyllophytina carried on evolving to form the next major stage in land plant evolution -
Asteroxylon grew up to 50 cm but species which followed represent the first successful wave of the development of massive plants. Asteroxylon looks similar to the earliest plants and first records place it in the Devonian. It had a rhizomatous axis which produced root-like dichotomizing branches with vertical stems reaching upwards covered in leaflike scales. The sporangia were flattened sacs and were intermixed with the scales. This plant comes form a group with members which are still living and looks very like some of our modern lycopods (club-mosses). The development of leaf-like structures was an extremely important step.
As the Devonian continued into the carboniferous we find Sigillaria, a 30m "tree". Even more impressive is Lepidodendron a tree up to 150'/50 m which formed very extensive forests which gave rise to present day coal deposits. Not really a tree, but a giant lycopod. A remarkable organism, presenting the zenith of diversification of the lycophytes - the highest point in their evolution. There is also no doubt that Lepidodendron also had a great influence on the evolution of other species. The forest communities would have created shady environments - creating selection for shade tolerant species or species which required shelter. The lycophyte nature of the tree can be seen by the pattern left from the shedding of leaf scales which covers the entire plant. It still branched dichotomously. Although it had modifications to help prevent desiccation, such as sunken stomata in its leaves. The so-called root system was limited suggesting it would be restricted to swampy environments where water was readily available. In fact the trunk rose from an expanded base organ - stigmarian system - usually consisting of four main branches which branched and had roots instead of leaves emerging towards the tips. They were shallow rooted which seemed to give adequate support in this environment it appears they were vulnerable to climatic change and as the climate became more arid they died out.
A second line which developed in the Late Devonian (365 Mya) and through into the Carboniferous was the arthrophytes. This appears to be a rather unusual group of plants. Probably represented by Pseudobormia - a main upright axis with spatially arranged branches (20m) tall. Arthrophytes were characterized by whorled leaves - whorled arrangement of everything and there were many herbaceous examples. This group goes through into the Carboniferous when Calamites was common - a 10-20 m shrub with whorled arrangements of leaves and sporangia, and a hollow stem. This would have provided the understory in those carboniferous forests. But these giant forms became extinct in the Permian. There is just one genus which has survived through to the present day - Equisetum.
Separated off early on but for a long time this line resembled the ancestors the rhyniophytes. Stauropteris was a plant of the Carboniferous age. The plants were probably bushy and small, with the main stems still no more than 3 mm in diameter. These species are known only as fossils and appear to be derived from the Trimerophyta.
A fourth line developing in Upper Devonian was characterized by Archaeopteris. Represents the first true tree, but in some ways resembled a fern. It had more complex foliage than the lycophytes or arthrophytes - true laminated leaves. It was this line that then saw the change in reproduction (plants such as Medullosa) which would lead to the evolution of the seed, the crucial evolutionary step - leading to both gymnosperms and angiosperms setting the path for the second evolutionary explosion. The gametophyte generation was always the weak point in the cycle - the vulnerable phase. Having motile gametes requires water and restricts reproduction to damp environments. The seed internalizes this generation, protecting it and allows for much greater rates of survival of these new species.
We can trace the conifers from species in the Upper Carboniferous and cycad ancestors are found in Lower Jurassic sediments in Yorkshire. By this time in the Mesozoic Era many of the fossil plants have equivalents in our modern flora. The angiosperms are recorded in the lower part of the Cretaceous period and evolved rapidly. Unfortunately, we have no series of fossil ancestors for the flowering plants.
Important to remember that the world looked v. different at this time and it is worth relating the features we see in evolution to the physical environment of the time. In the Cambrian Period evidence suggests that most of the continental land masses were in equatorial latitudes. However, we also see evidence for glaciation in many areas, so it appears that during this period there was world wide glaciation. During the Ordovician (500 - 440 Mya) - prior to the invasion of the land plants, we see provincialism in marine fossils, i.e. organisms had evolved separately in different parts of the world. This reflects the presence of a barrier at this time - in the form of the Iapetus Ocean. This gradually closed during the Ordovician Period and during the following Silurian Period. In contrast, evidence from the Silurian Period suggests remarkable biological uniformity on a world wide scale - the continental mass was all becoming interconnected with intervening oceans decreasing in size and all within low latitudes (equatorial) - warm and wet climates were clearly advantageous to the evolution of species of land plants which had such problems with desiccation.
By Devonian times (400 Mya) the Iapetus Ocean had completely disappeared, forming a large continent - forming a large continent - the Old Red Sandstone (Laurasia), including much of Europe, Greenland and N. America. This may have extended into Australia (link between Laurasia and Gondwanaland). It was the existence of this equatorial continent, with vast terrestrial areas containing extensive lakes, over the Silurian and Devonian Periods which seemed to have provided suitable conditions for the evolution of land plants and animals. It is for this reason that we find evidence for the first land plants such as Rhynia scattered all across our current continents, N. America, Europe, Russia and Africa.
As time progressed the continents shifted round to form one large horseshoe-shaped continent (Late Carboniferous and Permian). The spread of animals and plants across it was virtually unrestricted. The rise and fall of sea levels inundating these lands in the Carboniferous are what gave rise to our coal measures - found in N. America and W. Europe. These coals contain identical fossils of the flora and fauna of the time. The areas of the continent which had moved away from the equator were beginning to see evolution of new species. Siberia was isolated and more northerly. The fossils here have stems which show well-marked growth rings indicating alternation in winter and summer conditions.
Still the majority of Gondwanaland was low in altitude, with the only mountain ranges being low and confined to its periphery. Hence the range of ecological niches available was small. As conditions became harsher as most of Gondwanaland moved into medium and polar latitudes and the climate became colder, covering much of the area in ice sheets. It is at this time (Triassic - 230 Mya) we see a large number of extinctions of the original land flora. As time progressed and the climate improved once again, the single large continent broke up and movements created mountain ranges. As a consequence of increasing number of niches we see a second explosion in evolution, taking us all the way through to the flowering plants.
Spores have been found as microfossils from the time of the very earliest land plants. The problem is relating the spores to particular species. All of these plants produced trilete spores which are also seen to evolve over time. Certainly early plants had some form of waterproof jacket - shown by the presence of stomata. Examples of cuticles are found as fossils. The early stomata were simple, they became slightly more sophisticated over time, but have not altered a great deal. The fact they are almost universal throughout the land plants indicates they were one of the first general adaptations. Vascularization - again seen in the earliest plants in the form of a central stele/column. As plants became larger and more complex, this central column was divided into vascular bundles, supporting and transporting water to new and more complex structures.
Mycorrhizae - association of the first plants with fungal hyphae may seem unimportant, but there is a hypothesis that both the colonization of land and the following evolution of land plants was only possible as a result of these associations. The first symbiotic associations appear to be between a semi-aquatic ancestral alga and an aquatic fungus within the shallow bays and lagoons in Silurian times. The continents were expanding at this time due to increasing aridity and some of the more ancient algae became extinct. They were replaced by essentially modern algae and the land plants. Fungi were present and, being heterotrophic, were dependent on the photosynthetically produced carbohydrates, while the algae benefited from the ability of the fungus to extract essential nutrients from the surrounding environment. It seems that this association was obligate amongst the first plants, as it is with many species of lower plants today (though not always obligate - 90% of all plants are mycorrhizal). Certainly the partnership would have conferred a selective advantage for survival and greater potential for evolution.
Roots and leaves are two features described in 'basic plant design' which were not part of the strategy which enabled plants to first invade the land. The development of both these features occurred relatively late on. Roots - rhizoids sufficed for a long period, roots evolved with a drier climate (rhizoids were not bathed in swamps) and need for better anchorage. Leaves - Mid Devonian saw flat photosynthetic surfaces develop simultaneously in different phyla. These surfaces were required to increase the surface to volume ratios, and the need was tackled differently in the different groups of species. Some microphyllous plants - the Lycophyta - developed large, narrow leaves, others greatly increased the number of minute leaves. True megaphyllous leaves also developed at this time, e.g. Archaeopteris. This sudden appearance of leaves seems to be related to the increase in oxygen in the atmosphere providing enough protection from UV light to allow these more delicate tissues, in more direct sunlight to remain undamaged.