Embryophyte
Encyclopedia
The land plants or embryophytes, more formally Embryophyta or Metaphyta, are the most familiar group of plant
s. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae
that are primarily aquatic. The embryophytes include tree
s, flower
s, fern
s, moss
es, and various other green land plants. All are complex multicellular eukaryote
s with specialized reproductive organs. With very few exceptions, embryophytes obtain their energy through photosynthesis
(that is, by absorbing light); and they synthesize their food from carbon dioxide
.
era (which started around ). The Charales
or stoneworts appear to be the best living illustration of that developmental step. Embryophytes are primarily adapted for life on land, although some are secondarily aquatic
. Accordingly, they are often called land plants or terrestrial plants.
On a microscopic level, the cells of embryophytes remain very similar to those of green algae. They are eukaryotic
, with a cell wall
composed of cellulose
and plastid
s surrounded by two membranes. The latter usually take the form of chloroplast
s, which conduct photosynthesis and store food in the form of starch
, and are characteristically pigmented with chlorophylls a and b, generally giving them a bright green color. Embryophyte cells also generally have an enlarged central vacuole
or tonoplast, which maintains cell turgor and keeps the plant rigid. They lack flagella
and centriole
s except in certain gamete
s.
Embryophytes have three features related to their reproduction which collectively distinguish them from all other plant lineages. Firstly, they have a life cycle which involves 'alternation of generations
'. A multicellular generation with a single set of chromosome
s – the haploid gametophyte
– produces sperm and eggs which fuse and grow into a multicellular generation with twice the number of chromosomes – the diploid sporophyte
. The mature sporophyte produces spore
s which grow into a gametophyte, thus completing the cycle. Secondly, their gametophytes produce sperm and eggs in multicellular structures (called 'antheridia
' and 'archegonia
' respectively) containing sterile tissue (i.e. non-reproducing tissue). Thirdly, the fertilized egg (the zygote
) initially develops within the archegonium where it is protected and provided with nutrition. This last feature is the origin of the term 'embryophyte' – the fertilized egg develops into a protected embryo, rather than dispersing as a single cell.
Embryophytes also differ from algae by having metamers
. Metamers are repeated units of development, in which each unit derives from a single cell, but the resulting product tissue or part is largely the same for each cell. The whole organism is thus constructed from similar, repeating parts or metamers. Accordingly, these plants are sometimes termed 'metaphytes' and classified as the group Metaphyta.
, one name for which is Viridiplantae (i.e. 'green plants'). According to several molecular clock estimates the Viridiplantae split to into two clades: chlorophytes and streptophytes. The chlorophytes are considerably more diverse (with around 700 genera) and were originally marine, although some groups have since spread into fresh water. The streptophyte algae (i.e. the streptophyte clade minus the land plants) are less diverse (with around 122 genera) and adapted to fresh water very early in their evolutionary history. They have not spread into marine environments (only a few stoneworts, which belong to this group, tolerate brackish water). Some time during the Ordovician period (which started around ) one or more streptophytes invaded the land and began the evolution of the embryophyte land plants.
Becker and Marin speculate that land plants evolved from streptophytes rather than any other group of algae because streptophytes were adapted to living in fresh water. This prepared them to tolerate a range of environmental conditions found on land. Fresh water living made them tolerant of exposure to rain; living in shallow pools required tolerance to temperature variation, high levels of ultra-violet light and seasonal dehydration.
Relationships between the groups making up Viridiplantae are still being elucidated; views have changed considerably since 2000 and classifications have not yet caught up. However, the division between chlorophytes and streptophytes and the evolution of embryophytes from within the latter group, as shown in the cladogram below, are well established. Three approaches to classification are shown. Older classifications, as on the left, treated all green algae as a single division of the plant kingdom under the name Chlorophyta. Land plants were then placed in separate divisions. All the streptophyte algae can be grouped into one paraphyletic
taxon, as in the middle, allowing the embryophytes to form a taxon at the same level. Alternatively, the embryophytes can be sunk into a monophyletic
taxon comprising all the streptophytes, as on the right. A variety of names have been used for the different groups which result from these approaches; those used below are only one of a number of possibilities. The higher-level classification of the Viridiplantae varies considerably, resulting in widely different ranks being assigned to the embryophytes, from kingdom to class.
The preponderance of currently available molecular evidence suggests that the groups making up the embryophytes are related as shown in the cladogram below (based on Qiu et al. 2006 with additional names from Crane et al. 2004).
Studies based on morphology rather than on genes and proteins have regularly reached different conclusions; for example that neither the monilophytes (ferns and horsetails) nor the gymnosperms are a natural or monophyletic
group.
There is considerable variation in how these relationships are converted into a formal classification. Consider the angiosperms or flowering plant
s. Many botanists, following Lindley in 1830, have treated the angiosperms as a division. Researchers concerned with fossil plants have usually followed Banks in treating the tracheophytes or vascular plant
s as a division, so that the angiosperms become a class
or even a subclass. Two very different systems are shown below. The classification on the left is a traditional one, in which ten living groups are treated as separate divisions; the classification on the right (based on Kenrick and Crane's 1997 treatment) sharply reduces the rank
of groups such as the flowering plants. (More complex classifications are needed if extinct plants are included.)
s consist of all nonvascular land plants (embryophytes without vascular tissue
). All are relatively small and are usually confined to environments that are humid or at least seasonally moist. They are limited by their reliance on water needed to disperse their gamete
s, although only a few bryophytes are truly aquatic. Most species are tropical, but there are many arctic species as well. They may locally dominate the ground cover in tundra or the epiphyte flora in rain forest habitats.
The three living divisions are the mosses (Bryophyta), hornworts (Anthocerotophyta), and liverworts (Marchantiophyta). Originally, these three groups were included together as classes within the single division Bryophyta. However, they now are placed separately into three divisions since the bryophytes as a whole are known to be a paraphyletic (artificial) group instead of a single lineage. Instead, the three bryophyte groups form an evolutionary grade of those land plants that are not vascular. Some closely related green algae are also non-vascular, but are not considered "land plants."
Despite the fact that they are no longer classified as a single group, the bryophytes are still studied together because of their many biological similarities as non-vascular land plants. All three bryophyte groups share a haploid-dominant life cycle and unbranched sporophytes, These are traits that appear to be plesiotypic within the land plants, and thus were common to all early diverging lineages of plants on the land. The fact that the bryophytes have a life cycle in common is thus an artefact of being the oldest extant lineages of land plant, and not the result of close shared ancestry. (See the phylogeny above.)
The bryophyte life-cycle is strongly dominated by the haploid gametophyte generation. The sporophyte remains small and dependent on the parent gametophyte for its entire brief life. All other living groups of land plants have a life cycle dominated by the diploid sporophyte generation. It is in the diploid sporophyte that vascular tissue develops. Although some mosses have quite complex water-conducting vessels, bryophytes lack true vascular tissue.
Like the vascular plants, bryophytes do have differentiated stems, and although these are most often no more than a few centimeters tall, they do provide mechanical support. Most bryophytes also have leaves, although these typically are one cell thick and lack veins. Unlike the vascular plants, bryophytes lack true roots or any deep anchoring structures. Some species do grow a filamentous network of horizontal stems, but these have a primary function of mechanical attachment rather than extraction of soil nutrients (Palaeos 2008).
and Devonian
periods (around ), plants evolved which possessed true vascular tissue, including cells with walls strengthened by lignin (tracheid
s). Some extinct early plants appear to be between the grade of organization bryophytes and that of true vascular plants (eutracheophytes). Genera such as Horneophyton
have water-conducting tissue more like that of mosses, but a different life-cycle in which the sporophyte is more developed than the gametophyte. Genera such as Rhynia
have a similar life-cycle but have simple tracheids and so are a kind of vascular plant.
During the Devonian period, vascular plants diversified and spread to many different land environments. In addition to vascular tissues which transport water throughout the body, tracheophytes have an outer layer or cuticle that resists drying out
. The sporophyte is the dominant generation, and in modern species develops leaves
, stems
and root
s, while the gametophyte remains very small.
. It is believed that microphylls evolved from outgrowths on stems, such as spines, which later acquired veins (vascular traces).
Although the living lycophytes are all relatively small and inconspicuous plants, more common in the moist tropics than in temperate regions, during the Carboniferous
period tree-like lycophytes (such as Lepidodendron
) formed huge forests that dominated the landscape.
The euphyllophytes, making up more than 99% of living vascular plant species, have large 'true' leaves (megaphylls), which effectively grow from the sides or the apex, via marginal or apical meristems. One theory is that megaphylls developed from three-dimensional branching systems by first 'planation' – flattening to produce a two dimensional branched structure – and then 'webbing' – tissue growing out between the flattened branches. Others have questioned whether megaphylls developed in the same way in different groups.
Euphyllophytes are divided into two lineages: the ferns and horsetails (monilophytes) and the seed plants (spermatophytes). Like all the preceding groups, the monilophytes continue to use spores as their main method of dispersal. Traditionally, whisk ferns and horsetails were treated as distinct from 'true' ferns. Recent research suggests that they all belong together, although there are differences of opinion on the exact classification to be used. Living whisk ferns and horsetails do not have the large leaves (megaphylls) which would be expected of euphyllophytes. However, this has probably resulted from reduction, as evidenced by early fossil horsetails, in which the leaves are broad with branching veins.
Ferns are a large and diverse group, with some 12,000 species
. A stereotypical fern has broad, much divided leaves, which grow by unrolling.
-resistant capsules called seed
s. Starting from a plant which disperses by spores, highly complex changes are needed to produce seeds. The sporophyte has two kinds of spore-forming organs (sporangia). One kind, the megasporangium, produces only a single large spore (a megaspore). This sporangium is surrounded by one or more sheathing layers (integuments) which form the seed coat. Within the seed coat, the megaspore develops into a tiny gametophyte, which in turn produces one or more egg cells. Before fertilization, the sporangium and its contents plus its coat is called an 'ovule'; after fertilization a 'seed'. In parallel to these developments, the other kind of sporangium, the microsporangium, produces microspores. A tiny gametophyte develops inside the wall of a microspore, producing a pollen
grain. Pollen grains are physically transferred between plants by the wind or by insects. When a pollen grain reaches an ovule, it enters via a microscopic gap in the coat (the micropyle). The tiny gametophyte inside the pollen grain then produces sperm cells which move to the egg cell and fertilize it.
Seed plants include two groups with living members, the gymnosperms and the angiosperms or flowering plants. In gymnosperms, the ovules or seeds are not further enclosed. In angiosperms, they are enclosed in ovaries. A split ovary with a visible seed can be seen in the image to the right. Angiosperms typically also have other, secondary structures, such as petals, which together form a flower.
Extant seed plants are divided into five groups:
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s. They are called 'land plants' because they live primarily in terrestrial habitats, in contrast with the related green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...
that are primarily aquatic. The embryophytes include tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...
s, flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
s, fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s, moss
Moss
Mosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es, and various other green land plants. All are complex multicellular eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s with specialized reproductive organs. With very few exceptions, embryophytes obtain their energy through photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
(that is, by absorbing light); and they synthesize their food from carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
.
Description
The evolutionary origins of the embryophytes are discussed further below, but they are believed to have evolved from within a group of complex green algae during the PaleozoicPaleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era (which started around ). The Charales
Charales
Charales is an order of pondweeds, freshwater algae in the division Charophyta. They are green plants believed to be the closest relatives of the green land plants. Linnaeus established the genus Chara in 1753.-Description:...
or stoneworts appear to be the best living illustration of that developmental step. Embryophytes are primarily adapted for life on land, although some are secondarily aquatic
Aquatic plant
Aquatic plants are plants that have adapted to living in aquatic environments. They are also referred to as hydrophytes or aquatic macrophytes. These plants require special adaptations for living submerged in water, or at the water's surface. Aquatic plants can only grow in water or in soil that is...
. Accordingly, they are often called land plants or terrestrial plants.
On a microscopic level, the cells of embryophytes remain very similar to those of green algae. They are eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
, with a cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
composed of cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
and plastid
Plastid
Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s surrounded by two membranes. The latter usually take the form of chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s, which conduct photosynthesis and store food in the form of starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
, and are characteristically pigmented with chlorophylls a and b, generally giving them a bright green color. Embryophyte cells also generally have an enlarged central vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
or tonoplast, which maintains cell turgor and keeps the plant rigid. They lack flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
and centriole
Centriole
A Centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though it is absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
s except in certain gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s.
Embryophytes have three features related to their reproduction which collectively distinguish them from all other plant lineages. Firstly, they have a life cycle which involves 'alternation of generations
Alternation of generations
Alternation of generations is a term primarily used in describing the life cycle of plants . A multicellular sporophyte, which is diploid with 2N paired chromosomes , alternates with a multicellular gametophyte, which is haploid with N unpaired chromosomes...
'. A multicellular generation with a single set of chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s – the haploid gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
– produces sperm and eggs which fuse and grow into a multicellular generation with twice the number of chromosomes – the diploid sporophyte
Sporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
. The mature sporophyte produces spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s which grow into a gametophyte, thus completing the cycle. Secondly, their gametophytes produce sperm and eggs in multicellular structures (called 'antheridia
Antheridium
An antheridium or antherida is a haploid structure or organ producing and containing male gametes . It is present in the gametophyte phase of lower plants like mosses and ferns, and also in the primitive vascular psilotophytes...
' and 'archegonia
Archegonium
An archegonium , from the ancient Greek ἀρχή and γόνος , is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamete. The archegonium has a long neck canal and a swollen base...
' respectively) containing sterile tissue (i.e. non-reproducing tissue). Thirdly, the fertilized egg (the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
) initially develops within the archegonium where it is protected and provided with nutrition. This last feature is the origin of the term 'embryophyte' – the fertilized egg develops into a protected embryo, rather than dispersing as a single cell.
Embryophytes also differ from algae by having metamers
Metamerism (biology)
In biology, metamerism is a linear series of body segments fundamentally similar in structure, though not all such structures are entirely alike in any single life form because some of them perform special functions....
. Metamers are repeated units of development, in which each unit derives from a single cell, but the resulting product tissue or part is largely the same for each cell. The whole organism is thus constructed from similar, repeating parts or metamers. Accordingly, these plants are sometimes termed 'metaphytes' and classified as the group Metaphyta.
Phylogeny and classification
All green algae and land plants are now known to form a single evolutionary lineage or cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, one name for which is Viridiplantae (i.e. 'green plants'). According to several molecular clock estimates the Viridiplantae split to into two clades: chlorophytes and streptophytes. The chlorophytes are considerably more diverse (with around 700 genera) and were originally marine, although some groups have since spread into fresh water. The streptophyte algae (i.e. the streptophyte clade minus the land plants) are less diverse (with around 122 genera) and adapted to fresh water very early in their evolutionary history. They have not spread into marine environments (only a few stoneworts, which belong to this group, tolerate brackish water). Some time during the Ordovician period (which started around ) one or more streptophytes invaded the land and began the evolution of the embryophyte land plants.
Becker and Marin speculate that land plants evolved from streptophytes rather than any other group of algae because streptophytes were adapted to living in fresh water. This prepared them to tolerate a range of environmental conditions found on land. Fresh water living made them tolerant of exposure to rain; living in shallow pools required tolerance to temperature variation, high levels of ultra-violet light and seasonal dehydration.
Relationships between the groups making up Viridiplantae are still being elucidated; views have changed considerably since 2000 and classifications have not yet caught up. However, the division between chlorophytes and streptophytes and the evolution of embryophytes from within the latter group, as shown in the cladogram below, are well established. Three approaches to classification are shown. Older classifications, as on the left, treated all green algae as a single division of the plant kingdom under the name Chlorophyta. Land plants were then placed in separate divisions. All the streptophyte algae can be grouped into one paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
taxon, as in the middle, allowing the embryophytes to form a taxon at the same level. Alternatively, the embryophytes can be sunk into a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
taxon comprising all the streptophytes, as on the right. A variety of names have been used for the different groups which result from these approaches; those used below are only one of a number of possibilities. The higher-level classification of the Viridiplantae varies considerably, resulting in widely different ranks being assigned to the embryophytes, from kingdom to class.
| {clade|style=line-height:100%; font-size:100%; | label1=Viridiplantae Viridiplantae Viridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional Plant Kingdom to include the green algae... | 1= }} |
Plantae
|
Viridiplantae
|
Viridiplantae
|
The preponderance of currently available molecular evidence suggests that the groups making up the embryophytes are related as shown in the cladogram below (based on Qiu et al. 2006 with additional names from Crane et al. 2004).
Studies based on morphology rather than on genes and proteins have regularly reached different conclusions; for example that neither the monilophytes (ferns and horsetails) nor the gymnosperms are a natural or monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
group.
There is considerable variation in how these relationships are converted into a formal classification. Consider the angiosperms or flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s. Many botanists, following Lindley in 1830, have treated the angiosperms as a division. Researchers concerned with fossil plants have usually followed Banks in treating the tracheophytes or vascular plant
Vascular plant
Vascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the clubmosses, Equisetum, ferns, gymnosperms and angiosperms...
s as a division, so that the angiosperms become a class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
or even a subclass. Two very different systems are shown below. The classification on the left is a traditional one, in which ten living groups are treated as separate divisions; the classification on the right (based on Kenrick and Crane's 1997 treatment) sharply reduces the rank
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
of groups such as the flowering plants. (More complex classifications are needed if extinct plants are included.)
| Liverworts | Marchiantiophyta | Marchiantiophyta |
| Mosses | Bryophyta | Bryophyta |
| Hornworts | Anthocerotophyta | Anthocerotophyta |
| Tracheophyta | ||
| Lycophytes | Lycopodiophyta | Lycophytina |
| Euphyllophytina | ||
| Ferns and horsetails | Pteridophyta | Moniliformopses |
| Radiatopses | ||
| Cycads | Cycadophyta | Cycadatae |
| Conifers | Pinophyta | Coniferophytatae |
| Ginkgo | Ginkgophyta | Ginkgoatae |
| Gnetophytes | Gnetophyta | Anthophytatae |
| Flowering plants | Magnoliophyta |
Bryophytes
BryophyteBryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...
s consist of all nonvascular land plants (embryophytes without vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
). All are relatively small and are usually confined to environments that are humid or at least seasonally moist. They are limited by their reliance on water needed to disperse their gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s, although only a few bryophytes are truly aquatic. Most species are tropical, but there are many arctic species as well. They may locally dominate the ground cover in tundra or the epiphyte flora in rain forest habitats.
The three living divisions are the mosses (Bryophyta), hornworts (Anthocerotophyta), and liverworts (Marchantiophyta). Originally, these three groups were included together as classes within the single division Bryophyta. However, they now are placed separately into three divisions since the bryophytes as a whole are known to be a paraphyletic (artificial) group instead of a single lineage. Instead, the three bryophyte groups form an evolutionary grade of those land plants that are not vascular. Some closely related green algae are also non-vascular, but are not considered "land plants."
- MarchantiophytaMarchantiophytaThe Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....
(liverworts) - BryophytaMossMosses are small, soft plants that are typically 1–10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
(mosses) - AnthocerotophytaHornwortHornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...
(hornworts)
Despite the fact that they are no longer classified as a single group, the bryophytes are still studied together because of their many biological similarities as non-vascular land plants. All three bryophyte groups share a haploid-dominant life cycle and unbranched sporophytes, These are traits that appear to be plesiotypic within the land plants, and thus were common to all early diverging lineages of plants on the land. The fact that the bryophytes have a life cycle in common is thus an artefact of being the oldest extant lineages of land plant, and not the result of close shared ancestry. (See the phylogeny above.)
The bryophyte life-cycle is strongly dominated by the haploid gametophyte generation. The sporophyte remains small and dependent on the parent gametophyte for its entire brief life. All other living groups of land plants have a life cycle dominated by the diploid sporophyte generation. It is in the diploid sporophyte that vascular tissue develops. Although some mosses have quite complex water-conducting vessels, bryophytes lack true vascular tissue.
Like the vascular plants, bryophytes do have differentiated stems, and although these are most often no more than a few centimeters tall, they do provide mechanical support. Most bryophytes also have leaves, although these typically are one cell thick and lack veins. Unlike the vascular plants, bryophytes lack true roots or any deep anchoring structures. Some species do grow a filamentous network of horizontal stems, but these have a primary function of mechanical attachment rather than extraction of soil nutrients (Palaeos 2008).
Rise of vascular plants
During the SilurianSilurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
and Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
periods (around ), plants evolved which possessed true vascular tissue, including cells with walls strengthened by lignin (tracheid
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
s). Some extinct early plants appear to be between the grade of organization bryophytes and that of true vascular plants (eutracheophytes). Genera such as Horneophyton
Horneophyton
Horneophyton, a member of the Horneophytopsida, was an early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert.-Description:...
have water-conducting tissue more like that of mosses, but a different life-cycle in which the sporophyte is more developed than the gametophyte. Genera such as Rhynia
Rhynia
Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes, and was basal to modern vascular plants or eutracheophytes.-Description:R...
have a similar life-cycle but have simple tracheids and so are a kind of vascular plant.
During the Devonian period, vascular plants diversified and spread to many different land environments. In addition to vascular tissues which transport water throughout the body, tracheophytes have an outer layer or cuticle that resists drying out
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
. The sporophyte is the dominant generation, and in modern species develops leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
, stems
Plant stem
A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence , conifer cones, roots, other stems etc. The internodes distance one node from another...
and root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s, while the gametophyte remains very small.
Lycophytes and euphyllophytes
All the vascular plants which disperse through spores were once thought to be related (and were often grouped as 'ferns and allies'). However, recent research suggests that leaves evolved quite separately in two different lineages. The lycophytes or lycopodiophytes – modern clubmosses, spikemosses and quillworts – make up less than 1% of living vascular plants. They have small leaves, often called 'microphylls' or 'lycophylls', which are borne all along the stems in the clubmosses and spikemosses, and which effectively grow from the base, via an intercalary meristemMeristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....
. It is believed that microphylls evolved from outgrowths on stems, such as spines, which later acquired veins (vascular traces).
Although the living lycophytes are all relatively small and inconspicuous plants, more common in the moist tropics than in temperate regions, during the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period tree-like lycophytes (such as Lepidodendron
Lepidodendron
Lepidodendron is an extinct genus of primitive, vascular, arborescent plant related to the Lycopsids . It was part of the coal forest flora. They sometimes reached heights of over , and the trunks were often over in diameter, and thrived during the Carboniferous period...
) formed huge forests that dominated the landscape.
The euphyllophytes, making up more than 99% of living vascular plant species, have large 'true' leaves (megaphylls), which effectively grow from the sides or the apex, via marginal or apical meristems. One theory is that megaphylls developed from three-dimensional branching systems by first 'planation' – flattening to produce a two dimensional branched structure – and then 'webbing' – tissue growing out between the flattened branches. Others have questioned whether megaphylls developed in the same way in different groups.
Ferns and horsetails
Ferns are a large and diverse group, with some 12,000 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
. A stereotypical fern has broad, much divided leaves, which grow by unrolling.
Seed plants
Seed plants, which first appeared in the fossil record towards the end of the Paleozoic era, reproduce using desiccationDesiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
-resistant capsules called seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s. Starting from a plant which disperses by spores, highly complex changes are needed to produce seeds. The sporophyte has two kinds of spore-forming organs (sporangia). One kind, the megasporangium, produces only a single large spore (a megaspore). This sporangium is surrounded by one or more sheathing layers (integuments) which form the seed coat. Within the seed coat, the megaspore develops into a tiny gametophyte, which in turn produces one or more egg cells. Before fertilization, the sporangium and its contents plus its coat is called an 'ovule'; after fertilization a 'seed'. In parallel to these developments, the other kind of sporangium, the microsporangium, produces microspores. A tiny gametophyte develops inside the wall of a microspore, producing a pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
grain. Pollen grains are physically transferred between plants by the wind or by insects. When a pollen grain reaches an ovule, it enters via a microscopic gap in the coat (the micropyle). The tiny gametophyte inside the pollen grain then produces sperm cells which move to the egg cell and fertilize it.
Seed plants include two groups with living members, the gymnosperms and the angiosperms or flowering plants. In gymnosperms, the ovules or seeds are not further enclosed. In angiosperms, they are enclosed in ovaries. A split ovary with a visible seed can be seen in the image to the right. Angiosperms typically also have other, secondary structures, such as petals, which together form a flower.
Extant seed plants are divided into five groups:
- Gymnosperms
- PinophytaPinophytaThe conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority being...
- conifers - Cycadophyta - cycads
- Ginkgophyta - ginkgo
- GnetophytaGnetophytaThe plant division Gnetophyta or gnetophytes consists of three genera of woody plants grouped in the gymnosperms. The living Gnetophyta consists of around 70 species across the three genera Gnetum , Welwitschia , and Ephedra .The gnetophytes differ from other gymnosperms The plant division...
- gnetophytes
- Pinophyta
- Angiosperms
- MagnoliophytaFlowering plantThe flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
– flowering plants
- Magnoliophyta