Embryophyte

Embryophyte or ground plants are the most familiar group of green plants that form vegetation on Earth. Embryophyte is a clade within Phragmoplastophyta , a larger clade that also includes many groups including green algae ( Charophyceae and Coleochaetales ), and within this larger clade embryophyte ( m b r i f aɪ t s / ) are sister to Zygnematophyceae / Mesotaeniaceae and consist of bryophytes plus polysporangiophytes . [12] Living embryophyte therefore include hornworts , liverworts , mosses , ferns , lycophytes , gymnosperms and flowering plants .

Embryophyte are informally called land plants because they live mainly in terrestrial habitats, while related green algae are mainly aquatic. Embryophyte are complex multicellular eukaryotes with specialized reproductive organs. The name derives from their novel feature of nurturing the young embryonic sporophyte during the early stages of its multicellular development within the tissues of the parent gametophyte. With very few exceptions, embryophyte obtain their energy by photosynthesis, that is, using the energy of sunlight to synthesize their food from carbon dioxide and water.

Description

Moss, clubmoss, ferns and cycads in a greenhouse

The evolutionary origins of Embryophyte are discussed further below, but they are believed to have evolved from within a group of complex green algae during the Paleozoic Era (which began about 540  million years ago ) ^{[13] [14 ] ]} probably from terrestrial unicellular caryophytes. , similar to the extant Klebsormidiophyceae. ^[15] Embryophyte are mainly adapted to life on land, although some are secondary aquatic. Accordingly, they are often called land plants or terrestrial plants .

At the microscopic level, the cells of embryophyte are broadly similar to those of green algae, but differ in that in cell division the daughter nuclei are separated by a phragmoplast. ^[16] They are eukaryotic, having a cell wall composed of cellulose and plastids surrounded by two membranes. The latter include chloroplasts, which perform photosynthesis and store food in the form of starch, and are specifically pigmented with chlorophyll a and b , which typically give them a bright green color. Embryonic cells also typically have an enlarged central vacuole surrounded by a vacuolar membrane or tonoplast, which maintains cell turgor and keeps the plant rigid.

In common with all groups of multicellular algae, they have a life cycle that includes ‘alternation of generations’. A multicellular generation with one set of chromosomes – the haploid gametophyte – produces sperm and eggs that combine into a multicellular generation and have twice the number of chromosomes – the diploid sporophyte. The mature sporophyte produces haploid spores which develop into a gametophyte, thus completing the cycle. Embryophyte have two characteristics related to their reproductive cycle that differentiate them from all other plant lineages. First, its gametophytes produce sperm and eggs in multicellular structures (called ‘antheridia’ and ‘archegonia’), and fertilization of the ovum takes place within the archegonium rather than in the external environment. Others, And most importantly, the early stage of development in the diploid multicellular sporophyte of the fertilized egg (zygote), occurs within the archegonium where it both protects and provides nutrition. This second feature is the origin of the term ’embryophyte’ – the fertilized egg develops into a protected embryo rather than spreading as a single cell.[13] In bryophytes the sporophyte is dependent on the gametophyte, whereas in all other embryophyte the sporophyte generation is dominant and capable of independent existence.

Embryophyte also differ from algae by having metamers. Metamers are repeating units of development, in which each unit originates from a single cell, but the resulting product tissue or part is largely identical for each cell. Thus the whole organism is made up of similar, repeating parts or metamers . Accordingly, these plants are sometimes referred to as ‘metaphytes’ and classified as the group Metaphyta [17] (but Haeckel’s definition of metaphyta belongs to this group [18]).has some algae in it). All land plants form a disc-like structure called a phragmoplast where the cell will divide, a feature found only in land plants in the streptophyte lineage, some species within their relatives Coleochaetales, Charales and Zygnematales, as well as subaerial species within the algae order Trentpohalies , and appears to be essential in adaptation to a terrestrial lifestyle.

Phylogeny, Evolutionary History and Taxonomy

All green algae and land plants are now known to form a single evolutionary lineage or clade, a name for the Viridiplantae (i.e. ‘green plants’). According to several molecular clock estimates, the Viridiplantae split from 1,200 million years ago to 725 million years ago into two divisions : the chlorophytes and the streptophytes. Chlorophytes are significantly more diverse (with about 700 genera) and were marine in origin, although some groups have since spread to fresh water. Streptophyte algae (i.e. the streptophyte clade minus the land plants) are less diverse (with about 122 genera) and adapted to fresh water much earlier in their evolutionary history. They are not widespread in marine environments (only some stones, which belong to this group, tolerate salt water). Ordovician period (about490 million years ago ) one or more streptophytes invaded the land and began the evolution of Embryophyte land plants. [23] Today, the embryophyte form a monophyletic group known as the hemitracheophytes. [24]

Baker and Marin hypothesized that land plants evolved from streptophytes rather than from any other group of algae because streptophytes were adapted to live in fresh water. This prepared them to endure the many environmental conditions found on land. Living in fresh water made them tolerant of rain; Living in shallow pools requires tolerance to temperature variation, high levels of ultra-violet light, and seasonal dehydration. [25]

The relationships between the groups that make up the Viridiplantae are still being elucidated. Ideas have changed significantly since 2000 and the classification has not yet caught on. However, the division between chlorophytes and streptophytes and the evolution of embryophyte from within the latter group, as shown in the cladogram below, are well established. [23] [26] Three approaches to classification are shown. The older classification, as on the left, treated all green algae as a single division of the plant kingdom called Chlorophyta. [27] The land plants were then placed in separate divisions. All streptophyte algae can be grouped into a paraphyletic taxon, as in the middle, allowing embryophyte to form a taxon on a similar level. [ citation needed] Alternatively, embryophytes may be subdivided into a monophyletic taxon that includes all streptophytes, as shown below. [26] These approaches have resulted in the use of different names for different groups; Those used below are only one of many possibilities. The high-level classification of Viridiplantae varies greatly, resulting in widely varying ranks assigned to embryos, from kingdom to class.

As of March 2012 the exact relationships within streptophytes are less clear . Stoneworts (Charales) have traditionally been identified as closest to the embryophyte, but recent work suggests that a clade consisting of either Zygnematales or Zygnematales and Coleochaetales may be the sister group to land plants. ^[28] [29] Zygnematales (or Zygnematophyceae) are the closest algal relatives to land plants, underscored by a thorough phylogenetic analysis (phylogenomics) performed in 2014, ^[30] which phylogenies both plastid genomes. ^[31] as well as plastid gene content and properties . ^[32]

As of 2006 the preponderance of molecular evidence suggested that the groups making up 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 genes and proteins have routinely reached different conclusions; For example that neither monolophytes (ferns and horsetails) nor gymnosperms are a natural or monophyletic group. ^{[35] [36] [37]}

There is considerable variation in how these relationships are translated into formal classifications. Consider angiosperms or flowering plants. Many botanists who followed Lindley in 1830 have treated angiosperms as a division. ^[38] Paleontologists have generally followed Banks in treating tracheophytes or vascular plants as a division, ^[39] so that angiosperms become a class or a subclass. Two very different systems are shown below. The classification on the left is traditional, with the ten living groups being treated as separate divisions; ^{[ citation needed ]} The classification on the right (based on Kenrick and Crane’s 1997 treatment) sharply lowers the rank of groups such as flowering plants. ^[40](If extinct plants are included a more complex classification is needed.)

An updated phylogeny of embryophyte based on work by Novikov and Barabas-Krasny 2015 ^[41] and Hao and Xue 2013 ^[42] with plant taxon authors from Anderson, Anderson and Kleil 2007 ^[43] and from Pelletier 2012 et al. Some clade names. ^{[44] [ Self-published source? ] [45]} Putik et al./Nishiyama et al used the basal clade.

Variety

Bryophytes

Bryophytes consist of all non-vascular land plants (embryophyte 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 dependence on the water needed to disperse their gametes, although only a few bryophytes are truly aquatic. Most species are tropical, but there are also many arctic species. They may locally dominate land cover in tundra and arctic-alpine habitats or epiphyte vegetation in rain forest habitats.

The three living divisions are mosses (Bryophyta), hornworts (Anthoceratophyta), and liverworts (Marchantiophyta). Originally, these three groups were grouped together as classes within the single division Bryophyta. They are generally separated into three divisions under the assumption that bryophytes are a paraphyletic (more than one lineage) group, but new research supports the monophyletic (having a common ancestor) model. ^[48] ​​The three bryophyte groups form an evolutionary grade of land plants that are not vascular. Some closely related green algae are also non-vascular, but are not considered “land plants”.

• Merchantiophyta (Liverworts)
• Bryophyta (Moss)
• Anthoceratophyta (hornworts)

Despite their evolutionary origin, bryophytes are commonly studied together because of their many biological similarities as non-vascular land plants. All three Bryophyta groups share a shared haploid-dominant (gametophyte) life cycle and unbranched sporophytes (the diploid structure of the plant). These are traits that appear to be plesiotypic within land plants, and thus were common to all early divergent lineages of land plants. The fact that bryophytes have a similar life cycle may thus be an attribute of being the oldest extant lineage of land plant, and not the result of a closely shared lineage. (See phylogeny above.)

Like vascular plants, bryophytes have differentiated stems, and although these are often no more than a few centimeters long, they provide mechanical support. Most bryophytes also have leaves, although they are usually one cell thick and lack veins. Unlike vascular plants, bryophytes lack true roots or any deep anchoring structures. Some species develop a filamentous network of horizontal stems, but their primary function is mechanical attachment rather than extraction of soil nutrients (Palios 2008).

Rise of vascular plants

During the Silurian and Devonian periods (around 440 to 360 million years ago ), plants developed which possessed true vascular tissue, including cells with stronger walls than lignin (tracheids). Some extinct early plants appear to be between the grade of organization of the bryophytes and the true vascular plants (eutracophytes). Genera such as Horniophyton have more water-conducting tissues like moss , but a different life-cycle in which the sporophyte is more developed than the gametophyte. Genera such as Rhinia have a similar life cycle, but are simple tracheids and are therefore a type of vascular plant. [ citation needed ]It was believed that the gametophyte dominant stage seen in bryophytes used to be the ancestral condition in terrestrial plants, and that the sporophyte dominant stage in vascular plants was a derived feature. But research points to the possibility that both the gametophyte and sporophyte stages were equally independent of each other, and in that case both mosses and vascular plants are derived, and evolved in the opposite direction from the other. [49]

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. The sporophyte is the dominant genera, and leaves, stems, and roots develop in modern species, while the gametophyte remains much smaller.

Lycophytes and Euphilophytes

All vascular plants that propagate via spores were once thought to be related (and often classified as ‘ferns and allies’). However, recent research suggests that leaves evolved separately in the two different lineages. Lycophytes or lycopodiophytes – modern clubmosses, spikemosses and quillworts – make up less than 1% of living vascular plants. They have small leaves, often called ‘microfoils’ or ‘lycophylls’, which are borne along the stems in clubmoss and spikemoss, and which grow effectively from the base, through an interstitial meristem. [50] It is thought that microfoils develop by spreading on stems such as the spinal cord, which then acquire nerves (vascular scars). [51]

Although living lycophytes are all relatively small and inconspicuous plants, more common in the humid tropics than in temperate regions, during the Carboniferous period tree-like lycophytes (such as Lepidodendron ) formed the vast forests that dominated the landscape. [52]

Euphilophytes, which make up more than 99% of living vascular plant species, have large ‘true’ leaves (megaphylls), which grow effectively from the edges or apex, through the marginal or apical meristem. [50] One theory is that megafills evolved from three-dimensional branching systems by first ‘planation’ – flattening to form a two-dimensional branching structure – and then ‘webbing’ – spreading out between flattened branches. Tissues. [53] Others have questioned whether megaphylls have evolved in the same way in different groups.

Fern and Horsetail

Euphilophytes are divided into two lineages: ferns and horsetails (monilophytes) and seed plants (spermatophytes). Like all previous groups, monolophytes continue to use spores as their main method of dispersal. Traditionally, whisk ferns and horsetails were considered distinct from ‘true’ ferns. Recent research suggests that they all belong together, ^[55] although there are differences on the exact classification used. Live whisk ferns and horsetails do not have the large leaves (megaphylls) that would be expected from euphilophytes. However, this probably resulted from a reduction, as evidenced by an early fossil horsetail, in which the leaves are broad with branching veins. ^[56]

Ferns are a large and diverse group, containing about 12,000 species. ^[57] An orthodox fern has broad, highly divided leaves that grow erratic.

Seed plants

Seed plants, which first appeared in the fossil record at the end of the Paleozoic Era, reproduce using aridification-resistant capsules called seeds. Starting with a plant that propagates by spores, producing seeds requires highly complex transformations. The sporophyte has two types of spore-forming organs (sporangia). One type, the megasporangium, produces only one large spore (a megaspore). This spore is surrounded by one or more sheath layers (integuments) which form the seed coat. Within the seed cover, the megaspore develops into a small gametophyte, which in turn produces one or more egg cells. Before fertilization, the sporangium and its contents and its coat are called ‘ovules’; A ‘seed’ after fertilization. In parallel with these developments, other types of sporangium, microsporangium, produce microspores. A small gametophyte develops inside the wall of a microscopic spore, which produces the pollen grain. Pollen grains can be physically transferred between plants by wind or by animals, usually insects. Pollen grains can also transfer to the ovule of the same plant, either with the same flower or between two flowers of the same plant (self-fertilization). When the pollen grain reaches the ovule, it enters through a microscopic gap in the coat (micropyle). The small gametophyte inside the pollen grain then produces sperm cells that travel to the egg cell and fertilize it. Usually insects. Pollen grains can also transfer to the ovule of the same plant, either with the same flower or between two flowers of the same plant (self-fertilization). When the pollen grain reaches the ovule, it enters through a microscopic gap in the coat (micropyle). The small gametophyte inside the pollen grain then produces sperm cells that travel to the egg cell and fertilize it. Usually insects. Pollen grains can also transfer to the ovule of the same plant, either with the same flower or between two flowers of the same plant (self-fertilization). When the pollen grain reaches the ovule, it enters through a microscopic gap in the coat (micropyle). The small gametophyte inside the pollen grain then produces sperm cells that travel to the egg cell and fertilize it.^[58] Seed plants include two groups with living members, gymnosperms and angiosperms, or flowering plants. In gymnosperms, the ovule or seeds are not further attached. In angiosperms, they attach to the ovary. A divided ovary with a visible seed can be seen in the adjacent image. Angiosperms usually also have other, secondary structures, such as petals, that together make up a flower.

Existing seed plants are divided into five groups:gymnosperm

• pinophyta – coniferous
• Cycadophyta – Cycads
• Ginkgophyta – Ginkgo
• Gnetophyta – gnetophytes

angiosperms

• Magnoliophyta – flowering plants