The Embryophytes emerged either a half-billion years ago, at some time in the interval between the mid-Cambrian and earlyOrdovician, or almost a billion years ago, during the Tonian or Cryogenian,[15] probably from freshwatercharophytes, a clade of multicellulargreen algae similar to extantKlebsormidiophyceae.[16][17][18][19] The emergence of the Embryophytes depleted atmospheric CO2 (agreenhouse gas), leading toglobal cooling, and thereby precipitatingglaciations.[20] Embryophytes are primarily adapted for life on land, although some are secondarilyaquatic. Accordingly, they are often called land plants or terrestrial plants.
On a microscopic level, the cells of charophytes are broadly similar to those ofchlorophyte green algae, but differ in that in cell division the daughter nuclei are separated by aphragmoplast.[21] They areeukaryotic, with acell wall composed ofcellulose andplastids surrounded by two membranes. The latter includechloroplasts, which conduct photosynthesis and store food in the form ofstarch, and are characteristically pigmented with chlorophyllsa andb, generally giving them a bright green color. Embryophyte cells also generally have an enlarged centralvacuole enclosed by a vacuolar membrane or tonoplast, which maintains cellturgor and keeps the plant rigid.
In common with all groups of multicellular algae they have a life cycle which involvesalternation of generations. A multicellularhaploid generation with a single set ofchromosomes – thegametophyte – produces sperm and eggs which fuse and grow into adiploid multicellular generation with twice the number of chromosomes – thesporophyte which produces haploidspores at maturity. The spores divide repeatedly bymitosis and grow into a gametophyte, thus completing the cycle. Embryophytes have two features related to their reproductive cycles which distinguish them from all other plant lineages. Firstly, their 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. Secondly, the initial stage of development of the fertilized egg (thezygote) into a diploid multicellular sporophyte, takes place within the archegonium where it is both protected and provided with nutrition. This second feature is the origin of the term 'embryophyte' – the fertilized egg develops into a protected embryo, rather than dispersing as a single cell.[17] In thebryophytes the sporophyte remains dependent on the gametophyte, while in all other embryophytes the sporophyte generation is dominant and capable of independent existence.
Embryophytes also differ from algae by havingmetamers. 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 ormetamers. Accordingly, these plants are sometimes termed 'metaphytes' and classified as the group Metaphyta[22] (butHaeckel's definition of Metaphyta places some algae in this group[23]). In all land plants a disc-like structure called aphragmoplast forms where the cell willdivide, a trait only found in the land plants in thestreptophyte lineage, some species within their relativesColeochaetales,Charales andZygnematales, as well as withinsubaerial species of the algae orderTrentepohliales, and appears to be essential in the adaptation towards a terrestrial life style.[24][25][26][27]
The green algae and land plants form aclade, theViridiplantae. According tomolecular clock estimates, the Viridiplantae split1,200 million years ago to725 million years ago into two clades:chlorophytes andstreptophytes. The chlorophytes, with around 700 genera, were originally marine algae, although some groups have since spread intofresh water. The streptophyte algae (i.e. excluding the land plants) have around 122 genera; they adapted to fresh water very early in their evolutionary history and have not spread back into marine environments.[28][29][30]
Some time during theOrdovician, streptophytes invaded the land and began the evolution of the embryophyte land plants.[31] Present day embryophytes form a clade.[32] Becker and Marin speculate that land plants evolved from streptophytes because living in fresh water poolspre-adapted them to tolerate a range of environmental conditions found on land, such as exposure to rain, tolerance of temperature variation, high levels of ultra-violet light, and seasonal dehydration.[33]
The preponderance of molecular evidence as of 2006 suggested that the groups making up the embryophytes are related as shown in the cladogram below (based on Qiuet al. 2006 with additional names from Craneet al. 2004).[34][35]
An updated phylogeny of Embryophytes based on the work by Novíkov & Barabaš-Krasni 2015[36] and Hao and Xue 2013[37] with plant taxon authors from Anderson, Anderson & Cleal 2007[38] and some additional clade names.[39] Puttick et al./Nishiyama et al. are used for the basal clades.[13][40][41]
Bryophytes, such as these mosses, produce unbranched, stalked sporophytes from which their spores are released.
The non-vascular land plants, namely themosses (Bryophyta),hornworts (Anthocerotophyta), andliverworts (Marchantiophyta), are relatively small plants, often confined to environments that are humid or at least seasonally moist. They are limited by their reliance on water needed to disperse theirgametes; a few are truly aquatic. Most are tropical, but there are many arctic species. They may locally dominate the ground cover intundra andArctic–alpine habitats or the epiphyte flora in rain forest habitats.
They are usually studied together because of their many similarities. All three groups share ahaploid-dominant (gametophyte) life cycle and unbranchedsporophytes (the plant'sdiploidgeneration). These traits appear to be common to all early diverging lineages of non-vascular plants on the land. Their 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. In some ways, the term "non-vascular" is a misnomer. Some mosses and liverworts do produce a special type of vascular tissue composed of complex water-conducting cells.[42] However, this tissue differs from that of "vascular" plants in that these water-conducting cells are not lignified.[43] It is unlikely that the water-conducting cells in mosses are homologous with the vascular tissue in "vascular" plants.[42]
Like the vascular plants, they have differentiated stems, and although these are most often no more than a few centimeters tall, they provide mechanical support. Most have leaves, although these typically are one cell thick and lack veins. They lack true roots or any deep anchoring structures. Some species grow a filamentous network of horizontal stems, but these have a primary function of mechanical attachment rather than extraction of soil nutrients (Palaeos 2008).
During theSilurian andDevonian periods (around440 to 360 million years ago), plants evolved which possessed true vascular tissue, including cells with walls strengthened by lignin (tracheids). Some extinct early plants appear to be between the grade of organization of bryophytes and that of true vascular plants (eutracheophytes). Genera such asHorneophyton have water-conducting tissue more like that of mosses, but a different life-cycle in which the sporophyte is branched and more developed than the gametophyte. Genera such asRhynia have a similar life-cycle but have simple tracheids and so are a kind of vascular plant.[44] It was assumed that the gametophyte dominant phase seen in bryophytes used to be the ancestral condition in terrestrial plants, and that the sporophyte dominant stage in vascular plants was a derived trait. However, the gametophyte and sporophyte stages were probably equally independent from each other, and that the mosses and vascular plants in that case are both derived, and have evolved in opposite directions.[45]
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 resistsdrying out. The sporophyte is the dominant generation, and in modern species developsleaves,stems androots, while the gametophyte remains very small.
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 intercalarymeristem.[46] It is believed that microphylls evolved from outgrowths on stems, such as spines, which later acquired veins (vascular traces).[47]
Although the living lycophytes are all relatively small and inconspicuous plants, more common in the moist tropics than in temperate regions, during theCarboniferous period tree-like lycophytes (such asLepidodendron) formed huge forests that dominated the landscape.[48]
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.[46] One theory is that megaphylls evolved 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.[49] Others have questioned whether megaphylls evolved in the same way in different groups.[50]
The ferns and horsetails (the Polypodiophyta) form a clade; they use spores as their main method of dispersal. Traditionally, whisk ferns and horsetails were historically treated as distinct from 'true' ferns.[51] Living whisk ferns and horsetails do not have the large leaves (megaphylls) which would be expected of euphyllophytes. This has probably resulted from reduction, as evidenced by early fossil horsetails, in which the leaves are broad with branching veins.[52]
Ferns are a large and diverse group, with some 12,000species.[53] A stereotypical fern has broad, much divided leaves, which grow by unrolling.
Seed plants, which first appeared in the fossil record towards the end of thePaleozoic era, reproduce usingdesiccation-resistant capsules calledseeds. 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 or sporangia. One kind, the megasporangium, produces only a single large spore, a megaspore. This sporangium is surrounded by sheathing layers or 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 apollen grain. Pollen grains can be physically transferred between plants by thewind or animals, most commonlyinsects. Pollen grains can also transfer to an ovule of the same plant, either with the same flower or between two flowers of the same plant (self-fertilization). 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.[54] Seed plants include two clades with living members, thegymnosperms and theangiosperms or flowering plants. In gymnosperms, the ovules or seeds are not further enclosed. In angiosperms, they are enclosed within the carpel. Angiosperms typically also have other, secondary structures, such aspetals, which together form aflower.
Meiosis in sexual land plants provides a direct mechanism forrepairing DNA in reproductive tissues.[55]Sexual reproduction appears to be needed for maintaining long-termgenomic integrity and only infrequent combinations of extrinsic and intrinsic factors allow for shifts to asexuality.[55]
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