Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars from carbon dioxide and water, using the green pigment chlorophyll. Exceptions are parasitic plants that have lost the genes for chlorophyll and photosynthesis, and obtain their energy from other plants or fungi.
Historically, as in Aristotle's biology, the plant kingdom encompassed all living things that were not animals, and included algae and fungi. Definitions have narrowed since then; current definitions exclude the fungi and some of the algae. By the definition used in this article, plants form the clade Viridiplantae (green plants), which consists of the green algae and the embryophytes or land plants (hornworts, liverworts, mosses, lycophytes, ferns, conifers and other gymnosperms, and flowering plants). A definition based on genomes includes the Viridiplantae, along with the red algae and the glaucophytes, in the clade Archaeplastida.
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There are about 380,000 known species of plants, of which the majority, some 260,000, produce seeds. They range in size from single cells to the tallest trees. Green plants provide a substantial proportion of the world's molecular oxygen; the sugars they create supply the energy for most of Earth's ecosystems and other organisms, including animals, either consume plants directly or rely on organisms which do so.
Grain, fruit, and vegetables are basic human foods and have been domesticated for millennia. People use plants for many purposes, such as building materials, ornaments, writing materials, and, in great variety, for medicines. The scientific study of plants is known as botany, a branch of biology.
The ancestors of land plants evolved in water. An algal scum formed on the land 1,200 million years ago, but it was not until the Ordovician, around 450 million years ago, that the first land plants appeared, with a level of organisation like that of bryophytes.[34][35] However, fossils of organisms with a flattened thallus in Precambrian rocks suggest that multicellular freshwater eukaryotes existed over 1000 mya.[36]
Primitive land plants began to diversify in the late Silurian, around 420 million years ago. Bryophytes, club mosses, and ferns then appear in the fossil record.[37] Early plant anatomy is preserved in cellular detail in an early Devonian fossil assemblage from the Rhynie chert. These early plants were preserved by being petrified in chert formed in silica-rich volcanic hot springs.[38]
By the end of the Devonian, most of the basic features of plants today were present, including roots, leaves and secondary wood in trees such as Archaeopteris.[39][40] The Carboniferous Period saw the development of forests in swampy environments dominated by clubmosses and horsetails, including some as large as trees, and the appearance of early gymnosperms, the first seed plants.[41] The Permo-Triassic extinction event radically changed the structures of communities.[42] This may have set the scene for the evolution of flowering plants in the Triassic (~200 million years ago), with an adaptive radiation in the Cretaceous so rapid that Darwin called it an "abominable mystery".[43][44][45] Conifers diversified from the Late Triassic onwards, and became a dominant part of floras in the Jurassic.[46][47]
In 2019, a phylogeny based on genomes and transcriptomes from 1,153 plant species was proposed.[48] The placing of algal groups is supported by phylogenies based on genomes from the Mesostigmatophyceae and Chlorokybophyceae that have since been sequenced. Both the "chlorophyte algae" and the "streptophyte algae" are treated as paraphyletic (vertical bars beside phylogenetic tree diagram) in this analysis, as the land plants arose from within those groups.[49][50] The classification of Bryophyta is supported both by Puttick et al. 2018,[51] and by phylogenies involving the hornwort genomes that have also since been sequenced.[52][53]
Most plants are multicellular. Plant cells differentiate into multiple cell types, forming tissues such as the vascular tissue with specialized xylem and phloem of leaf veins and stems, and organs with different physiological functions such as roots to absorb water and minerals, stems for support and to transport water and synthesized molecules, leaves for photosynthesis, and flowers for reproduction.[55]
This causes plants to release oxygen into the atmosphere. Green plants provide a substantial proportion of the world's molecular oxygen, alongside the contributions from photosynthetic algae and cyanobacteria.[57][58][59]
Frost and dehydration can damage or kill plants. Some plants have antifreeze proteins, heat-shock proteins and sugars in their cytoplasm that enable them to tolerate these stresses.[64] Plants are continuously exposed to a range of physical and biotic stresses which cause DNA damage, but they can tolerate and repair much of this damage.[65]
When reproducing sexually, plants have complex lifecycles involving alternation of generations. One generation, the sporophyte, which is diploid (with 2 sets of chromosomes), gives rise to the next generation, the gametophyte, which is haploid (with one set of chromosomes). Some plants also reproduce asexually via spores. In some non-flowering plants such as mosses, the sexual gametophyte forms most of the visible plant.[67] In seed plants (gymnosperms and flowering plants), the sporophyte forms most of the visible plant, and the gametophyte is very small. Flowering plants reproduce sexually using flowers, which contain male and female parts: these may be within the same (hermaphrodite) flower, on different flowers on the same plant, or on different plants. The pollen produces male gametes that enter the ovule to fertilize the egg cell of the female gametophyte. Fertilization takes place within the carpels or ovaries, which develop into fruits that contain seeds. Fruits may be dispersed whole, or they may split open and the seeds dispersed individually.[68]
Plants reproduce asexually by growing any of a wide variety of structures capable of growing into new plants. At the simplest, plants such as mosses or liverworts may be broken into pieces, each of which may regrow into whole plants. The propagation of flowering plants by cuttings is a similar process. Structures such as runners enable plants to grow to cover an area, forming a clone. Many plants grow food storage structures such as tubers or bulbs which may each develop into a new plant.[69]
Plants are distributed almost worldwide. While they inhabit several biomes which can be divided into a multitude of ecoregions,[79] only the hardy plants of the Antarctic flora, consisting of algae, mosses, liverworts, lichens, and just two flowering plants, have adapted to the prevailing conditions on that southern continent.[80]
Plants are often the dominant physical and structural component of the habitats where they occur. Many of the Earth's biomes are named for the type of vegetation because plants are the dominant organisms in those biomes, such as grassland, savanna, and tropical rainforest.[81]
The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis, at first by cyanobacteria and later by photosynthetic eukaryotes, radically changed the composition of the early Earth's anoxic atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems.[82] Plants form about 80% of the world biomass at about 450 gigatonnes (4.41011 long tons; 5.01011 short tons) of carbon.[83]
Numerous animals have coevolved with plants; flowering plants have evolved pollination syndromes, suites of flower traits that favour their reproduction. Many, including insect and bird partners, are pollinators, visiting flowers and accidentally transferring pollen in exchange for food in the form of pollen or nectar.[84]
Many animals disperse seeds that are adapted for such dispersal. Various mechanisms of dispersal have evolved. Some fruits offer nutritious outer layers attractive to animals, while the seeds are adapted to survive the passage through the animal's gut; others have hooks that enable them to attach to a mammal's fur.[85] Myrmecophytes are plants that have coevolved with ants. The plant provides a home, and sometimes food, for the ants. In exchange, the ants defend the plant from herbivores and sometimes competing plants. Ant wastes serve as organic fertilizer.[86]
The majority of plant species have fungi associated with their root systems in a mutualistic symbiosis known as mycorrhiza. The fungi help the plants gain water and mineral nutrients from the soil, while the plant gives the fungi carbohydrates manufactured in photosynthesis.[87]Some plants serve as homes for endophytic fungi that protect the plant from herbivores by producing toxins. The fungal endophyte Neotyphodium coenophialum in tall fescue grass has pest status in the American cattle industry.[88]
Many legumes have Rhizobium nitrogen-fixing bacteria in nodules of their roots, which fix nitrogen from the air for the plant to use; in return, the plants supply sugars to the bacteria.[89] Nitrogen fixed in this way can become available to other plants, and is important in agriculture; for example, farmers may grow a crop rotation of a legume such as beans, followed by a cereal such as wheat, to provide cash crops with a reduced input of nitrogen fertilizer.[90]
Some 1% of plants are parasitic. They range from the semi-parasitic mistletoe that merely takes some nutrients from its host, but still has photosynthetic leaves, to the fully-parasitic broomrape and toothwort that acquire all their nutrients through connections to the roots of other plants, and so have no chlorophyll. Full parasites can be extremely harmful to their plant hosts.[91] 0852c4b9a8
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