Plants are the eukaryotic organisms that constitute the kingdom Plantae (/'plænteɪ/). They are predominantly photosynthetic or came from photosynthetic ancestors, meaning that they obtain their energy from sunlight by using the green pigment chlorophyll in their chloroplasts to produce sugars from carbon dioxide and water. Over the course of time, some plants have evolved to be parasitic, meaning they take energy from other organisms. Except for glaucophytes, some green algae and a few red algae, most plants are multicellular. 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 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. 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 are herbivores or rely on other organisms. Grain, fruit, and vegetables are basic human foods and have been domesticated for millennia. People use plants for many purposes, such as building materials, such as wood, ornaments, writing materials, and, in great variety, for medicines. The scientific study of plants is known as botany.
Definition
Taxonomic history
All living things were traditionally placed into one of two groups, plants and animals. This classification dates from Aristotle (384–322 BC), who distinguished different levels of beings in his biology, based on whether living things had a "sensitive soul" or like plants only a "vegetative soul". Theophrastus, Aristotle's student, continued his work in plant taxonomy and classification. Much later, Carl Linnaeus (1707–1778) created the basis of the modern system of scientific classification, but retained the animal and plant kingdoms, naming the plant kingdom the Vegetabilia.
Alternative concepts When the name Plantae or plant is applied to a specific group of organisms or taxa, it usually refers to one of four concepts. From least to most inclusive, these four groupings are:
Evolution
Diversity
There are about 382,000 accepted species of plants, of which the great majority, some 283,000, produce seeds. The table below shows some species count estimates of different green plant (Viridiplantae) divisions. About 85–90% of all plant species are flowering plants. Several projects are currently attempting to collect records on all plant taxa in online databases, e.g. the World Flora Online. Plants range in scale from single-celled organisms such as desmids (from 10 micrometres (μm) across) and picozoa (less than 3 μm across), to the largest trees (megaflora) such as the conifer Sequoia sempervirens (up to 120 metres (380 ft) tall) and the angiosperm Eucalyptus regnans (up to 100 m (325 ft) tall).
The naming of plants is governed by the International Code of Nomenclature for algae, fungi, and plants and the International Code of Nomenclature for Cultivated Plants.
Evolutionary history
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. However, fossils of organisms with a flattened thallus in Precambrian rocks suggest that multicellular freshwater eukaryotes existed over 1000 million years ago. 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. 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. 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. The Carboniferous period of Paleozoic 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. The Permo-Triassic extinction event radically changed the structures of communities. This may have enabled the evolution of flowering plants in the Triassic (~200 million years ago), with an adaptive radiation in the Cretaceous so rapid that Charles Darwin called it "an abominable mystery". Conifers diversified from the Late Triassic onwards, and became a dominant part of floras in the Jurassic.
Phylogeny In 2019, a phylogeny based on genomes and transcriptomes from 1,153 plant species was proposed. 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. The classification of Bryophyta is supported both by Puttick et al. 2018, and by phylogenies involving the hornwort genomes that have also since been sequenced.
Physiology
Plant cells
Plant cells have distinctive features that other eukaryotic cells (such as those of animals) lack. These include the large water-filled central vacuole, chloroplasts, and the strong flexible cell wall, which is outside the cell membrane. Chloroplasts are derived from what was once a symbiosis of a non-photosynthetic cell and photosynthetic cyanobacteria. The cell wall, made mostly of cellulose, allows plant cells to swell up with water without bursting. The vacuole allows the cell to change in size while the amount of cytoplasm stays the same.
Plant structure
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