Plant

Plants are the eukaryotic organisms that constitute the kingdom Plantae. They are predominantly photosynthetic, meaning that they obtain their energy from sunlight. They do that using the green pigment chlorophyll in their chloroplasts to produce sugars from carbon dioxide and water. Exceptions are parasitic plants that have lost the genes for chlorophyll and photosynthesis, and obtain their energy from other plants or fungi. Most plants are multicellular, except for some green algae.
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, which consists of the green algae and the embryophytes or land 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 eat 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.

Definition

Taxonomic history

All living things were traditionally placed into one of two groups, plants and animals. This classification dates from Aristotle, 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, Linnaeus 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:

Name	Scope	Organisation	Description
Land plants, also known as Embryophyta	Plantae sensu strictissimo	Multicellular	Plants in the strictest sense include liverworts, hornworts, mosses, and vascular plants, as well as fossil plants similar to these surviving groups.
Green plants, also known as Viridiplantae, Viridiphyta, Chlorobionta or Chloroplastida	Plantae sensu stricto	Some unicellular, some multicellular	Plants in a strict sense include the green algae, and land plants that emerged within them, including stoneworts. The relationships between plant groups are still being worked out, and the names given to them vary considerably. The clade Viridiplantae encompasses a group of organisms that have cellulose in their cell walls, possess chlorophylls a and b and have plastids bound by only two membranes that are capable of photosynthesis and of storing starch. This clade is the main subject of this article.
Archaeplastida, also known as Plastida or Primoplantae	Plantae sensu lato	Some unicellular, some multicellular	Plants in a broad sense comprise the green plants listed above plus the red algae and the glaucophyte algae that store Floridean starch outside the plastids, in the cytoplasm. This clade includes all of the organisms that eons ago acquired their primary chloroplasts directly by engulfing cyanobacteria.
Old definitions of plant	Plantae sensu amplo	Some unicellular, some multicellular	Plants in the widest sense included the unrelated groups of algae, fungi and bacteria on older, obsolete classifications.

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 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 and picozoa, to the largest trees such as the conifer Sequoia sempervirens and the angiosperm Eucalyptus regnans.

Informal group	Division name	Common name	No. of living species
Green algae	Chlorophyta	Green algae	3800–4300
Green algae	Charophyta	Green algae	2800–6000
Bryophytes	Marchantiophyta	Liverworts	6000–8000
Bryophytes	Anthocerotophyta	Hornworts	100–200
Bryophytes	Bryophyta	Mosses	12000
Pteridophytes	Lycopodiophyta	Clubmosses	1200
Pteridophytes	Polypodiophyta	Ferns, whisk ferns & horsetails	11000
Spermatophytes	Cycadophyta	Cycads	160
Spermatophytes	Ginkgophyta	Ginkgo	1
Spermatophytes	Pinophyta	Conifers	630
Spermatophytes	Gnetophyta	Gnetophytes	70
Spermatophytes	Angiospermae	Flowering plants	258650

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, but it was not until the Ordovician, around, 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 mya.
Primitive land plants began to diversify in the late Silurian, around. 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 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 set the scene for the evolution of flowering plants in the Triassic, with an adaptive radiation in the Cretaceous so rapid that 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 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 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

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.

Photosynthesis

Plants photosynthesize, manufacturing food molecules using energy obtained from light. Plant cells contain chlorophylls inside their chloroplasts, which are green pigments that are used to capture light energy. The end-to-end chemical equation for photosynthesis is:
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.
Plants that have secondarily adopted a parasitic lifestyle may lose the genes involved in photosynthesis and the production of chlorophyll.