Plant

Plants are living organisms that belong to the kingdom Plantae. They include organisms, such as trees, herbs, bushes, grass, vines, ferns, mosses, and green algae. The study of plants, known as botany, has identified about 350,000 species of plants that are still alive. As of 2004, about 287,655 species of plants were identified, with 258,650 to be flowering, and about 18,000 to be bryophytes. Green plants, or Viridiplantae obtain most of their energy from sunlight through photosynthesis.

Definition
Aristotle divided every living thing between plants (organisms that don't move) and animals (organisms that are mobile, and catch their food). In Carolus Linnaeus's system, these kingdoms became the Vegetabilia (later known as Metaphyta or Plantae) and Animalia (known also as Metazoa). Since then, it has been clear that plants, as originally defined included several unrelated groups, such as the fungi and algae, which were moved to new kingdoms. These are still often considered plants, in both technical and popular culture.

Current definitions
When the name Plantae or plants is applied to a taxon, it is usually referring to one of these three concepts. From smallest to largest in inclusiveness, these are:

Outside of formal science contexts, the term plant is usually associated with certain traits, such as multicellularity, cellulose, and photosynthesis. Many classification controversies involve organisms that are rarely encountered or of little economic significance, but are crucial in developing an understanding for the evolution of modern flora.

Algae

 * For more information, see Algae

Most algae are no longer classified as being in the plant kingdom. The algae comprise of several groups of organisms that produce energy through photosynthesis, each of which arose from separate non-photosynthetic ancestors. The most conspicuous among all the algae are the seaweed, which are multicellular algae that may resemble water dwelling plants. These are classified among the green, red, and brown algae.

The two groups of green algae are close relatives of the land plants. The first of these groups is the Charophyta, which the land plants may have evolved from. The sister group to the embryophytes and charophytes are the members of the Chlorophyta. The plant kingdom is often times monophyletic when grouping. With some exceptions with the green algae, all forms of plants have cell walls containing cellulose, have chloroplasts which store chlorophyll, and store food in the form of starch. They undergo mitosis without centrioles, and usually have mitochondria with flat cristae.

The chloroplasts of Viridiplantae are surrounded by two membranes, which suggests that they directly originated from endosymbiotic cyanobacteria. This is also true with two other groups of algae, the red algae and Glaucophytes. All three groups are believed to have a common origin, and are placed together in the Archaeplastida clade. In contrast, most other algae have chloroplasts surrounded by atleast 3 membranes. These are not closely related to green algae, probably acquiring the chloroplasts separately from ingested or symbiotic red or green algae.

Fungi

 * For more information, see Fungi

The classification of the fungi has been controversal until quite recently in the study of biology. Carolus Linnaeus originally placed the fungi with the plants. With later developments with microbiology, Ernst Haeckel thought that a third kingdom was needed to classify newly-discovered microorganisms. The introduction the the Protists kingdom brought confusion about whether the fungi be placed in the plant or protist kingdom. Haeckel found it different to decide and it wasn't until 1969 that Robert Whittaker suggested the creation of a Fungi kingdom. Molecular evidence shows that the last common ancestor of fungi was probably was probably more similar to the Animalia kingdom than any other kingdom.

Whittaker's original reclassification was based upon the difference of nutrition between the fungi and plants. Unlike plants, which gain their energy through photosynthesis, fungi gain their energy by breaking down and absorbing nearby materials. In addition, the substructure of multicellular fungi takes the form of strands made of chitin known as hyphae, which may be further subdivided into cells or a syncytium with several eukaryotic nuclei. Fruiting bodies (mushrooms being a common example) are reproductive structures of fungi.

Diversity
About 350,000 species of plants are estimated to exist currently. As of 2004, about 287,655 species have been identified, of which, 258,650 are flowering plants, 16,000 are bryophytes, 11,000 are ferns, and 8,000 are green algae.

The naming of plants is governed by the International Code of Botanical Nomenclature and the International Code of Nomenclature for Cultivated Plants (see cultivated plant taxonomy).

Phylogeny
A proposed phylogenetic tree after Kenrick and Crane, is shown below, with modifications to the Pteridophyta by Smith and other people. The Prasinophyceae may be a paraphyletic basal group to all Viridiplantae.

Embryophytes

 * For more information see Embryophyte

The plants that are most familiar to humans are multicellular land plants known as embryophytes. This includes the vascular plants (plants with full systems of leaves, stems, and roots) and bryophytes.

All of these plants have eukaryotic cells with cellulose in their cell walls and make energy through photosynthesis using light and carbon dioxide. About 300 species of plants do not undergo photosynthesis, and a parasitic, feeding off of the photosynthetic plants. Plants are distinguished from green algae, which represent a form of photosynthetic life similar to the kind modern plants have evolved from, with reproductive organs protected by non-reproductive tissues.

Bryophytes are believed to have first appeared during the early Paleozoic. These can only live where moisture is available for significant periods, though some are desiccation tolerant. Most species of bryophytes remain small throughout their whole life. This involves alternation between two generations, a gametophyte, or haploid stage, and a sporophyte, or a diploid stage. The sporophyte lives a short time, and depends on its parent, the gametophyte.

Vascular plants first appeared during the Silurian period, and have diversified and spread into many different environments by the Devonian. These have several adaptations that allow them to overcome the limits of the bryophytes. Some include a cuticle resistant to desiccation, and vascular tissues which transfer water and nutrients throughout the plant. In most, the sporophyte is a different individual, while the gametophyte remains small.

The first primitive seed plants, the Pteridosperms and Cordaites appeared in the Devonian, and existed through diversified through the Carboniferous, evolving later in the Permian and Triassic periods. In these, the gametophytes are completely reduced, and the sporophytes begin life in an enclosure known as a seed, which develops on the parent plant, and is fertilized by pollen grains. Other vascular plants, such as ferns reproduce using spores and need moisture to develop, some seed plants can reproduce in extremely dry conditions.

Early seed plants, known as gymnosperms have a seed embryo that is not enclosed in a protective structure during pollination, making the pollen land directly on the embryo. Four surviving groups remain widespread now, mainly the conifers, which dominate several biomes. The angiosperms, or flowering plant were the last major group of plants to appear, emerging from the gymnosperms during the Jurassic, and diversifying quickly through the Cretaceous. With the angiosperms, the seed embryo is enclosed, so the pollen has to flow through a tube to penetrate the seed coat. These are predominate in most biomes today.

Fossils

 * See Paleobotany and Evolutionary history of plants



Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber. Fossilized land plants are found in terrestrial, lacustrine, fluvial and near shore sediments. Pollen, spores, and algae are used for dating sedimentary rock sequences. Remains of plants in fossils are not as common as fossilized animal remains, though there are places where there is an abundence of plant fossils.

The earliest fossils obviously belonging to the plant kingdom are green algae from the Cambrian. These resemble calcified multicellular members of the Dasycladales. Earlier Precambrian fossils that resemble single-celled green algae are known, but it is unclear which group of algae these are classified as.

The oldest known embryophyte fossils date back to the Ordovician. These fossils are usually fragmentary. By the Silurian, fossils of whole plants are preserved. From the Devonian, detailed fossils of Rhyniophyta have been found. The Devonian may have also given rise to what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with fronds of a fern, but it produced no seeds.

The Coal measures are a major source of Paleozoic plant fossils, with many groups of plants existing at this time. Spoil heaps of coal mines are the best places to collect; coal is actually the remains of fossilized plants, though structural detail of plants is rarely ever visible on the coal. In Glasgow at Victoria Park, the stumps of Lepidodendron trees are found in their original growth positions.

The fossilized remains of conifer and angiosperm roots, stems, and branches may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Cenozoic. Sequoia and its allies, Magnolia, Quercus (oak), and Arecaceae (palms) are often found.

Petrified wood is common in some areas of the world, and is most frequent in arid or desert areas where it is more readily exposed to erosion. Petrified wood is often times silicified, and the impregnated tissue is often preserved in detail. Fossil forests of petrified wood have been found on all of the 7 continents.

Fossils of seed ferns are widely distributed throughout several continents in the Southern Hemisphere, a fact that supported Alfred Wegener's ideas of the Continental drift theory.

Structure, growth, and development

 * For more information, see Morphology

Most of the solid material in a plant is taken from the atmosphere. Through photosynthesis, most plants use energy from the sunlight to convert carbon dioxide from the atmosphere and water into simple sugars. Parasitic plants, on the other hand, use the resources of its host to grow and make food. These sugars are then used to form the main structural component of a plant. Chlorophyll, a green-colored pigment containing magnesium, is important in the process; it is generally located in leaves and often times in other parts of the plant as well.

Plants usually rely on soil for support and water (in quantitative terms), but also obtain compounds of nitrogen, phosphorus, and other needed elemental nutrients. Epiphyte and lithophytes depend on rainwater or other sources for nutrients, while carnivorous plants require insects as prey for their nutrient requirements. For most plants to grow successfully, they require oxygen in the atmosphere and around their roots for respiration. However, some plants grow underwater, using oxygen dissolved in the surrounding water, and a few specialized vascular plants, such as mangroves, can grow with their roots in anoxic waters.

Factors affecting growth
A plant's genotype affects its growth, for example, some varieties of wheat grow rapidly, maturing within 110 days, while others, in the same environmental conditions, grow slower, maturing in 155 days.

Growth is also determined by environmental factors, such as the amount of sunlight, water, nutrients, and temperature. Any change in the availability of any of these factors may affect the growth of the plant.

Biotic factors can also affect plant growth. Plants often times compete with other plants for needed space, sunlight, water, and nutrients. Plants can sometimes be so crowded, that no single individual produces normal growth. Plant growth can also be affected by grazing animals, suboptimal soil condition, lack of micorrhizal fungi, and attacks caused by plant diseases caused by insects, bacteria, fungi, viruses, and nematodes.

Simple plants like green algae may have short life spans individually, but their populations are usually seasonal. Other plants may be organized by their seasonal growth pattern: annual plants live and reproduce in one growing season, while biennial plants live for two growing seasons, and usually reproduce in the second growing season. Perennial plants, on the other hand, live for many growing seasons, and continue reproducing after they mature. These designations often times are affected by climate and other environmental factors; plants that are annual in alpine and temperate regions can be biennial or perennial in warmer climates. Among vascular plants, perennial plants include both evergreens, which keep their leaves throughout the whole year, and deciduous plants, which lose their leaves or some parts of it. In temperate and boreal environments, deciduous plants generally lose their leaves in winter; many tropical deciduous plants lose their leaves during the dry season.

The growth rates of plants is extremely variable. Some mosses grow 0.001 mm per hour, while most trees grow 0.025-0.250 mm per hour. Some climbing species, such as Pueraria lobata (Kudzu), which don't need to produce thick protective tissues, may grow up to 12.5 mm per hour.

Plants protect themselves from frost and dehydration stress with antifreeze proteins (AFPs), heat-shock proteins (HSPs), and sugars (commonly sucrose). Late Embryogenesis Abundant proteins (LEA) expression is included by stresses, and protects other proteins from aggregation from desiccation and freezing.

Plant cell

 * See Plant cell

Plant cells are typically distinguished by their large water-filled vacuole, chloroplasts, and cell walls made of cellulose, hemicellulose, and pectin. Cell division is also characterized by the creation of a phragmoplast for the construction of a cell plate in the middle of cytokinesis. Just like animal cells, plant cells differentiate and develop into multiple cell types. Totipotent meristematic cells can differentiate into vascular, storage, protective, or reproductive tissues, with some primitive plants lacking some of these tissues.

Physiology

 * See Physiology

Photosynthesis

 * ''See Photosynthesis and Biological pigment

Plants are photosynthetic, meaning they manufacture their own food molecules using energy taken from light. The main mechanism plants have for capturing energy from light is chlorophyll. All green plants contain two types of chlorophyll, chlorophyll a and chlorophyll b (latter of these pigments aren't found in red or brown algae).

Immune system
By means of cells that act as nerves, plants receive and distribute inside their systems information about incident light intensity and quality. Incident light stimulates a chemical reaction in one leaf, which will cause a chain reaction of signals to the whole plant from a specialized cell known as a "bundle sheath cell". Researcers from the Warsaw University of Life Sciences found that plants have a specific memory for varying light conditions, which prepares their immune systems against seasonal pathogens.

Internal distribution

 * ''See Vascular tissue

Vascular plants differ from other plants, because vascular plants transport nutrients between different parts through specialized structures called xylem and phloem. They also have roots for taking up water and minerals. The xylem transports water and minerals through the plant, while the phloem transports sugars and other nutrients taken from the leaves down to the roots.

Ecology

 * See Ecology

The photosynthesis of the land plants and green algae is the ultimate source of energy and organic material in nearly every ecosystem. Photosynthesis greatly changed the composition of the early Earth's atmosphere, which as a result is now 21% oxygen. Animals and most other organisms alive are aerobic, relying upon oxygen. The small percentage of organisms that don't rely on oxygen are found in anaerobic environments, which are rarely found on Earth. Plants are the primary producers in most ecosystems, and form the basis of the food web in its system. Many animals rely on plants for shelter as well as oxygen and energy.

Land plants are major components to the water cycle and several biogeochemical cycles. Some plants have coevolved with nitrogen fixing bacteria, which makes plants very important in the Nitrogen cycle. Plant roots play an important role in the development of soil and the prevention of erosion of soil.

Distribution
Plants are distributed throughout the world in numbers that vary. While they inhabit several biomes and ecoregions, few plants can be found beyond tundras at the most northern regions of continental shelves. At southern extremes, plants have adapted tenaceiously to the prevailing conditions.

Plants are often times the dominant physical and structural component of habitats where they occur. Many of Earth's biomes are named for the type of vegetation because plants are the dominant organisms in those biomes, such as grasslands and forests.

Ecological relationships
A large number of animals have coevolved with plants. Several animals pollinate flowers in exchange for food in the form of pollen or nectar. After an animal eats a fruit, the seeds inside the fruit may be dispersed through its feces. Myrmecophytes are plants that have coevolved with ants. The ants are benefited by the plant, because it may provide a home, and sometimes food, while the plant gets benefited by the ants, because they often times defend the plant from several herbivores and competing plants. The wastes of the ants also provide an organic fertilizer.

Most plant species have various types of fungi associated with their root systems in a kind of mutualistic symbiosis known as mycorrhiza. The plant is benefited because the fungi helps the plants gain water and mineral nutrients, while the fungi is being benefited with carbohydrates manufactured through photosynthesis. Some plants are homes for endophytic fungi, which benefit the plant by producing toxins that keep herbivores away. The endophyte, Neotyphodium coenophialum, in Festuca arundinacea (Tall Fescue) does severe economic damage to the cattle industry in the United States.

Various forms of parasitism are also fairly common among plants, such as the semi-parasitic mistletoe (order Santalales), which takes some nutrients from its host, but still has photosynthetic leaves, and the fully parasitic broomrape and toothwort, which acquire all their nutrients through connections of their roots to the roots of other plants, because they have no chlorophyll. Some plants, known as myco-heterotrophs, parasitize myccorhizal fungi, and act as epiparasites on plants.

Many plants are epiphytic, meaning the grow on other plants without parasitizing them. Epiphytes may indirectly harm their host by intercepting mineral nutrients and light that the host would usually receive. A large number of epiphytes may break tree limbs, due to their combined weight. Hemiepiphytic plants, such as the strangler fig begin as epiphytes, but will eventually set their own roots, and overpower and kill their host. Many orchids, bromeliads, ferns, and mosses may also grow as epiphytes. Bromeliad epiphytes store water in leaf axils to form phytotelmata, which are complex, aquatic food webs.

Approximately 630 species of plants are carnivorous, such as the Venus flytrap (Dionaea muscipula), and sundew (Drosera species). These trap small animals, and digest them to obtain certain nutrients.

Importance
The study of plant uses by people is known as economic botany or ethnobotany; some people consider economic botany to focus on modern cultivated plants, while ethnobotany focuses on indigenous plants cultivated and used by native people. Cultivation of plants by humans is part of agriculture, which is the basis of human civilization. Plant agriculture is subdivided into agronomy, horticulture, and forestry.

Food
Humans depend on land plants for nutrition. People get nutrition from plants either directly or indirectly. Human nutrition depends to a large extent on cereals, especially maize (corn), wheat, and rice. Other staple crops include potatos, cassava, and legumes. Human food also includes vegetables, spices, fruits, nuts, herbs, and edible flowers.

Beverages, such as coffee, tea, wine, beer, and alcohol are produced from plants.

Sugar is obtained commonly from the sugar cane or sugar beet.

Some types of cooking oils and margarine come from maize (corn), soybeans, rapeseed, safflowers, sunflowers, olives, and others.

Plants are also used as food additives. Some examples are gum arabic, guar gum, locust bean gum, starch, and pectin.

Livestock animals, such as cows, pigs, sheep, and goats feed mainly or entirely on cereal plants, especially grass.

Nonfood products
Wood is used for buildings, furniture, paper, cardboard, musical instruments, sports equipment, and more. Cloth is often times made with cotton, flax, or synthetic fibers derived from cellulose. Plants produce renewable fuels, such as firewood and peat. Coal and petroleum are derived from plants. Medicines, such as aspirin, taxol, morphine, quinine, reserpine, colchicine, digitalis, and vincristine are derived from plants. There are several herbal supplements, such as ginkgo, Echinacea, feverfew, and Saint John's wort. Pesticides that are derived from plants include nicotine, rotenone, strychnine, and pyrethrins. Drugs that are obtained from plants include opium, cocaine, and marijuana. Poisons obtained from plants include ricin, hemlock, and curare. Plants are also sources of natural products, such as fibers, essential oils, dyes, pigments, waxes, tannin, latex, gums, resins, alkaloids, amber, and cork. Products derived from plants include soaps, paints, shampoos, paints, perfumes, cosmetics, turpentine, rubber, varnish, lubricants, linoleum, plastics, inks, chewing gum, and hemp rope. Plants are also a source of basic chemicals for the industrial synthesis of a vast array of organic chemicals. These chemicals are often used in a vast variety of studies and experiments.

Aesthetic uses
Thousands of species of plants are cultivated for aesthetic purposes as well to provide shade, modify temperatures, reduce wind, abate noise, provide privacy, and prevent soil erosion. People use cut flowers, dried flowers, and houseplants indoors and in greenhouses. In outdoor gardens, common plants used for aesthetic uses are lawn grasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials, bedding plants, and more. Images of plants are often used in art, architecture, humor, language, and photography on textiles, money, stamps, flags, and coat of arms. Living plant art forms include topiaries, bonsais, and ikebana. Plants are the basis of a multi-billion dollar per year tourism industry, in which people travel and visit to arboretums, botanical gardens, historic gardens, national parks, tulip festivals, rainforests, forests, and the National Cherry Blossom Festival. Examples of plants sold as novelties include the Venus flytrap (Dionaea muscipula), sensitive plant (Mimosa pudica), and the resurrection plant.

Scientific and cultural uses
The study of dendrochronology uses tree rings to date in archaeology, which also serve as a record of past climates. Basic biological research has often been done with plants, such as the pea plants used to derive Gregor Mendel's laws of genetics. Space stations or space colonies may someday rely on plants for life support. Plants are used in national and state emblems. Ancient trees are revered, and many are now famous. Several world records are ones regarding plants. Plants are often times used as memorials, gifts, and to mark special occasions, such as births, deaths, weddings, and holidays. Plants are commonly found in mythology, religion, and literature. Ethnobotany studies the uses of plants used by indigenous people, which helps to conserve endangered species as well as discover new medicinal plants. Gardening is a popular leisure activity in the United States. Working with plants or horticultural therapy is very beneficial for people recovering from disabilities. Certain plants contain psychotropic chemicals, which are extracted, and placed into the body, such as tobacco, cannabis (marijuana), and opium.

Negative effects
Weeds are plants that grow where people don't want them. People often times spread plants beyond the place where they are native, and some of these plants become invasive, which will damage existing ecosystems by displacing the native species. Invasive plants and weeds cause billions of dollars in crop losses annually, displacing the crops. This increases the cost of production, and the use of chemical means to control them may be harmful to the environment.

Plants may also cause harm to people and animals directly. Some plant species produce windblown pollen, which may invoke allergic reactions in people that suffer from hay fever. Several types of plants are poisonous to people and/or animals. Plants, such as poison ivy (Toxicodendron radicans) can cause skin irritations when touched. Certain plants contain psychotropic chemicals, which are extracted and ingested or smoked, such as tobacco, cannabis (marijuana), cocaine, and opium. Smoking causes damage to someone's health, and may eventually lead to death, while some drugs may also be harmful or fatal to people. Both illegal and legal drugs derived from plants may have negative effects on the economy, affecting worker productivity and law enforcement costs. Some plants cause allergic reactions to people and animals when ingested, while others cause food intolerances that negatively affect health.