Plant

Plants are mainly multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, plants were treated as one of two kingdoms including all living things that were not animals, and all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes (the archaea and bacteria). By one definition, plants form the clade Viridiplantae (Latin name for "green plants"), a group that includes the flowering plants, conifers and other gymnosperms, ferns and their allies, hornworts, liverworts, mosses and the green algae, but excludes the red and brown algae.

Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria. Their chloroplasts contain chlorophylls a and b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is also common.

There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants (see the table below).[5] Green plants provide a substantial proportion of the world's molecular oxygen[6] and are the basis of most of Earth's ecosystems, especially on land. Plants that produce grain, fruit and vegetables form humankind's basic foods, and have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs. The scientific study of plants is known as botany, a branch of biology.

Plants
Temporal range: Mesoproterozoic–present
Diversity of plants image version 5
Scientific classification
Domain: Eukaryota
(unranked): Diaphoretickes
(unranked): Archaeplastida
Kingdom: Plantae
sensu Copeland, 1956
Superdivisions
Synonyms
  • Viridiplantae Cavalier-Smith 1981[1]
  • Chlorobionta Jeffrey 1982, emend. Bremer 1985, emend. Lewis and McCourt 2004[2]
  • Chlorobiota Kenrick and Crane 1997[3]
  • Chloroplastida Adl et al., 2005 [4]
  • Phyta Barkley 1939 emend. Holt & Uidica 2007
  • Cormophyta Endlicher, 1836
  • Cormobionta Rothmaler, 1948
  • Euplanta Barkley, 1949
  • Telomobionta Takhtajan, 1964
  • Embryobionta Cronquist et al., 1966
  • Metaphyta Whittaker, 1969

Definition

All living things were traditionally placed into one of two groups, plants and animals. This classification may date from Aristotle (384 BC – 322 BC), who made the distincton between plants, which generally do not move, and animals, which often are mobile to catch their food. Much later, when Linnaeus (1707–1778) created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia (later Metaphyta or Plantae) and Animalia (also called Metazoa). Since then, it has become clear that the plant kingdom as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these organisms are still often considered plants, particularly in popular contexts.

The term "plant" generally implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts.[7][8]

Current definitions of Plantae

When the name Plantae or plant is applied to a specific group of organisms or taxon, it usually refers to one of four concepts. From least to most inclusive, these four groupings are:

Name(s) Scope Description
Land plants, also known as Embryophyta Plantae sensu strictissimo Plants in the strictest sense include the liverworts, hornworts, mosses, and vascular plants, as well as fossil plants similar to these surviving groups (e.g., Metaphyta Whittaker, 1969,[9] Plantae Margulis, 1971[10]).
Green plants, also known as Viridiplantae, Viridiphyta, Chlorobionta or Chloroplastida Plantae sensu stricto 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 (e.g., Plantae Copeland, 1956[11]).
Archaeplastida, also known as Plastida or Primoplantae Plantae sensu lato Plants in a broad sense comprise the green plants listed above plus the red algae (Rhodophyta) and the glaucophyte algae (Glaucophyta 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 (e.g., Plantae Cavalier-Smith, 1981[12]).
Old definitions of plant (obsolete) Plantae sensu amplo Plants in the widest sense refers to older, obsolete classifications that placed diverse algae, fungi or bacteria in Plantae (e.g., Plantae or Vegetabilia Linnaeus,[13] Plantae Haeckel 1866,[14] Metaphyta Haeckel, 1894,[15] Plantae Whittaker, 1969[9]).

Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships. These are not yet completely settled, but one accepted relationship between the three groups described above is shown below.[16][17][18][19][20][21] Those which have been called "plants" are in bold (some minor groups have been omitted).

Archaeplastida 

Rhodophyta (red algae)

Glaucophyta (glaucophyte algae)

green plants

Mesostigmatophyceae  

Chlorokybophyceae  

Spirotaenia  

Chlorophyta  

Streptophyta

Charales (stoneworts)  

land plants or embryophytes

groups traditionally
called green algae

The way in which the groups of green algae are combined and named varies considerably between authors.

Algae

Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom. The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, and they are no longer classified as plants as defined here.[22][23]

The Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common; primary chloroplasts derived from cyanobacteria containing chlorophylls a and b, cell walls containing cellulose, and food stores in the form of starch contained within the plastids. They undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria.

Two additional groups, the Rhodophyta (red algae) and Glaucophyta (glaucophyte algae), also have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour. These groups also differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event. This is the broadest modern definition of the term 'plant'.

In contrast, most other algae (e.g. brown algae/diatoms, haptophytes, dinoflagellates, and euglenids) not only have different pigments but also have chloroplasts with three or four surrounding membranes. They are not close relatives of the Archaeplastida, presumably having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in even the broadest modern definition of the plant kingdom, although they were in the past.

The green plants or Viridiplantae were traditionally divided into the green algae (including the stoneworts) and the land plants. However, it is now known that the land plants evolved from within a group of green algae, so that the green algae by themselves are a paraphyletic group, i.e. a group that excludes some of the descendants of a common ancestor. Paraphyletic groups are generally avoided in modern classifications, so that in recent treatments the Viridiplantae have been divided into two clades, the Chlorophyta and the Streptophyta (including the land plants and Charophyta).[24][25]

The Chlorophyta (a name that has also been used for all green algae) are the sister group to the Charophytes, from which the land plants evolved. There are about 4,300 species,[26] mainly unicellular or multicellular marine organisms such as the sea lettuce, Ulva.

The other group within the Viridiplantae are the mainly freshwater or terrestrial Streptophyta, which consists of the land plants together with the Charophyta, itself consisting of several groups of green algae such as the desmids and stoneworts. Streptophyte algae are either unicellular or form multicellular filaments, branched or unbranched.[25] The genus Spirogyra is a filamentous streptophyte alga familiar to many, as it is often used in teaching and is one of the organisms responsible for the algal "scum" on ponds. The freshwater stoneworts strongly resemble land plants and are believed to be their closest relatives. Growing immersed in fresh water, they consist of a central stalk with whorls of branchlets.

Fungi

Linnaeus' original classification placed the fungi within the Plantae, since they were unquestionably neither animals or minerals and these were the only other alternatives. With 19th century developments in microbiology, Ernst Haeckel introduced the new kingdom Protista in addition to Plantae and Animalia, but whether fungi were best placed in the Plantae or should be reclassified as protists remained controversial. In 1969, Robert Whittaker proposed the creation of the kingdom Fungi. Molecular evidence has since shown that the most recent common ancestor (concestor), of the Fungi was probably more similar to that of the Animalia than to that of Plantae or any other kingdom.[27]

Whittaker's original reclassification was based on the fundamental difference in nutrition between the Fungi and the Plantae. Unlike plants, which generally gain carbon through photosynthesis, and so are called autotrophs, fungi do not possess chloroplasts and generally obtain carbon by breaking down and absorbing surrounding materials, and so are called heterotrophic saprotrophs. In addition, the substructure of multicellular fungi is different from that of plants, taking the form of many chitinous microscopic strands called hyphae, which may be further subdivided into cells or may form a syncytium containing many eukaryotic nuclei. Fruiting bodies, of which mushrooms are the most familiar example, are the reproductive structures of fungi, and are unlike any structures produced by plants.

Diversity

The table below shows some species count estimates of different green plant (Viridiplantae) divisions. It suggests there are about 300,000 species of living Viridiplantae, of which 85–90% are flowering plants. (Note: as these are from different sources and different dates, they are not necessarily comparable, and like all species counts, are subject to a degree of uncertainty in some cases.)

Diversity of living green plant (Viridiplantae) divisions
Informal group Division name Common name No. of living species Approximate No. in informal group
Green algae Chlorophyta green algae (chlorophytes) 3,800–4,300 [28][29] 8,500

(6,600–10,300)

Charophyta green algae (e.g. desmids & stoneworts) 2,800–6,000 [30][31]
Bryophytes Marchantiophyta liverworts 6,000–8,000 [32] 19,000

(18,100–20,200)

Anthocerotophyta hornworts 100–200 [33]
Bryophyta mosses 12,000 [34]
Pteridophytes Lycopodiophyta club mosses 1,200 [23] 12,000

(12,200)

Pteridophyta ferns, whisk ferns & horsetails 11,000 [23]
Seed plants Cycadophyta cycads 160 [35] 260,000

(259,511)

Ginkgophyta ginkgo 1 [36]
Pinophyta conifers 630 [23]
Gnetophyta gnetophytes 70 [23]
Magnoliophyta flowering plants 258,650 [37]

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

Evolution

The evolution of plants has resulted in increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns to the complex gymnosperms and angiosperms of today. Plants in all of these groups continue to thrive, especially in the environments in which they evolved.

An algal scum formed on the land 1,200 million years ago, but it was not until the Ordovician Period, around 450 million years ago, that land plants appeared.[38] However, new evidence from the study of carbon isotope ratios in Precambrian rocks has suggested that complex photosynthetic plants developed on the earth over 1000 m.y.a.[39] For more than a century it has been assumed that the ancestors of land plants evolved in aquatic environments and then adapted to a life on land, an idea usually credited to botanist Frederick Orpen Bower in his 1908 book "The Origin of a Land Flora". A recent alternative view, supported by genetic evidence, is that they evolved from terrestrial single-celled algae.[40] Primitive land plants began to diversify in the late Silurian Period, around 420 million years ago, and the results of their diversification are displayed in remarkable detail in an early Devonian fossil assemblage from the Rhynie chert. This chert preserved early plants in cellular detail, petrified in volcanic springs. By the middle of the Devonian Period most of the features recognised in plants today are present, including roots, leaves and secondary wood, and by late Devonian times seeds had evolved.[41] Late Devonian plants had thereby reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued in the Carboniferous and later geological periods and is ongoing today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the evolution of flowering plants in the Triassic (~200 million years ago), which exploded in the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO
2
and warm, dry conditions of the tropics over the last 10 million years.

A 1997 proposed phylogenetic tree of Plantae, after Kenrick and Crane,[42] is as follows, with modification to the Pteridophyta from Smith et al.[43] The Prasinophyceae are a paraphyletic assemblage of early diverging green algal lineages, but are treated as a group outside the Chlorophyta:[44] later authors have not followed this suggestion.

Prasinophyceae (micromonads)

Streptobionta
Embryophytes
Stomatophytes
Polysporangiates
Tracheophytes
Eutracheophytes
Euphyllophytina
Lignophyta

Spermatophytes (seed plants)

Progymnospermophyta †

Pteridophyta

Pteridopsida (true ferns)

Marattiopsida

Equisetopsida (horsetails)

Psilotopsida (whisk ferns & adders'-tongues)

Cladoxylopsida †

Lycophytina

Lycopodiophyta

Zosterophyllophyta †

Rhyniophyta †

Aglaophyton †

Horneophytopsida †

Bryophyta (mosses)

Anthocerotophyta (hornworts)

Marchantiophyta (liverworts)

Charophyta

Chlorophyta

Trebouxiophyceae (Pleurastrophyceae)

Chlorophyceae

Ulvophyceae

A newer proposed classification follows Leliaert et al. 2011[45] and modified with Silar 2016[46][20][21][47] for the green algae clades and Novíkov & Barabaš-Krasni 2015[48] for the land plants clade. Notice that the Prasinophyceae are here placed inside the Chlorophyta.

Viridiplantae

Mesostigmatophyceae  

Chlorokybophyceae  

Spirotaenia  

Chlorobionta

Chlorophyta inc. Prasinophyceae  

Streptobionta

Streptofilum  

Klebsormidiophyta  

Phragmoplastophyta

Charophyta Rabenhorst 1863 emend. Lewis & McCourt 2004 (Stoneworts)  

Coleochaetophyta  

Zygnematophyta  

Embryobiotes

Marchantiophyta (Liverworts)

Stomatophyta

Bryophyta (True mosses)

Anthocerotophyta (Non-flowering hornworts)

Polysporangiophyta

Horneophyta

Aglaophyta

Tracheophyta (Vascular Plants)

Embryophytes

The plants that are likely most familiar to us are the multicellular land plants, called embryophytes. Embryophytes include the vascular plants, such as ferns, conifers and flowering plants. They also include the bryophytes, of which mosses and liverworts are the most common.

All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light, water and carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Embryophytes are distinguished from green algae, which represent a mode of photosynthetic life similar to the kind modern plants are believed to have evolved from, by having specialized reproductive organs protected by non-reproductive tissues.

Bryophytes first appeared during the early Paleozoic. They mainly live in habitats where moisture is available for significant periods, although some species, such as Targionia, are desiccation-tolerant. Most species of bryophytes remain small throughout their life-cycle. This involves an alternation between two generations: a haploid stage, called the gametophyte, and a diploid stage, called the sporophyte. In bryophytes, the sporophyte is always unbranched and remains nutritionally dependent on its parent gametophyte. The embryophytes have the ability to secrete a cuticle on their outer surface, a waxy layer that confers resistant to desiccation. In the mosses and hornworts a cuticle is usually only produced on the sporophyte. Stomata are absent from liverworts, but occur on the sporangia of mosses and hornworts, allowing gas exchange.

Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different terrestrial environments. They developed a number of adaptations that allowed them to spread into increasingly more arid places, notably the vascular tissues xylem and phloem, that transport water and food throughout the organism. Root systems capable of obtaining soil water and nutrients also evolved during the Devonian. In modern vascular plants, the sporophyte is typically large, branched, nutritionally independent and long-lived, but there is increasing evidence that Paleozoic gametophytes were just as complex as the sporophytes. The gametophytes of all vascular plant groups evolved to become reduced in size and prominence in the life cycle.

In seed plants, the microgametophyte is reduced from a multicellular free-living organism to a few cells in a pollen grain and the miniaturised megagametophyte remains inside the megasporangium, attached to and dependent on the parent plant. A megasporangium enclosed in a protective layer called an integument is known as an ovule. After fertilisation by means of sperm produced by pollen grains, an embryo sporophyte develops inside the ovule. The integument becomes a seed coat, and the ovule develops into a seed. Seed plants can survive and reproduce in extremely arid conditions, because they are not dependent on free water for the movement of sperm, or the development of free living gametophytes.

The first seed plants, pteridosperms (seed ferns), now extinct, appeared in the Devonian and diversified through the Carboniferous. They were the ancestors of modern gymnosperms, of which four surviving groups are widespread today, particularly the conifers, which are dominant trees in several biomes. The name gymnosperm comes from the Greek composite word γυμνόσπερμος (γυμνός gymnos, "naked" and σπέρμα sperma, "seed"), as the ovules and subsequent seeds are not enclosed in a protective structure (carpels or fruit), but are borne naked, typically on cone scales.

Fossils

Petrified forest log 1 md
A petrified log in Petrified Forest National Park, Arizona

Plant fossils include roots, wood, leaves, seeds, fruit, pollen, spores, phytoliths, and amber (the fossilized resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide.

The earliest fossils clearly assignable to Kingdom Plantae are fossil green algae from the Cambrian. These fossils resemble calcified multicellular members of the Dasycladales. Earlier Precambrian fossils are known that resemble single-cell green algae, but definitive identity with that group of algae is uncertain.

The earliest fossils attributed to green algae date from the Precambrian (ca. 1200 mya).[49][50] The resistant outer walls of prasinophyte cysts (known as phycomata) are well preserved in fossil deposits of the Paleozoic (ca. 250–540 mya). A filamentous fossil (Proterocladus) from middle Neoproterozoic deposits (ca. 750 mya) has been attributed to the Cladophorales, while the oldest reliable records of the Bryopsidales, Dasycladales) and stoneworts are from the Paleozoic.[44][51]

The oldest known fossils of embryophytes date from the Ordovician, though such fossils are fragmentary. By the Silurian, fossils of whole plants are preserved, including the simple vascular plant Cooksonia in mid-Silurian and the much larger and more complex lycophyte Baragwanathia longifolia in late Silurian. From the early Devonian Rhynie chert, detailed fossils of lycophytes and rhyniophytes have been found that show details of the individual cells within the plant organs and the symbiotic association of these plants with fungi of the order Glomales. The Devonian period also saw the evolution of leaves and roots, and the first modern tree, Archaeopteris. This tree with fern-like foliage and a trunk with conifer-like wood was heterosporous producing spores of two different sizes, an early step in the evolution of seeds.[52]

The Coal measures are a major source of Paleozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to collect; coal itself is the remains of fossilised plants, though structural detail of the plant fossils is rarely visible in coal. In the Fossil Grove at Victoria Park in Glasgow, Scotland, 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 eras. Sequoia and its allies, magnolia, oak, and palms are often found.

Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas where it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced by silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents.

Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the Southern Hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory.

Structure, growth and development

Leaf 1 web
The leaf is usually the primary site of photosynthesis in plants.

Most of the solid material in a plant is taken from the atmosphere. Through the process of photosynthesis, most plants use the energy in sunlight to convert carbon dioxide from the atmosphere, plus water, into simple sugars. These sugars are then used as building blocks and form the main structural component of the plant. Chlorophyll, a green-colored, magnesium-containing pigment is essential to this process; it is generally present in plant leaves, and often in other plant parts as well. Parasitic plants, on the other hand, use the resources of their host to provide the materials needed for metabolism and growth.

Plants usually rely on soil primarily for support and water (in quantitative terms), but they also obtain compounds of nitrogen, phosphorus, potassium, magnesium and other elemental nutrients from the soil. Epiphytic and lithophytic plants depend on air and nearby debris for nutrients, and carnivorous plants supplement their nutrient requirements, particularly for nitrogen and phosphorus, with insect prey that they capture. For the majority of plants to grow successfully they also require oxygen in the atmosphere and around their roots (soil gas) for respiration. Plants use oxygen and glucose (which may be produced from stored starch) to provide energy.[53] Some plants grow as submerged aquatics, using oxygen dissolved in the surrounding water, and a few specialized vascular plants, such as mangroves and reed (Phragmites australis),[54] can grow with their roots in anoxic conditions.

Factors affecting growth

The genome of a plant controls its growth. For example, selected varieties or genotypes of wheat grow rapidly, maturing within 110 days, whereas others, in the same environmental conditions, grow more slowly and mature within 155 days.[55]

Growth is also determined by environmental factors, such as temperature, available water, available light, carbon dioxide and available nutrients in the soil. Any change in the availability of these external conditions will be reflected in the plant's growth and the timing of its development.

Biotic factors also affect plant growth. Plants can be so crowded that no single individual produces normal growth, causing etiolation and chlorosis. Optimal plant growth can be hampered by grazing animals, suboptimal soil composition, lack of mycorrhizal fungi, and attacks by insects or plant diseases, including those caused by bacteria, fungi, viruses, and nematodes.[55]

Eenbruinigherfstblad
There is no photosynthesis in deciduous leaves in autumn.

Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Annual plants grow and reproduce within one growing season, biennial plants grow for two growing seasons and usually reproduce in second year, and perennial plants live for many growing seasons and once mature will often reproduce annually. These designations often depend on climate and other environmental factors. Plants that are annual in alpine or temperate regions can be biennial or perennial in warmer climates. Among the vascular plants, perennials include both evergreens that keep their leaves the entire year, and deciduous plants that lose their leaves for some part of it. In temperate and boreal climates, they generally lose their leaves during the winter; many tropical plants lose their leaves during the dry season.

The growth rate of plants is extremely variable. Some mosses grow less than 0.001 millimeters per hour (mm/h), while most trees grow 0.025-0.250 mm/h. Some climbing species, such as kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h.

Plants protect themselves from frost and dehydration stress with antifreeze proteins, heat-shock proteins and sugars (sucrose is common). LEA (Late Embryogenesis Abundant) protein expression is induced by stresses and protects other proteins from aggregation as a result of desiccation and freezing.[56]

Effects of freezing

When water freezes in plants, the consequences for the plant depend very much on whether the freezing occurs within cells (intracellularly) or outside cells in intercellular spaces.[57] Intracellular freezing, which usually kills the cell[58] regardless of the hardiness of the plant and its tissues, seldom occurs in nature because rates of cooling are rarely high enough to support it. Rates of cooling of several degrees Celsius per minute are typically needed to cause intracellular formation of ice.[59] At rates of cooling of a few degrees Celsius per hour, segregation of ice occurs in intercellular spaces.[60] This may or may not be lethal, depending on the hardiness of the tissue. At freezing temperatures, water in the intercellular spaces of plant tissue freezes first, though the water may remain unfrozen until temperatures drop below −7 °C (19 °F).[57] After the initial formation of intercellular ice, the cells shrink as water is lost to the segregated ice, and the cells undergo freeze-drying. This dehydration is now considered the fundamental cause of freezing injury.

DNA damage and repair

Plants are continuously exposed to a range of biotic and abiotic stresses. These stresses often cause DNA damage directly, or indirectly via the generation of reactive oxygen species.[61] Plants are capable of a DNA damage response that is a critical mechanism for maintaining genome stability.[62] The DNA damage response is particularly important during seed germination, since seed quality tends to deteriorate with age in association with DNA damage accumulation.[63] During germination repair processes are activated to deal with this accumulated DNA damage.[64] In particular, single- and double-strand breaks in DNA can be repaired.[65] The DNA checkpoint kinase ATM has a key role in integrating progression through germination with repair responses to the DNA damages accumulated by the aged seed.[66]

Plant cells

Plant cell structure-en
Plant cell structure

Plant cells are typically distinguished by their large water-filled central vacuole, chloroplasts, and rigid cell walls that are made up of cellulose, hemicellulose, and pectin. Cell division is also characterized by the development of a phragmoplast for the construction of a cell plate in the late stages of cytokinesis. Just as in animals, plant cells differentiate and develop into multiple cell types. Totipotent meristematic cells can differentiate into vascular, storage, protective (e.g. epidermal layer), or reproductive tissues, with more primitive plants lacking some tissue types.[67]

Physiology

Photosynthesis

Plants are photosynthetic, which means that they manufacture their own food molecules using energy obtained from light. The primary mechanism plants have for capturing light energy is the pigment chlorophyll. All green plants contain two forms of chlorophyll, chlorophyll a and chlorophyll b. The latter of these pigments is not found in red or brown algae. The simple equation of photosynthesis is as follows:

Immune system

By means of cells that behave like nerves, plants receive and distribute within their systems information about incident light intensity and quality. Incident light that stimulates a chemical reaction in one leaf, will cause a chain reaction of signals to the entire plant via a type of cell termed a bundle sheath cell. Researchers, from the Warsaw University of Life Sciences in Poland, found that plants have a specific memory for varying light conditions, which prepares their immune systems against seasonal pathogens.[68] Plants use pattern-recognition receptors to recognize conserved microbial signatures. This recognition triggers an immune response. The first plant receptors of conserved microbial signatures were identified in rice (XA21, 1995)[69] and in Arabidopsis thaliana (FLS2, 2000).[70] Plants also carry immune receptors that recognize highly variable pathogen effectors. These include the NBS-LRR class of proteins.

Internal distribution

Vascular plants differ from other plants in that nutrients are transported between their different parts through specialized structures, called xylem and phloem. They also have roots for taking up water and minerals. The xylem moves water and minerals from the root to the rest of the plant, and the phloem provides the roots with sugars and other nutrient produced by the leaves.[67]

Genomics

Plants have some of the largest genomes among all organisms.[71] The largest plant genome (in terms of gene number) is that of wheat (Triticum asestivum), predicted to encode ≈94,000 genes[72] and thus almost 5 times as many as the human genome. The first plant genome sequenced was that of Arabidopsis thaliana which encodes about 25,500 genes.[73] In terms of sheer DNA sequence, the smallest published genome is that of the carnivorous bladderwort (Utricularia gibba) at 82 Mb (although it still encodes 28,500 genes)[74] while the largest, from the Norway Spruce (Picea abies), extends over 19,600 Mb (encoding about 28,300 genes).[75]

Ecology

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. Many animals rely on plants for shelter as well as oxygen and food.

Land plants are key components of the water cycle and several other biogeochemical cycles. Some plants have coevolved with nitrogen fixing bacteria, making plants an important part of the nitrogen cycle. Plant roots play an essential role in soil development and the prevention of soil erosion.

Distribution

Plants are distributed almost worldwide. While they inhabit a multitude of biomes and ecoregions, few can be found beyond the tundras at the northernmost regions of continental shelves. At the southern extremes, plants of the Antarctic flora have adapted tenaciously to the prevailing conditions.

Plants are often the dominant physical and structural component of 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 grasslands, taiga and tropical rainforest.

Ecological relationships

Venus Flytrap showing trigger hairs
The Venus flytrap, a species of carnivorous plant.

Numerous animals have coevolved with plants. Many animals pollinate flowers in exchange for food in the form of pollen or nectar. Many animals disperse seeds, often by eating fruit and passing the seeds in their feces. 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 provide organic fertilizer.

The majority of plant species have various kinds of fungi associated with their root systems in a kind of 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. 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 (Festuca arundinacea) does tremendous economic damage to the cattle industry in the U.S.

Various forms of parasitism are also fairly common among plants, 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. Some plants, known as myco-heterotrophs, parasitize mycorrhizal fungi, and hence act as epiparasites on other plants.

Many plants are epiphytes, meaning they grow on other plants, usually trees, without parasitizing them. Epiphytes may indirectly harm their host plant by intercepting mineral nutrients and light that the host would otherwise receive. The weight of large numbers of epiphytes may break tree limbs. Hemiepiphytes like the strangler fig begin as epiphytes but eventually set their own roots and overpower and kill their host. Many orchids, bromeliads, ferns and mosses often grow as epiphytes. Bromeliad epiphytes accumulate water in leaf axils to form phytotelmata that may contain complex aquatic food webs.[76]

Approximately 630 plants are carnivorous, such as the Venus Flytrap (Dionaea muscipula) and sundew (Drosera species). They trap small animals and digest them to obtain mineral nutrients, especially nitrogen and phosphorus.[77]

Importance

The study of plant uses by people is called economic botany or ethnobotany.[78] Human cultivation of plants is part of agriculture, which is the basis of human civilization.[79] Plant agriculture is subdivided into agronomy, horticulture and forestry.[80]

Food

Harvest Time - geograph.org.uk - 747095
Mechanical harvest of oats.

Humans depend on plants for food, either directly or as feed for domestic animals. Agriculture deals with the production of food crops, and has played a key role in the history of world civilizations. Agriculture includes agronomy for arable crops, horticulture for vegetables and fruit, and forestry for timber.[81] About 7,000 species of plant have been used for food, though most of today's food is derived from only 30 species. The major staples include cereals such as rice and wheat, starchy roots and tubers such as cassava and potato, and legumes such as peas and beans. Vegetable oils such as olive oil provide lipids, while fruit and vegetables contribute vitamins and minerals to the diet.[82]

Medicines

Medicinal plants are a primary source of organic compounds, both for their medicinal and physiological effects, and for the industrial synthesis of a vast array of organic chemicals.[83] Many hundreds of medicines are derived from plants, both traditional medicines used in herbalism[84][85] and chemical substances purified from plants or first identified in them, sometimes by ethnobotanical search, and then synthesised for use in modern medicine. Modern medicines derived from plants include aspirin, taxol, morphine, quinine, reserpine, colchicine, digitalis and vincristine. Plants used in herbalism include ginkgo, echinacea, feverfew, and Saint John's wort. The pharmacopoeia of Dioscorides, De Materia Medica, describing some 600 medicinal plants, was written between 50 and 70 AD and remained in use in Europe and the Middle East until around 1600 AD; it was the precursor of all modern pharmacopoeias.[86][87][88]

Nonfood products

Timber DonnellyMills2005 SeanMcClean
Timber in storage for later processing at a sawmill

Plants grown as industrial crops are the source of a wide range of products used in manufacturing, sometimes so intensively as to risk harm to the environment.[89] Nonfood products include essential oils, natural dyes, pigments, waxes, resins, tannins, alkaloids, amber and cork. Products derived from plants include soaps, shampoos, perfumes, cosmetics, paint, varnish, turpentine, rubber, latex, lubricants, linoleum, plastics, inks, and gums. Renewable fuels from plants include firewood, peat and other biofuels.[90][91] The fossil fuels coal, petroleum and natural gas are derived from the remains of aquatic organisms including phytoplankton in geological time.[92]

Structural resources and fibres from plants are used to construct dwellings and to manufacture clothing. Wood is used not only for buildings, boats, and furniture, but also for smaller items such as musical instruments and sports equipment. Wood is pulped to make paper and cardboard.[93] Cloth is often made from cotton, flax, ramie or synthetic fibres such as rayon and acetate derived from plant cellulose. Thread used to sew cloth likewise comes in large part from cotton.[94]

Aesthetic uses

Rose espalier Niedernhall
A rose espalier at Niedernhall in Germany.

Thousands of plant species are cultivated for aesthetic purposes as well as to provide shade, modify temperatures, reduce wind, abate noise, provide privacy, and prevent soil erosion. Plants are the basis of a multibillion-dollar per year tourism industry, which includes travel to historic gardens, national parks, rainforests, forests with colorful autumn leaves, and festivals such as Japan's[95] and America's cherry blossom festivals.[96]

Luxor, West Bank, Ramesseum, column top decorations, Egypt, Oct 2004
Capitals of ancient Egyptian columns decorated to resemble papyrus plants. (at Luxor, Egypt)

While some gardens are planted with food crops, many are planted for aesthetic, ornamental, or conservation purposes. Arboretums and botanical gardens are public collections of living plants. In private outdoor gardens, lawn grasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials and bedding plants are used. Gardens may cultivate the plants in a naturalistic state, or may sculpture their growth, as with topiary or espalier. Gardening is the most popular leisure activity in the U.S., and working with plants or horticulture therapy is beneficial for rehabilitating people with disabilities.

Plants may also be grown or kept indoors as houseplants, or in specialized buildings such as greenhouses that are designed for the care and cultivation of living plants. Venus Flytrap, sensitive plant and resurrection plant are examples of plants sold as novelties. There are also art forms specializing in the arrangement of cut or living plant, such as bonsai, ikebana, and the arrangement of cut or dried flowers. Ornamental plants have sometimes changed the course of history, as in tulipomania.[97]

Architectural designs resembling plants appear in the capitals of Ancient Egyptian columns, which were carved to resemble either the Egyptian white lotus or the papyrus.[98] Images of plants are often used in painting and photography, as well as on textiles, money, stamps, flags and coats of arms.

Scientific and cultural uses

Barbara McClintock (1902-1992) shown in her laboratory in 1947
Barbara McClintock (1902–1992) was a pioneering cytogeneticist who used maize (or corn) to study the mechanism of inheritance of traits.

Basic biological research has often been done with plants. In genetics, the breeding of pea plants allowed Gregor Mendel to derive the basic laws governing inheritance,[99] and examination of chromosomes in maize allowed Barbara McClintock to demonstrate their connection to inherited traits.[100] The plant Arabidopsis thaliana is used in laboratories as a model organism to understand how genes control the growth and development of plant structures.[101] NASA predicts that space stations or space colonies will one day rely on plants for life support.[102]

Ancient trees are revered and many are famous. Tree rings themselves are an important method of dating in archeology, and serve as a record of past climates.

Plants figure prominently in mythology, religion and literature. They are used as national and state emblems, including state trees and state flowers. Plants are often used as memorials, gifts and to mark special occasions such as births, deaths, weddings and holidays. The arrangement of flowers may be used to send hidden messages.

Negative effects

Weeds are unwanted plants growing in managed environments such as farms, urban areas, gardens, lawns, and parks. People have spread plants beyond their native ranges and some of these introduced plants become invasive, damaging existing ecosystems by displacing native species, and sometimes becoming serious weeds of cultivation.

Plants may cause harm to animals, including people. Plants that produce windblown pollen invoke allergic reactions in people who suffer from hay fever. A wide variety of plants are poisonous. Toxalbumins are plant poisons fatal to most mammals and act as a serious deterrent to consumption. Several plants cause skin irritations when touched, such as poison ivy. Certain plants contain psychotropic chemicals, which are extracted and ingested or smoked, including nicotine from tobacco, cannabinoids from Cannabis sativa, cocaine from Erythroxylon coca and opium from opium poppy. Smoking causes damage to health or even death, while some drugs may also be harmful or fatal to people.[103][104] Both illegal and legal drugs derived from plants may have negative effects on the economy, affecting worker productivity and law enforcement costs.[105][106]

See also

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Further reading

General
  • Evans, L.T. (1998). Feeding the Ten Billion – Plants and Population Growth. Cambridge University Press. Paperback, 247 pages. ISBN 0-521-64685-5.
  • Kenrick, Paul & Crane, Peter R. (1997). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D.C.: Smithsonian Institution Press. ISBN 1-56098-730-8.
  • Raven, Peter H.; Evert, Ray F.; & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W.H. Freeman and Company. ISBN 0-7167-1007-2.
  • Taylor, Thomas N. & Taylor, Edith L. (1993). The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-651589-4.
  • Trewavas A (2003). "Aspects of Plant Intelligence". Annals of Botany. 92 (1): 1–20. doi:10.1093/aob/mcg101. PMC 4243628. PMID 12740212.
Species estimates and counts
  • International Union for Conservation of Nature and Natural Resources (IUCN) Species Survival Commission (2004). IUCN Red List [2].
  • Prance G.T. (2001). "Discovering the Plant World". Taxon. 50 (2, Golden Jubilee Part 4): 345–359. doi:10.2307/1223885. JSTOR 1223885.

External links

Botanical and vegetation databases
Botany

Botany, also called plant science(s), plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek word βοτάνη (botanē) meaning "pasture", "grass", or "fodder"; βοτάνη is in turn derived from βόσκειν (boskein), "to feed" or "to graze". Traditionally, botany has also included the study of fungi and algae by mycologists and phycologists respectively, with the study of these three groups of organisms remaining within the sphere of interest of the International Botanical Congress. Nowadays, botanists (in the strict sense) study approximately 410,000 species of land plants of which some 391,000 species are vascular plants (including approximately 369,000 species of flowering plants), and approximately 20,000 are bryophytes.Botany originated in prehistory as herbalism with the efforts of early humans to identify – and later cultivate – edible, medicinal and poisonous plants, making it one of the oldest branches of science. Medieval physic gardens, often attached to monasteries, contained plants of medical importance. They were forerunners of the first botanical gardens attached to universities, founded from the 1540s onwards. One of the earliest was the Padua botanical garden. These gardens facilitated the academic study of plants. Efforts to catalogue and describe their collections were the beginnings of plant taxonomy, and led in 1753 to the binomial system of Carl Linnaeus that remains in use to this day.

In the 19th and 20th centuries, new techniques were developed for the study of plants, including methods of optical microscopy and live cell imaging, electron microscopy, analysis of chromosome number, plant chemistry and the structure and function of enzymes and other proteins. In the last two decades of the 20th century, botanists exploited the techniques of molecular genetic analysis, including genomics and proteomics and DNA sequences to classify plants more accurately.

Modern botany is a broad, multidisciplinary subject with inputs from most other areas of science and technology. Research topics include the study of plant structure, growth and differentiation, reproduction, biochemistry and primary metabolism, chemical products, development, diseases, evolutionary relationships, systematics, and plant taxonomy. Dominant themes in 21st century plant science are molecular genetics and epigenetics, which are the mechanisms and control of gene expression during differentiation of plant cells and tissues. Botanical research has diverse applications in providing staple foods, materials such as timber, oil, rubber, fibre and drugs, in modern horticulture, agriculture and forestry, plant propagation, breeding and genetic modification, in the synthesis of chemicals and raw materials for construction and energy production, in environmental management, and the maintenance of biodiversity.

Cassava

Manihot esculenta, commonly called cassava (), manioc, yuca, macaxeira, mandioca, aipim and Brazilian arrowroot, is a woody shrub native to South America of the spurge family, Euphorbiaceae. It is extensively cultivated as an annual crop in tropical and subtropical regions for its edible starchy tuberous root, a major source of carbohydrates. Though it is often called yuca in Spanish and in the United States, it is not related to yucca, a shrub in the family Asparagaceae. Cassava, when dried to a powdery (or pearly) extract, is called tapioca; its fried, granular form is named garri.

Cassava is the third-largest source of food carbohydrates in the tropics, after rice and maize. Cassava is a major staple food in the developing world, providing a basic diet for over half a billion people. It is one of the most drought-tolerant crops, capable of growing on marginal soils. Nigeria is the world's largest producer of cassava, while Thailand is the largest exporter of dried cassava.

Cassava is classified as either sweet or bitter. Like other roots and tubers, both bitter and sweet varieties of cassava contain antinutritional factors and toxins, with the bitter varieties containing much larger amounts. It must be properly prepared before consumption, as improper preparation of cassava can leave enough residual cyanide to cause acute cyanide intoxication, goiters, and even ataxia, partial paralysis, or death. The more toxic varieties of cassava are a fall-back resource (a "food security crop") in times of famine or food insecurity in some places. Farmers often prefer the bitter varieties because they deter pests, animals, and thieves.

Endangered species

An endangered species is a species which has been categorized as very likely to become extinct. Endangered (EN), as categorized by the International Union for Conservation of Nature (IUCN) Red List, is the second most severe conservation status for wild populations in the IUCN's schema after Critically Endangered (CR).

In 2012, the IUCN Red List featured 3,079 animal and 2,655 plant species as endangered (EN) worldwide. The figures for 1998 were, respectively, 1,102 and 1,197.

Many nations have laws that protect conservation-reliant species: for example, forbidding hunting, restricting land development or creating protected areas. Population numbers, trends and species' conservation status can be found at the lists of organisms by population.

Family (biology)

Family (Latin: familia, plural familiae) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy; it is classified between order and genus. A family may be divided into subfamilies, which are intermediate ranks between the ranks of family and genus. The official family names are Latin in origin; however, popular names are often used: for example, walnut trees and hickory trees belong to the family Juglandaceae, but that family is commonly referred to as being the "walnut family".

What does or does not belong to a family—or whether a described family should be recognized at all—are proposed and determined by practicing taxonomists. There are no hard rules for describing or recognizing a family. Taxonomists often take different positions about descriptions, and there may be no broad consensus across the scientific community for some time. The publishing of new data and opinion often enables adjustments and consensus.

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Magnoliophyta, also called angiosperms). The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs. Flowers may facilitate outcrossing (fusion of sperm and eggs from different individuals in a population) or allow selfing (fusion of sperm and egg from the same flower). Some flowers produce diaspores without fertilization (parthenocarpy). Flowers contain sporangia and are the site where gametophytes develop. Many flowers have evolved to be attractive to animals, so as to cause them to be vectors for the transfer of pollen. After fertilization, the ovary of the flower develops into fruit containing seeds.

In addition to facilitating the reproduction of flowering plants, flowers have long been admired and used by humans to bring beauty to their environment, and also as objects of romance, ritual, religion, medicine and as a source of food.

Flowering plant

The flowering plants, also known as angiosperms, Angiospermae or Magnoliophyta, are the most diverse group of land plants, with 64 orders, 416 families, approximately 13,164 known genera and c. 369,000 known species. Like gymnosperms, angiosperms are seed-producing plants. However, they are distinguished from gymnosperms by characteristics including flowers, endosperm within the seeds, and the production of fruits that contain the seeds. Etymologically, angiosperm means a plant that produces seeds within an enclosure; in other words, a fruiting plant. The term comes from the Greek words angeion ("case" or "casing") and sperma ("seed").

The ancestors of flowering plants diverged from gymnosperms in the Triassic Period, 245 to 202 million years ago (mya), and the first flowering plants are known from 160 mya. They diversified extensively during the Early Cretaceous, became widespread by 120 mya, and replaced conifers as the dominant trees from 100 to 60 mya.

Horticulture

Horticulture has been defined as the culture of plants for food, comfort and beauty. A more precise definition can be given as "The cultivation, processing, and sale of fruits, nuts, vegetables, ornamental plants, and flowers as well as many additional services". It also includes plant conservation, landscape restoration, soil management, landscape and garden design, construction, and maintenance, and arboriculture. In contrast to agriculture, horticulture does not include large-scale crop production or animal husbandry.

Horticulturists apply their knowledge, skills, and technologies used to grow intensively produced plants for human food and non-food uses and for personal or social needs. Their work involves plant propagation and cultivation with the aim of improving plant growth, yields, quality, nutritional value, and resistance to insects, diseases, and environmental stresses. They work as gardeners, growers, therapists, designers, and technical advisors in the food and non-food sectors of horticulture. Horticulture even refers to the growing of plants in a field or garden.

Hydroelectricity

Hydroelectricity is electricity produced from hydropower. In 2015, hydropower generated 16.6% of the world's total electricity and 70% of all renewable electricity, and was expected to increase about 3.1% each year for the next 25 years.

Hydropower is produced in 150 countries, with the Asia-Pacific region generating 33 percent of global hydropower in 2013. China is the largest hydroelectricity producer, with 920 TWh of production in 2013, representing 16.9 percent of domestic electricity use.

The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity. The hydro station consumes no water, unlike coal or gas plants. The average cost of electricity from a hydro station larger than 10 megawatts is 3 to 5 U.S. cents per kilowatt hour. With a dam and reservoir it is also a flexible source of electricity since the amount produced by the station can be varied up or down very rapidly (as little as a few seconds) to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, and in many cases, has a considerably lower output level of greenhouse gases than fossil fuel powered energy plants.

Led Zeppelin

Led Zeppelin were an English rock band formed in London in 1968. The group consisted of guitarist Jimmy Page, singer Robert Plant, bassist/keyboardist John Paul Jones, and drummer John Bonham. Along with Black Sabbath and Deep Purple,

the band's heavy, guitar-driven sound has led them to be cited as one of the progenitors of heavy metal. Their style drew from a wide variety of influences, including blues, psychedelia, and folk music.

After changing their name from the New Yardbirds, Led Zeppelin signed a deal with Atlantic Records that afforded them considerable artistic freedom. Although the group were initially unpopular with critics, they achieved significant commercial success with eight studio albums released over eleven years, from Led Zeppelin (1969) to In Through the Out Door (1979). Their untitled fourth studio album, commonly known as Led Zeppelin IV (1971) and featuring the song "Stairway to Heaven", is among the most popular and influential works in rock music, and it helped to secure the group's popularity.

Page wrote most of Led Zeppelin's music, particularly early in their career, while Plant generally supplied the lyrics. Jones' keyboard-based compositions later became central to the group's catalogue, which featured increasing experimentation. The latter half of their career saw a series of record-breaking tours that earned the group a reputation for excess and debauchery. Although they remained commercially and critically successful, their output and touring schedule were limited during the late 1970s, and the group disbanded following Bonham's death from alcohol-related asphyxia in 1980. In the decades that followed, the surviving members sporadically collaborated and participated in one-off Led Zeppelin reunions. The most successful of these was the 2007 Ahmet Ertegun Tribute Concert in London, with Jason Bonham taking his late father's place behind the drums.

Many critics consider Led Zeppelin to be one of the most successful, innovative, and influential rock groups in history. They are one of the best-selling music artists in the history of audio recording; various sources estimate the group's record sales at 200 to 300 million units worldwide. With RIAA-certified sales of 111.5 million units, they are the third-best-selling band in the US. Each of their nine studio albums placed in the top 10 of the Billboard album chart and six reached the number-one spot. They achieved eight consecutive UK number-one albums. Rolling Stone magazine described them as "the heaviest band of all time", "the biggest band of the Seventies", and "unquestionably one of the most enduring bands in rock history". They were inducted into the Rock and Roll Hall of Fame in 1995; the museum's biography of the band states that they were "as influential" during the 1970s as the Beatles were during the 1960s.

Legume

A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant (also called a pulse). Legumes are grown agriculturally, primarily for human consumption, for livestock forage and silage, and as soil-enhancing green manure. Well-known legumes include alfalfa, clover, peas, chickpeas, lentils, lupin bean, mesquite, carob, soybeans, peanuts and tamarind.

Legumes produce a botanically unique type of fruit – a simple dry fruit that develops from a simple carpel and usually dehisces (opens along a seam) on two sides. A common name for this type of fruit is a pod, although the term "pod" is also applied to a number of other fruit types, such as that of vanilla (a capsule) and of the radish (a silique).

Legumes are notable in that most of them have symbiotic nitrogen-fixing bacteria in structures called root nodules. For that reason, they play a key role in crop rotation.

Organ (anatomy)

Organs are groups of tissues with similar functions. Plant and animal life relies on many organs that coexist in organ systems.Organs are composed of main tissue, parenchyma, and "sporadic" tissues, stroma. The main tissue is that which is unique for the specific organ, such as the myocardium, the main tissue of the heart, while sporadic tissues include the nerves, blood vessels, and connective tissues. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. Functionally-related organs often cooperate to form whole organ systems. Organs exist in most multicellular organisms. In single-celled organisms such as bacteria, the functional analogue of an organ is known as an organelle. In plants there are three main organs. A hollow organ is an internal organ that forms a hollow tube, or pouch such as the stomach, intestine, or bladder.

In the study of anatomy, the term viscus is used to refer to an internal organ, and viscera is the plural form. 79 organs have been identified in the human body.

Perennial plant

A perennial plant or simply perennial is a plant that lives more than two years. Some sources cite perennial plants being plants that live more than three years. The term (per- + -ennial, "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widely used to distinguish plants with little or no woody growth from trees and shrubs, which are also technically perennials.Perennials, especially small flowering plants, that grow and bloom over the spring and summer, die back every autumn and winter, and then return in the spring from their rootstock, are known as herbaceous perennials. However, depending on the rigors of local climate, a plant that is a perennial in its native habitat, or in a milder garden, may be treated by a gardener as an annual and planted out every year, from seed, from cuttings or from divisions. Tomato vines, for example, live several years in their natural tropical/subtropical habitat but are grown as annuals in temperate regions because they don't survive the winter.

There is also a class of evergreen, or non-herbaceous, perennials, including plants like Bergenia which retain a mantle of leaves throughout the year. An intermediate class of plants is known as subshrubs, which retain a vestigial woody structure in winter, e.g. Penstemon. The local climate may dictate whether plants are treated as shrubs or perennials. For instance, many varieties of Fuchsia are shrubs in warm regions, but in colder temperate climates may be cut to the ground every year as a result of winter frosts.

The symbol for a perennial plant, based on Species Plantarum by Linnaeus, is , which is also the astronomical symbol for the planet Jupiter.

Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.

The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons. Cyanobacteria appeared later; the excess oxygen they produced contributed directly to the oxygenation of the Earth, which rendered the evolution of complex life possible. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about eight times the current power consumption of human civilization.

Photosynthetic organisms also convert around 100–115 billion tonnes (91-104 petagrams) of carbon into biomass per year.

Potato

The potato is a starchy, tuberous crop from the perennial nightshade Solanum tuberosum. In many contexts, potato refers to the edible tuber, but it can also refer to the plant itself. Common or slang terms include tater, tattie and spud. Potatoes were introduced to Europe in the second half of the 16th century by the Spanish. Today they are a staple food in many parts of the world and an integral part of much of the world's food supply. As of 2014, potatoes were the world's fourth-largest food crop after maize (corn), wheat, and rice.Wild potato species can be found throughout the Americas, from the United States to southern Chile. The potato was originally believed to have been domesticated independently in multiple locations, but later genetic testing of the wide variety of cultivars and wild species traced a single origin for potatoes. In the area of present-day southern Peru and extreme northwestern Bolivia, from a species in the Solanum brevicaule complex, potatoes were domesticated approximately 7,000–10,000 years ago. In the Andes region of South America, where the species is indigenous, some close relatives of the potato are cultivated.

Following millennia of selective breeding, there are now over 1,000 different types of potatoes. Over 99% of presently cultivated potatoes worldwide descended from varieties that originated in the lowlands of south-central Chile, which have displaced formerly popular varieties from the Andes.The importance of the potato as a food source and culinary ingredient varies by region and is still changing. It remains an essential crop in Europe, especially eastern and central Europe, where per capita production is still the highest in the world, while the most rapid expansion in production over the past few decades has occurred in southern and eastern Asia, with China and India leading the world in overall production as of 2014.

Being a nightshade similar to tomatoes, the vegetative and fruiting parts of the potato contain the toxin solanine and are not fit for human consumption. Normal potato tubers that have been grown and stored properly produce glycoalkaloids in amounts small enough to be negligible to human health, but if green sections of the plant (namely sprouts and skins) are exposed to light, the tuber can accumulate a high enough concentration of glycoalkaloids to affect human health.

Power station

A power station, also referred to as a power plant or powerhouse and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Most power stations contain one or more generators, a rotating machine that converts mechanical power into electrical power. The relative motion between a magnetic field and a conductor creates an electrical current. The energy source harnessed to turn the generator varies widely. Most power stations in the world burn fossil fuels such as coal, oil, and natural gas to generate electricity. Others use nuclear power, but there is an increasing use of cleaner renewable sources such as solar, wind, wave and hydroelectric.

Robert Plant

Robert Anthony Plant (born 20 August 1948) is an English singer, songwriter, and musician, best known as the lead singer and lyricist of the rock band Led Zeppelin. Plant is regarded as one of the greatest vocalists in the history of rock music.Plant enjoyed great success with Led Zeppelin throughout the 1970s and developed a compelling image as the charismatic rock-and-roll front man, similar to contemporaries such as Roger Daltrey of the Who, Freddie Mercury of Queen, Mick Jagger of the Rolling Stones and Jim Morrison of the Doors. With his mane of long blond hair and powerful, bare-chested appearance, Plant helped to create the "god of rock and roll" or "rock god" archetype. Although Led Zeppelin dissolved in 1980, Plant occasionally collaborated with Jimmy Page on various projects through this period, including forming a short-lived all-star group with Page and Jeff Beck in 1984, called the Honeydrippers. They released an album called The Honeydrippers: Volume One, and the band had a No. 3 hit with a remake of the Phil Phillips' tune "Sea of Love", plus a follow-up hit with a cover of Roy Brown's "Rockin' at Midnight".

A powerful and wide vocal range (particularly evident in his high-pitched vocals) has given him a successful singing career spanning over 50 years. In 2008, Rolling Stone editors ranked him number 15 on their list of the 100 best singers of all time. In 2011, Rolling Stone readers ranked Plant the greatest of all lead singers. In 2006, magazine Hit Parader named Plant the "Greatest Metal Vocalist of All Time". In 2009, Plant was voted "the greatest voice in rock" in a poll conducted by Planet Rock.

Sewage treatment

Sewage treatment is the process of removing contaminants from municipal wastewater, containing mainly household sewage plus some industrial wastewater. Physical, chemical, and biological processes are used to remove contaminants and produce treated wastewater (or treated effluent) that is safe enough for release into the environment. A by-product of sewage treatment is a semi-solid waste or slurry, called sewage sludge. The sludge has to undergo further treatment before being suitable for disposal or application to land.

Sewage treatment may also be referred to as wastewater treatment. However, the latter is a broader term which can also refer to industrial wastewater. For most cities, the sewer system will also carry a proportion of industrial effluent to the sewage treatment plant which has usually received pre-treatment at the factories themselves to reduce the pollutant load. If the sewer system is a combined sewer then it will also carry urban runoff (stormwater) to the sewage treatment plant. Sewage water can travel towards treatment plants via piping and in a flow aided by gravity and pumps. The first part of filtration of sewage typically includes a bar screen to filter solids and large objects which are then collected in dumpsters and disposed of in landfills. Fat and grease is also removed before the primary treatment of sewage.

Succulent plant

In botany, succulent plants, also known as succulents, are plants that have some parts that are more than normally thickened and fleshy, usually to retain water in arid climates or soil conditions. The word "succulent" comes from the Latin word sucus, meaning juice, or sap. Succulent plants may store water in various structures, such as leaves and stems. Some definitions also include roots, thus geophytes that survive unfavorable periods by dying back to underground storage organs may be regarded as succulents. In horticultural use, the term "succulent" is sometimes used in a way which excludes plants that botanists would regard as succulents, such as cacti. Succulents are often grown as ornamental plants because of their striking and unusual appearance.

Many plant families have multiple succulents found within them (over 25 plant families). In some families, such as Aizoaceae, Cactaceae, and Crassulaceae, most species are succulents. The habitats of these water preserving plants are often in areas with high temperatures and low rainfall. Succulents have the ability to thrive on limited water sources, such as mist and dew, which makes them equipped to survive in an ecosystem which contains scarce water sources.

Tomato

The tomato is the edible, often red, berry of the plant Solanum lycopersicum, commonly known as a tomato plant. The species originated in western South America. The Nahuatl (Aztec language) word tomatl gave rise to the Spanish word tomate, from which the English word tomato derived. Its use as a cultivated food may have originated with the indigenous peoples of Mexico. The Spanish encountered the tomato from their contact with the Aztec during the Spanish colonization of the Americas and brought it to Europe. From there, the tomato was introduced to other parts of the European-colonized world during the 16th century.The tomato is consumed in diverse ways, raw or cooked, in many dishes, sauces, salads, and drinks. While tomatoes are fruits — botanically classified as berries — they are commonly used as a vegetable ingredient or side dish.Numerous varieties of the tomato plant are widely grown in temperate climates across the world, with greenhouses allowing for the production of tomatoes throughout all seasons of the year. Tomato plants typically grow to 1–3 meters (3–10 ft) in height. They are vines that have a weak stem that sprawls and typically needs support. Indeterminate tomato plants are perennials in their native habitat, but are cultivated as annuals. Determinate, or bush, plants are annuals that stop growing at a certain height and produce a crop all at once. The size of the tomato varies according to the cultivar, with a range of 0.5–4 inches (1.3–10.2 cm) in width.

Subdisciplines
Plant groups
Plant morphology
(glossary)
Plant growth and habit
Reproduction
Plant taxonomy
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Rhodophyta
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Glaucocystophyta
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Viridiplantae
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& land plants)
Diaphoretickes
Discoba
Loukozoa
Podiata
Hemimastigophora
Incertae sedis
Elements of nature
Universe
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Gardening
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Extant Life phyla/divisions by domain

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