Botany called plant science, plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist; the term "botany" comes from the Ancient Greek word βοτάνη meaning "pasture", "grass", or "fodder". Traditionally, botany has included the study of fungi and algae by mycologists and phycologists with the study of these three groups of organisms remaining within the sphere of interest of the International Botanical Congress. Nowadays, botanists study 410,000 species of land plants of which some 391,000 species are vascular plants, 20,000 are bryophytes. Botany originated in prehistory as herbalism with the efforts of early humans to identify – and cultivate – edible and poisonous plants, making it one of the oldest branches of science. Medieval physic gardens 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, 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 and differentiation, reproduction and primary metabolism, chemical products, diseases, evolutionary relationships 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, rubber and drugs, in modern horticulture and forestry, plant propagation and genetic modification, in the synthesis of chemicals and raw materials for construction and energy production, in environmental management, the maintenance of biodiversity. Botany originated as the study and use of plants for their medicinal properties. Many records of the Holocene period date early botanical knowledge as far back as 10,000 years ago; this early unrecorded knowledge of plants was discovered in ancient sites of human occupation within Tennessee, which make up much of the Cherokee land today. The early recorded history of botany includes many ancient writings and plant classifications. Examples of early botanical works have been found in ancient texts from India dating back to before 1100 BC, in archaic Avestan writings, in works from China before it was unified in 221 BC.
Modern botany traces its roots back to Ancient Greece to Theophrastus, a student of Aristotle who invented and described many of its principles and is regarded in the scientific community as the "Father of Botany". His major works, Enquiry into Plants and On the Causes of Plants, constitute the most important contributions to botanical science until the Middle Ages seventeen centuries later. Another work from Ancient Greece that made an early impact on botany is De Materia Medica, a five-volume encyclopedia about herbal medicine written in the middle of the first century by Greek physician and pharmacologist Pedanius Dioscorides. De Materia Medica was read for more than 1,500 years. Important contributions from the medieval Muslim world include Ibn Wahshiyya's Nabatean Agriculture, Abū Ḥanīfa Dīnawarī's the Book of Plants, Ibn Bassal's The Classification of Soils. In the early 13th century, Abu al-Abbas al-Nabati, Ibn al-Baitar wrote on botany in a systematic and scientific manner. In the mid-16th century, "botanical gardens" were founded in a number of Italian universities – the Padua botanical garden in 1545 is considered to be the first, still in its original location.
These gardens continued the practical value of earlier "physic gardens" associated with monasteries, in which plants were cultivated for medical use. They supported the growth of botany as an academic subject. Lectures were given about the plants grown in the gardens and their medical uses demonstrated. Botanical gardens came much to northern Europe. Throughout this period, botany remained subordinate to medicine. German physician Leonhart Fuchs was one of "the three German fathers of botany", along with theologian Otto Brunfels and physician Hieronymus Bock. Fuchs and Brunfels broke away from the tradition of copying earlier works to make original observations of their own. Bock created his own system of plant classification. Physician Valerius Cordus authored a botanically and pharmacologically important herbal Historia Plantarum in 1544 and a pharmacopoeia of lasting importance, the Dispensatorium
Spider monkeys are New World monkeys belonging to the genus Ateles, part of the subfamily Atelinae, family Atelidae. Like other atelines, they are found in tropical forests of Central and South America, from southern Mexico to Brazil; the genus contains seven species. Disproportionately long limbs and long prehensile tails make them one of the largest New World monkeys and give rise to their common name. Spider monkeys live in the upper layers of the rainforest, forage in the high canopy, from 25 to 30 m, they eat fruits, but will occasionally consume leaves and insects. Due to their large size, spider monkeys require large tracts of moist evergreen forests, prefer undisturbed primary rainforest, they are social animals and live in bands of up to 35 individuals but will split up to forage during the day. Recent meta-analyses on primate cognition studies indicated spider monkeys are the most intelligent New World monkeys, they will "bark" when threatened. They are an important food source due to their large size, so are hunted by local human populations.
Spider monkeys are used in laboratory studies of the disease. The population trend for spider monkeys is decreasing. Theories abound about the evolution of the atelines; this theory is not supported by fossil evidence. Other theories include Brachyteles and Ateles in an unresolved trichotomy, two clades, one composed of Ateles and Lagothrix and the other of Alouatta and Brachyteles. More recent molecular evidence suggests the Atelinae split in the middle to late Miocene, separating spider monkeys from the woolly spider monkeys and the woolly monkeys; the genus name Ateles derives from the ancient greek word ἀτέλεια, meaning "incomplete, imperfect", in reference to the reduced or non-existent thumbs of spider monkeys. The genus contains seven species, seven subspecies. Family Atelidae Subfamily Alouattinae: howler monkeys Subfamily Atelinae Genus Ateles: spider monkeys Red-faced spider monkey, Ateles paniscus White-fronted spider monkey, Ateles belzebuth Peruvian spider monkey, Ateles chamek Brown spider monkey, Ateles hybridus White-cheeked spider monkey, Ateles marginatus Black-headed spider monkey, Ateles fusciceps Brown-headed spider monkey, Ateles fusciceps fusciceps Colombian spider monkey, Ateles fusciceps rufiventris Geoffroy's spider monkey, Ateles geoffroyi Hooded spider monkey Ateles geoffroyi grisescens Yucatan spider monkey, Ateles geoffroyi yucatanensis Mexican spider monkey, Ateles geoffroyi vellerosus Nicaraguan spider monkey, Ateles geoffroyi geoffroyi Ornate spider monkey, Ateles geoffroyi ornatus Genus Brachyteles: muriquis Genus Lagothrix: woolly monkeys Genus Oreonax: the yellow-tailed woolly monkey Spider monkeys are among the largest New World monkeys.
Disproportionately long, spindly limbs inspired the spider monkey's common name. Their deftly prehensile tails, which may be up to 89 cm long, have flexible, hairless tips and skin grooves similar to fingerprints; this adaptation to their arboreal lifestyle serves as a fifth hand. When the monkey walks, its arms drag on the ground. Unlike many monkeys, they do not use their arms for balance when walking, instead relying on their tails; the hands are long and hook-like, have reduced or non-existent thumbs. The fingers are recurved, their hair is coarse, ranging in color from ruddy gold to brown and black, or white in a rare number of specimens. The hands and feet are black. Heads are small with hairless faces; the nostrils are far apart, a distinguishing feature of spider monkeys. Spider monkeys are agile, they are said to be second only to the gibbons in this respect, they have been seen in the wild jumping from tree to tree. Female spider monkeys have a clitoris, developed; this urine is emptied at the bases of the clitoris, collects in skin folds on either side of a groove on the perineal.
Researchers and observers of spider monkeys of South America look for a scrotum to determine the animal sex because these female spider monkeys have pendulous and erectile clitorises long enough to be mistaken for a penis. Spider monkeys form loose groups with 15 to 25 individuals, but sometimes up to 30 or 40. During the day, groups break up into subgroups; the size of subgroups and the degree to which they avoid each other during the day depends on food competition and the risk of predation. The average subgroup size can sometimes be up to 17 animals. Less common in primates, females rather than males disperse at puberty to join new groups. Males tend to stick to
Alpine plants are plants that grow in an alpine climate, which occurs at high elevation and above the tree line. There are many different plant species and taxon that grow as a plant community in these alpine tundra; these include perennial grasses, forbs, cushion plants and lichens. Alpine plants are adapted to the harsh conditions of the alpine environment, which include low temperatures, ultraviolet radiation, a short growing season; some alpine plants serve as medicinal plants. Alpine plants occur in a tundra: a type of natural biome that does not contain trees. Alpine tundra occurs in mountains worldwide, it transitions to subalpine forests below the tree line. With increasing elevation it ends at the snow line where ice persist through summer. Alpine plants are not limited to higher elevations. However, high-elevation areas have different ecology than those located at higher latitudes. One of the biggest distinctions is that the lower bound of a tropical alpine area is difficult to define due to a mixture of human disturbances, dry climates, a lacking tree line.
The other major difference between tropical and arctic alpine ecology is the temperature differences. The tropics have a summer/winter cycle every day, where as the higher latitudes stay cold both day and night. In the northern latitudes, the main factor to overcome is the cold. Intense frost action processes have a profound effect on what little soil there is and the vegetation of arctic alpine regions. Tropical alpine regions are subject to these conditions as well, but they happen; because northern alpine areas cover a massive area it can be difficult to generalize the characteristics that define the ecology. One factor in alpine ecology is wind in an area. Wind pruning is a common sight within northern alpine regions. Along with wind pruning, wind erosion of vegetation mats is a common sight throughout Alaska. Long-lived perennial herbs are the most common type of plant in alpine environments, with most having a large, well-developed root and/or rhizome system; these underground systems store carbohydrates through the winter which are used in the spring for new shoot development.
Some species of saxifrages are evergreen. The leaves of these plants store energy in the form of lipids. Alpine plants go into vegetative dormancy at the end of the growing period, forming perennating buds with the shortening photoperiod. Seedling establishment is slow and occurs less than vegetative reproduction. In the first year of growth of perennial alpine plants, most of the photosynthate is used in establishing a stable root system, used to help prevent desiccation and for carbohydrate storage over winter. In this year, the plant may produce a few true leaves, but only the cotyledons are produced, it takes a few years for plants to become well established. Alpine plants can exist at high elevations, from 300 to 6,000 metres, depending on location. For example, there is a moss. Arenaria bryophylla is the highest flowering plant in the world, occurring as high as 6,180 m. In order to survive, alpine plants are adapted to the conditions at high altitudes, including cold, high levels of ultraviolet radiation, difficulty of reproduction.
Most alpine plants are faced with low temperature extremes at some point in their lives. There are a number of ways. Plants can avoid exposure to low temperature by using different forms of seasonal phenology, morphology, or by variable growth form preference, they can avoid the freezing of their exposed tissues by increasing the amount of solutes in their tissues, known as freezing-point depression. Another, somewhat similar, method plants may use to avoid freezing is supercooling, which prevents ice crystallization within plant tissues; these methods are only sufficient. In the alpine zone, temperatures are low enough that these methods are not sufficient; when plants need a more permanent solution, they can develop freeze tolerance. Plants can dehydrate their cells by moving water into intercellular spaces; this causes ice formation outside of the cell. When all of these strategies fail to prevent frost damage, alpine plants have the capacity to repair or replace the organs damaged; as it is difficult to prevent damage, many alpine plants depend on the replacement of their organs.
They help make this possible by placing their meristems below ground, where temperatures are warmer. Photosynthesis and respiration rates are not uniform throughout the growing season. At the start of the growing season, new shoots have low net photosynthesis rates and high respiration rates due to rapid growth of new shoots; as the temperature rises in a plants microclimate, the net photosynthesis rates will increase as long as ample water is available and will peak during flowering. Alpine plants are able to start photosynthesizing and reach maximum photosynthesis rates at lower temperatures compared to plants adapted to lower elevations and warmer climates; this is due to the combined effects of environmental factors. In alpine areas, water availability is variable. Bryophytes and lichens exhibit high desiccation tolerance, which contributes to their abundance throughout all alpine areas habitats. Among higher plants, tissue desiccation is rare at high altitudes. If it does occur, it happens to plants growing on exposed sites, where wind stress is increased.
Alpine plants avoid water loss by deep rooting and in
Zoology is the branch of biology that studies the animal kingdom, including the structure, evolution, classification and distribution of all animals, both living and extinct, how they interact with their ecosystems. The term is derived from Ancient Greek ζῷον, zōion, i.e. "animal" and λόγος, logos, i.e. "knowledge, study". The history of zoology traces the study of the animal kingdom from ancient to modern times. Although the concept of zoology as a single coherent field arose much the zoological sciences emerged from natural history reaching back to the biological works of Aristotle and Galen in the ancient Greco-Roman world; this ancient work was further developed in the Middle Ages by Muslim physicians and scholars such as Albertus Magnus. During the Renaissance and early modern period, zoological thought was revolutionized in Europe by a renewed interest in empiricism and the discovery of many novel organisms. Prominent in this movement were Vesalius and William Harvey, who used experimentation and careful observation in physiology, naturalists such as Carl Linnaeus, Jean-Baptiste Lamarck, Buffon who began to classify the diversity of life and the fossil record, as well as the development and behavior of organisms.
Microscopy revealed the unknown world of microorganisms, laying the groundwork for cell theory. The growing importance of natural theology a response to the rise of mechanical philosophy, encouraged the growth of natural history. Over the 18th, 19th, 20th centuries, zoology became an professional scientific discipline. Explorer-naturalists such as Alexander von Humboldt investigated the interaction between organisms and their environment, the ways this relationship depends on geography, laying the foundations for biogeography and ethology. Naturalists began to reject essentialism and consider the importance of extinction and the mutability of species. Cell theory provided a new perspective on the fundamental basis of life; these developments, as well as the results from embryology and paleontology, were synthesized in Charles Darwin's theory of evolution by natural selection. In 1859, Darwin placed the theory of organic evolution on a new footing, by his discovery of a process by which organic evolution can occur, provided observational evidence that it had done so.
Darwin gave a new direction to morphology and physiology, by uniting them in a common biological theory: the theory of organic evolution. The result was a reconstruction of the classification of animals upon a genealogical basis, fresh investigation of the development of animals, early attempts to determine their genetic relationships; the end of the 19th century saw the fall of spontaneous generation and the rise of the germ theory of disease, though the mechanism of inheritance remained a mystery. In the early 20th century, the rediscovery of Mendel's work led to the rapid development of genetics, by the 1930s the combination of population genetics and natural selection in the modern synthesis created evolutionary biology. Cell biology studies the structural and physiological properties of cells, including their behavior and environment; this is done on both the microscopic and molecular levels, for single-celled organisms such as bacteria as well as the specialized cells in multicellular organisms such as humans.
Understanding the structure and function of cells is fundamental to all of the biological sciences. The similarities and differences between cell types are relevant to molecular biology. Anatomy considers the forms of macroscopic structures such as organs and organ systems, it focuses on how organs and organ systems work together in the bodies of humans and animals, in addition to how they work independently. Anatomy and cell biology are two studies that are related, can be categorized under "structural" studies. Physiology studies the mechanical and biochemical processes of living organisms by attempting to understand how all of the structures function as a whole; the theme of "structure to function" is central to biology. Physiological studies have traditionally been divided into plant physiology and animal physiology, but some principles of physiology are universal, no matter what particular organism is being studied. For example, what is learned about the physiology of yeast cells can apply to human cells.
The field of animal physiology extends the tools and methods of human physiology to non-human species. Physiology studies how for example nervous, endocrine and circulatory systems and interact. Evolutionary research is concerned with the origin and descent of species, as well as their change over time, includes scientists from many taxonomically oriented disciplines. For example, it involves scientists who have special training in particular organisms such as mammalogy, herpetology, or entomology, but use those organisms as systems to answer general questions about evolution. Evolutionary biology is based on paleontology, which uses the fossil record to answer questions about the mode and tempo of evolution, on the developments in areas such as population genetics and evolutionary theory. Following the development of DNA fingerprinting techniques in the late 20th century, the application of these techniques in zoology has increased the understanding of animal populations. In the 1980s, developmental biology re-entered evolutionary biology from its initial exclusion from the modern synthesis through the study of evolutionary developmental biology.
Related fields considered part of evolutionary biology are phylogenetics and taxonomy. Scientific classification in zoology, is a method by which
Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea related to starfish. They crawl across the sea floor using their flexible arms for locomotion; the ophiuroids have five long, whip-like arms which may reach up to 60 cm in length on the largest specimens. They are known as serpent stars; the Ophiuroidea contain two large clades and Euryalida. Over 2,000 species of brittle stars live today. More than 1200 of these species are found in deep waters, greater than 200 m deep; the ophiuroids diverged in the Early Ordovician, about 500 million years ago. Ophiuroids can be found today from the poles to the tropics. Basket stars are confined to the deeper parts of this range. However, brittle stars are common members of reef communities, where they hide under rocks and within other living organisms. A few ophiuroid species can tolerate brackish water, an ability otherwise unknown among echinoderms. A brittle star's skeleton is made up of embedded ossicles; the 1900 extant species are placed in 230 genera, grouped in the three orders living: Oegophiurida and Ophiurida.
A Paleozoic order exists, the Stenurida. The relationships among ophiuroids and all other echinoderms provide an enduring problem in invertebrate evolution. Developmental and other studies based on modern organisms imply that asteroidea and ophiuroids are not related within the echinoderms. Stenurid morphology, in contrast, suggests a close common ancestry for the two. Of all echinoderms, the Ophiuroidea may have the strongest tendency toward five-segment radial symmetry; the body outline is similar to that of starfish, in that ophiuroids have five arms joined to a central body disk. However, in ophiuroids, the central body disk is marked off from the arms; the disk contains all of the viscera. That is, the internal organs of digestion and reproduction never enter the arms, as they do in the Asteroidea; the underside of the disk contains the mouth, which has five toothed jaws formed from skeletal plates. The madreporite is located within one of the jaw plates, not on the upper side of the animal as it is in starfish.
The ophiuroid coelom is reduced in comparison to other echinoderms. The vessels of the water vascular system end in tube feet; the water vascular system has one madreporite. Others, such as certain Euryalina, have one per arm on the aboral surface. Still other forms have no madreporite at all. Suckers and ampullae are absent from the tube feet; the nervous system consists of a main nerve ring. At the base of each arm, the ring attaches to a radial nerve; the nerves in each limb run through a canal at the base of the vertebral ossicles. Most ophiuroids have other specialised sense organs. However, they have several types of sensitive nerve endings in their epidermis, are able to sense chemicals in the water and the presence or absence of light. Moreover, tube feet may sense light as well as odors; these are found at the ends of their arms, detecting light and retreating into crevices. The mouth is rimmed with five jaws, serves as an anus as well as a mouth. Behind the jaws is a short esophagus and a large, blind stomach cavity which occupies much of the dorsal half of the disk.
Ophiuroids have neither an anus. Digestion occurs within 10 pouches or infolds of the stomach, which are ceca, but unlike in sea stars never extend into the arms; the stomach wall contains glandular hepatic cells. Ophiuroids are scavengers or detritivores. Small organic particles are moved into the mouth by the tube feet. Ophiuroids may prey on small crustaceans or worms. Basket stars in particular may be capable of suspension feeding, using the mucus coating on their arms to trap plankton and bacteria, they use the other four as anchors. Brittle stars will eat small suspended organisms. In large, crowded areas, brittle stars eat suspended matter from prevailing seafloor currents. In basket stars, the arms are used to rhythmically sweep food to the mouth. Pectinura consumes beech pollen in the New Zealand fjords. Eurylina clings to coral branches to browse on the polyps. Gas exchange and excretion occur through cilia-lined sacs called bursae. Ten bursae are found, each fits between two stomach digestive pouches.
Water flows through the bursae by means of cilia or muscular contraction. Oxygen is transported through the body by the hemal system, a series of sinuses and vessels distinct from the water vascular system; the bursae are also the main organs of excretion, with phagocytic "coelomocytes" collecting waste products in the body cavity and migrating to the bursae for expulsion from the body. Like all echinoderms, the Ophiuroidea possess a skeleton of calcium carbonate in the form of calcite. In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test; the plates are covered by the epidermis. In most species, the joints between the ossicles and superficial plates allow the arm to bend to the side, but not to bend upwards. However, in the basket stars, the arms are flexible in all directions. Bot
Marine biology is the scientific study of marine life, organisms in the sea. Given that in biology many phyla and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather than on taxonomy. A large proportion of all life on Earth lives in the ocean; the exact size of this large proportion is unknown, since many ocean species are still to be discovered. The ocean is a complex three-dimensional world covering 71% of the Earth's surface; the habitats studied in marine biology include everything from the tiny layers of surface water in which organisms and abiotic items may be trapped in surface tension between the ocean and atmosphere, to the depths of the oceanic trenches, sometimes 10,000 meters or more beneath the surface of the ocean. Specific habitats include coral reefs, kelp forests, seagrass meadows, the surrounds of seamounts and thermal vents, muddy and rocky bottoms, the open ocean zone, where solid objects are rare and the surface of the water is the only visible boundary.
The organisms studied range from microscopic phytoplankton and zooplankton to huge cetaceans 25–32 meters in length. Marine ecology is the study of how marine organisms interact with the environment. Marine life is a vast resource, providing food and raw materials, in addition to helping to support recreation and tourism all over the world. At a fundamental level, marine life helps determine the nature of our planet. Marine organisms contribute to the oxygen cycle, are involved in the regulation of the Earth's climate. Shorelines are in part shaped and protected by marine life, some marine organisms help create new land. Many species are economically important to humans, including shellfish, it is becoming understood that the well-being of marine organisms and other organisms are linked in fundamental ways. The human body of knowledge regarding the relationship between life in the sea and important cycles is growing, with new discoveries being made nearly every day; these cycles include those of matter and of air.
Large areas beneath the ocean surface still remain unexplored. The study of marine biology dates back to Aristotle, who made many observations of life in the sea around Lesbos, laying the foundation for many future discoveries. In 1768, Samuel Gottlieb Gmelin published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the new binomial nomenclature of Linnaeus, it included elaborate illustrations of seaweed and marine algae on folded leaves. The British naturalist Edward Forbes is regarded as the founder of the science of marine biology; the pace of oceanographic and marine biology studies accelerated during the course of the 19th century. The observations made in the first studies of marine biology fueled the age of discovery and exploration that followed. During this time, a vast amount of knowledge was gained about the life that exists in the oceans of the world. Many voyages contributed to this pool of knowledge. Among the most significant were the voyages of HMS Beagle where Charles Darwin came up with his theories of evolution and on the formation of coral reefs.
Another important expedition was undertaken by HMS Challenger, where findings were made of unexpectedly high species diversity among fauna stimulating much theorizing by population ecologists on how such varieties of life could be maintained in what was thought to be such a hostile environment. This era was important for the history of marine biology but naturalists were still limited in their studies because they lacked technology that would allow them to adequately examine species that lived in deep parts of the oceans; the creation of marine laboratories was important because it allowed marine biologists to conduct research and process their specimens from expeditions. The oldest marine laboratory in the world, Station biologique de Roscoff, was established in France in 1872. In the United States, the Scripps Institution of Oceanography dates back to 1903, while the prominent Woods Hole Oceanographic Institute was founded in 1930; the development of technology such as sound navigation ranging, scuba diving gear and remotely operated vehicles allowed marine biologists to discover and explore life in deep oceans, once thought to not exist.
As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for all the photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen and other nutrients and trace elements. Microscopic life undersea is diverse and still poorly understood. For example, the role of viruses in marine ecosystems is being explored in the beginning of the 21st century; the role of phytoplankton is better understood due to their critical position as the most numerous primary producers on Earth. Phytoplankton are categorized into cyanobacteria, various types of algae, dinoflagellates, coccolithophorids, chrysophytes, chlorophytes and silicoflagellates. Zooplankton tend to be somewhat larger, not all are microscopic. Many Protozoa are zooplankton, including dinoflagellates, zooflagellates and radiolarians; some of these are phytoplankton.
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, 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. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts 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 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 common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, 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 a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: 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 settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. 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, they are no longer classified as plants as defined here; 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, they undergo closed mitosis without centrioles, 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 and Glaucophyta 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 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 not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in 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