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The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence.…

Typically, an aquatic vertebrate with scales, gills, and paired fins. Fishes (Fig. 1) are found throughout the world's waterways, from saltwater oceans to freshwater streams and ponds. About 35,000 species of fishes have been described to date. Many…

The use of pollen and spore evidence in legal cases. In its broader application, the field of forensic palynology also includes legal information derived from the analysis of a broad range of microscopic organisms that can be found in both fresh and…

A space-traversing unmanned vehicle which contains the means for controlling its flight path and that delivers an explosive warhead with high accuracy. Various kinds of missiles are guided in the sense that they can aerodynamically adjust after…

A subdiscipline of paleobiology that investigates the processes of preservation and their influence on information in the fossil record. Taphonomy is the study of fossilization and considers the processes at work in the incorporation of living…

A plant, Musa textilis, of the banana family that is valuable for its hard fiber (also known as Manila hemp). Abaca (Musa textilis) is a flowering plant native to the Philippines and Borneo. Taxonomically, it is assigned to the order Zingiberales.…

A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C15H20O4 (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various…

An informal assemblage of predominantly aquatic eukaryotes that carry out oxygen-evolving photosynthesis, but lack specialized water-conducting and food-conducting tissues. Algae (Fig. 1) are eukaryotic organisms (having an organized nucleus).…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. The time necessary to transform resin…

A genus of fossil birds in the extinct order Archaeopterygiformes, characterized by flight feathers like those of modern birds. Archaeopteryx is generally accepted as the oldest known fossil bird. It represents a unique snapshot of evolution; most…

Any of a class of plant growth substances (phytohormones or plant hormones) that have an essential role in coordination of many growth and developmental processes during the life cycle of plants, especially the ability to increase the rate of shoot…

A class of nonflagellate unicellular algae, commonly called diatoms, with boxlike silicified walls. Members of the algal class Bacillariophyceae are commonly termed diatoms (Fig. 1). Diatoms range in maximum dimension from 4 μm (0.00004 in.) to more…

Any of several leguminous plants, or their seeds, long utilized as food by humans or livestock. There are more than 40,000 varieties of beans, with only a small number of these being mass-produced for human or animal consumption. In particular,…

The drug as well as the plant known as deadly nightshade (Atropa belladonna). The belladonna plant (Atropa belladonna; see illustration) belongs to the nightshade family, Solanaceae, in the order Solanales. The plant is a coarse and poisonous…

Fungi that exhibit the natural phenomenon of bioluminescence (the ability of an organism to emit visible light). Bioluminescence is the emission of photons of visible light by living organisms via the chemical reaction of molecular oxygen (O2) with…

A garden for the culture of plants collected chiefly for scientific, educational, and esthetic purposes. A botanical garden (see illustration) is an establishment in which various plants, including trees, are collected, cultivated, and grown for…

The branch of biological science that embraces the study of plants and plant life. Botany is the scientific study of plants (see illustration), encompassing investigations of plant anatomy, physiology, genetics, phylogeny, ecology, pathology, and…

A division that consists of small and relatively simple plants commonly known as mosses, peat mosses, granite mosses, liverworts, and hornworts. Members of the division Bryophyta are referred to as bryophytes (see illustration). These nonvascular…

A genus comprising yeasts that cause a variety of human diseases referred to as candidiasis. All Candida species are single-celled yeasts (fungal organisms) that are ovoid in shape and 3–5 μm in size, and they multiply by a process called budding.…

An order of flowering plants (angiosperms) including the mustard and caper families. The plant order Capparales contains various families of eurosid dicotyledons. Most notably, mustards, capers, and various ornamental plants (see illustration) are…

A member of a class of common pigments, typically of yellow, orange, or red hues, that serve in both light absorption and the protection against too much light in photosynthesis as well as in the vision process. Carotenoids comprise a class of…

A class in the phylum Mollusca that is exclusively marine and includes squids, octopuses, and cuttlefishes. The molluscan class Cephalopoda is one of the major taxonomic groupings of marine animals and contains both extant members—squids (Fig. 1),…

An order of the class Reptilia, including the turtles, terrapins, and tortoises; also termed Testudines. The order Chelonia (often referred to as Testudines) comprises one of the oldest groups of reptiles on Earth. Members of the Chelonia…

A type of sweet or sour fruit of the Prunus genus, as well as the tree that bears the fruit. Cherry trees belong to the Prunus genus in the order Rosales, and the sweet and sour fruits of these trees are called cherries (Fig. 1). There are…

A large palm, Cocos nucifera, widely grown throughout the tropics and valuable for its fruit and fiber. The coconut (Cocos nucifera) belongs to the plant family Arecaceae. The name coconut can refer to the tree and its fruit. Usually found near the…

Any of various tropical trees belonging to the genus Cola. Members of the genus Cola (family Malvaceae, order Malvales), which are native to tropical Africa, are called cola trees (see illustration). In particular, two species, C. acuminata and C.…

Any living or fossil member of the plant division Pinophyta, consisting chiefly of woody trees or shrubs that produce cones and evergreen needles. Conifers are predominantly cone-bearing, evergreen trees and shrubs belonging to the order Pinales, as…

A fiber obtained from the cotton plant Gossypium. The cotton plant (genus Gossypium, order Malvales; Fig. 1) has been used by humans for more than 5000 years. The fiber (cotton) obtained from this plant is the most widely used natural fiber because…

A woody higher plant that regularly loses its leaves at the end of each growing season. Deciduous plants drop (shed) their leaves seasonally, most commonly during the fall (autumn). In general, the dropping of leaves occurs at the inception of an…

A major clade or natural group of plants, comprising the land plants. The members of the Embryobionta or Embryophyta are commonly termed embryobionts or embryophytes, and they constitute the land plants. Embryophytes form a well-marked clade (a…

A volatile, fragrant oil obtained from plants. Essential oils are distinguished from those known as fixed oils, which are mainly triglycerides of fatty acids. Essential oils have been obtained from thousands of plants (Fig. 1) and are designated and…

A former order of rosid flowering plants, division Magnoliophyta (Angiospermae), in the class Magnoliopsida (dicotyledons). The order Euphorbiales consisted of the large family Euphorbiaceae (with about 7500 species) and 3 small satellite families…

A plant that retains its green foliage throughout the year. An evergreen plant, or simply evergreen, is a tree, shrub, or plant whose leaves are not shed in the winter and instead remain green year-round. In contrast, deciduous trees and plants…

The death and disappearance of a species. The extinction of a species usually represents an end point in a long series of population extinctions. During the extinction process, unique evolutionary history is lost at every stage, but the death of the…

A plant grown for its fibrous material, which is used for many commercial purposes and for home industry. Many plants are grown as fiber crops because of their content or yield of fibrous material (see illustration), which is used in the manufacture…

The segment of horticulture concerned with the commercial production, marketing, and sale of plants grown and used for decorative or display purposes. Floriculture is a branch of horticulture, that is, the art and science of growing plants,…

A record of earlier life buried in rock. Originally meaning any distinctive object that has been dug up (from Latin fodio, dig), the term “fossil” soon came to refer particularly to things resembling animals and plants. These objects were known…

A hypersensitivity or altered reactivity caused by fungal allergens. More than 20% of the population in industrialized countries suffers from allergic disease, and diverse allergens (substances foreign to the body that cause allergy) associated with…

A member of a family of diterpenoid carboxylic acids that are notable for being plant hormones. Gibberellins are diterpenoid carboxylic acids. Most of them possess ent-gibberellane or ent-20-norgibberellane carbon (C) skeletons. Gibberellins occur…

The common name for plants of the genus Vitis characterized by climbing stems and grown for the edible, pulpy berries. Grapes (genus Vitis) comprise one of the world's most significant and economically important crops. The common grapevine is…

An evergreen tree (Citrus paradisi) and its edible citrus fruit. Grapefruit (Citrus paradisi) is a subtropical evergreen tree belonging to the order Sapindales. It has a well-rounded top and is cultivated for its large and globose citrus fruit. The…

Any member of the family Poaceae (Gramineae) cultivated as forage and grain for consumption. Grasses are arguably the most useful of all the plants that cover the Earth. The cereal grass crops [wheat (Fig. 1), rice, corn (maize), rye, barley, oats,…

A morphologically distinct cell that is found in the epidermis of terrestrial plant shoots and that functions in regulating stoma size. Guard cells are paired, crescent-shaped epidermal cells that regulate water loss and gas exchange in terrestrial…

Optical instruments that establish an optical line or axis for the purpose of aiming a weapon. The axis includes the observer's eye, a suitable mark in the instrument, and the target. For a pistol, the traditional iron sights employ a rear notch and…

A class of lower green plants called liverworts that belong to the division Bryophyta. The class Hepaticopsida, also called Marchantiopsida, comprises the liverworts, which are nonvascular land plants. Although there is a great diversity of external…

The collective techniques for supplying nutrients and water directly to the roots of plants, without soil or other media. Hydroponics is concerned with the cultivation and growing of plants without soil or other media. Hydroponic systems (Fig. 1)…

Intentional generation of interfering signals in the electromagnetic spectrum by powerful transmitters as a countermeasure intended to block a communication, radar, or infrared system or to impair its effectiveness appreciably. Radio broadcasts or…

The common name for a member of the agnathan fishes. The fish superclass Agnatha (subphylum Vertebrata, phylum Chordata) comprises the jawless vertebrates (agnathans). Members include cyclostomes (modern lampreys and hagfishes), various extinct…

An order of flowering plants composed of seven eumagnoliid families of tropical tree and shrub species. The plant order Laurales, with approximately 2500–2800 species, contains numerous trees and shrubs having ecological significance. Members of the…

Citrus limon, a small evergreen tree belonging to the order Sapindales that is cultivated for its fruit. In commercial terms, the lemon (Citrus limon; order Sapindales) is the most important of the acid citrus fruits (see illustration). It probably…

One of the largest orders of the rosid eudicotyledons. The order Malpighiales is an important flowering plant group with a worldwide distribution. Depending on the classification, it contains approximately 35 to 40 families, totaling about 16,000…

An order of flowering plants in the core eudicots. The plant order Malvales consists of 10 families and approximately 6000 species (see illustration). The circumscription of the order Malvales has been altered greatly as a result of deoxyribonucleic…

A lineage of true elephants in the extinct genus Mammuthus that occupied Eurasia and North America in the late Cenozoic Era. Mammoths (genus Mammuthus) are extinct elephants assigned to the mammalian order Proboscidea (Fig. 1). They inhabited…

The widespread, catastrophic loss of many species in a relatively short period of geological time. Earth has supported life for approximately 3.5 billion years, and it is normal for species to become extinct and for new species to arise through…

A major phylum of the animal kingdom comprising a diverse group of unsegmented, soft-bodied invertebrates. The phylum Mollusca is one of the most significant groupings of invertebrate animals. Members of the Mollusca, termed mollusks or molluscs,…

A mammalian order containing the egg-laying platypus and echidnas. Members of the order Monotremata (monotremes) are egg-laying mammals belonging to the subclass Prototheria (in some alternative classifications, the monotremes have been assigned to…

A macroscopic fungus with a fruiting body. It has been estimated that 1.5 million to 5 million species of fungi exist on Earth. Yet, only about 150,000 species have been described to date. Approximately 50,000 fungal species are considered…

Weapons that arm warships. Of the wide variety of such weapons, three important categories are exclusively naval: antiship missiles, torpedoes, and mines. Other weapons all correspond to land-based types, but submarine-launched ballistic missiles…

An order of flowering plants that have previously been included in the subclass Magnoliidae. Deoxyribonucleic acid (DNA) sequence studies have demonstrated that Nymphaeales as previously defined contain two families, Ceratophyllaceae (water…

A genus (Quercus) of trees, some of which are shrubby, with about 200 species, mainly in the Northern Hemisphere. About 50 species of oaks are native in the United States. All oaks have scaly winter buds, usually clustered at the ends of the twigs,…

An agricultural cereal crop of the genus Avena grown for its grain and straw. Oats (Fig. 1) belong to the genus Avena in the family Poaceae (Gramineae) of the order Poales (Cyperales). The most important crop species is Avena sativa, known as the…

A small extinct order of articulated brachiopods that ranges in age from Early to Middle Cambrian and includes the earliest known calcitic brachiopods. Classification The order Obolellida is included within the class Obolellata, subphylum…

An order of coleoid cephalopods, characterized by eight appendages (which encircle the mouth), a saclike body, and an internal shell that is much modified or reduced from that of its ancestors. Members of the order Octopoda (subclass Coleoidea,…

An order of filamentous freshwater green algae (Chlorophyceae) with unique morphological features including (1) an elaborate method of cell division that results in the accumulation of apical caps, (2) zoospores and antherozoids with a subapical…

A morphologically distinct type of trace fossil left by boring organisms in hard biogenic substrates. Oichnus Bromley 1981 is a formal taxonomic name with the rank of ichnogenus (a trace fossil genus). Oichnus is a Latin-derived term that…

A suborder of trilobites (a class of extinct arthropods) and a diverse clade consisting of roughly 100 distinct species. The trilobites constituting the suborder Olenellina belong to the order Redlichiida in the class Trilobita. These extinct…

A cool-season biennial, Allium cepa, that is cultivated for its edible bulb. The onion is of Asiatic origin and belongs to the plant order Asparagales; it was formerly included in the order Liliales. Related species are leek (A. porrum), garlic (A.…

A class of the subphylum Asterozoa (phylum Echinodermata), comprising the brittle stars and basket stars, in which the arms are usually clearly demarcated from a central disk and perform whiplike locomotor movements, and the tube feet are…

Any of various evergreen trees of the genus Citrus, cultivated for its edible fruit. Orange trees (genus Citrus) belong to the family Rutaceae of the order Sapindales. The various trees classified as orange trees are most notable for producing…

Any member of the plant family Orchidaceae, having complex, specialized irregular flowers usually with only one or two stamens. The orchid family, Orchidaceae, is among the largest families of plants and is estimated to contain more than 25,000…

An order of flowering plants, division Magnoliophyta (Angiospermae), in the order Asparagales (monocotyledons). The order Orchidales consists of four families: the Orchidaceae, with 20,000–25,000 species; the Burmanniaceae, with more than 100…

An herb, also known as wild marjoram. The dried leaves of several species of aromatic plants are known as oregano; thus, oregano is a common name for a general flavor and aroma rather than the name of a specific plant. MarjoramEuropean oregano…

A plant that is grown and used for decorative or display purposes. The use of ornamental plants (also termed ornamentals) [see illustration] developed early in history for it is known that primitive peoples planted them near their dwellings. There…

The genus Maclura of the mulberry family, with one species, M. pomifera. The osage orange tree may attain a height of 60 ft (18 m) and has yellowish bark, milky sap, simple entire leaves, strong axillary thorns, and aggregate green fruit that are…

A taxonomic fish group comprising the bony fishes. The Osteichthyes grouping (often considered a superclass) consists of the classes Actinopterygii (ray-finned fishes) and Sarcopterygii (lungfishes and lobefins, or coelacanths, but excluding…

Fossil lobe-finned (sarcopterygian) fishes of Devonian to Permian age (395–270 million years ago), the ancestral group for the land vertebrates, or “tetrapods.” Phylogeny The term “osteolepiforms” is applied to a range of fossil fishes, found…

An order of extinct jawless vertebrate fishes, also called Cephalaspida, known from the Middle Silurian to Upper Devonian of Europe, Asia, and North America. They were mostly small, about 2 in. (5 cm) to 2 ft (60 cm) in length. The head and part of…

A major taxon of the Crustacea containing small bivalved animals 0.004–1.4 in. (0.1–33 mm) long, with most between 0.04 and 0.08 in. (1 and 2 mm). Members of the Ostracoda inhabit aquatic environments in nearly all parts of the world.…

A popular name applied to several groups of extinct jawless vertebrates (fishes). Most ostracoderms were covered with an external skeleton or armor of bone, from which is derived their name, meaning “shell-skinned” (see illustration). They are known…

An order of flowering plants (angiosperms) in the eurosid I group of the rosid dicots. The order is previously unrecognized in classifications of the angiosperms, but is indicated by numerous studies of DNA sequences. Oxalidales consist of five…

A peculiar arthropod, evidently related to the Pycnogonida and represented by a number of well-preserved fossils from the Devonian Hunsrück shales. Palaeoisopus was formerly considered by a number of paleontologists to have an anterior jointed…

A large, extinct assemblage of primitive ray-finned (actinopterygian) fishes known from the Silurian to Cretaceous, which were most numerous in Carboniferous and Permian times. They are known as fossils from all continents except Antarctica.…

A tiny fossil fish found only in rocks of Middle Devonian age within a small geographic region of Caithness, Scotland, and principally at one site, Achanarras Quarry, where it is surprisingly abundant. One of the most enigmatic of all fossil fishes,…

The study of chemical processes used by organisms that lived in the geological past. Most information on the nature of life in the geological past comes from the study of fossils; a record of biochemical processes that occurred can be found in the…

The study of pollen grains and spores, both extant and extinct, as well as other organic microfossils. Although the origin of the discipline dates back to the seventeenth century, when modern pollen was first examined microscopically, the term…

An order of monocotyledons, the composition of which only recently has been revealed by deoxyribonucleic acid (DNA) sequence studies of four genes. Included are four families, two of which have been generally viewed as closely related: Pandanaceae…

A fossil lobe-finned (sarcopterygian) fish from rock strata of the Baltic region dating to the Middle–Late Devonian Period (about 372–368 million years ago), which provides key evidence about the evolutionary transition from fish to land vertebrates…

An extinct order of relatively large placental mammals represented by Paleocene-Eocene fossils from western Europe, North America, and eastern Asia. Pantodonts were an early evolutionary experiment in large-bodied herbivory by primitive placental…

An order of flowering plants, division Magnoliophyta (Angiospermae), in the subclass Magnoliidae of the class Magnoliopsida (dicotyledons). The order Papaverales consists of two families: Papaveraceae, with approximately 200 species; and…

Carica papaya, a semiwoody, usually single-stemmed, herbaceous plant cultivated for its fruit in tropical and subtropical regions. The papaya plant (Fig. 1) originated along the Caribbean lowlands of Mesoamerica and is now extensively and…

A medium-sized class of extinct blastozoan echinoderms containing about 17–19 genera, ranging from the late Early Ordovician to the Early Silurian throughout North America and in northern Europe. Their maximum standing diversity (13 genera) occurred…

An annual leafy leguminous vine, Pisum sativum (order Fabales), cultivated for its smooth or wrinkled, round edible seeds, which are borne in dehiscent pods. The garden pea (Fig. 1) is one of the world's oldest cultivated crops. It is a native of…

A deciduous fruit tree species, Prunus persica, of the order Rosales. The peach (see illustration), taxonomically known as Prunus persica (order Rosales), is a notable fruit tree species that originated and was first cultivated in western China. It…

An order of herbivorous, odd-toed, hoofed mammals, including the living horses, zebras, donkeys, rhinoceroses, tapirs, and their extinct relatives. Members of the order Perissodactyla, or perissodactyls, are odd-toed ungulates (mammals with hooves).…

A long-season cultivar (cultivated variety) of muskmelon, Cucumis melo, of the gourd family. Persian melons (see illustration) are members of the family Cucurbitaceae (which includes melons, squashes, pumpkins, and cucumbers) in the order…

A deciduous fruit tree species, Diospyros kaki, in the ebony family (Ebenaceae). Persimmon trees (Fig. 1) originated in the subtropical regions of China, but were cultivated more extensively in Japan. At the present time, China, South Korea, and…

A concentration of appreciable numbers of petrified tree trunks exposed at the earth surface, standing upright in the position of growth, lying prostrate, or both. Petrifaction is a form of mineral emplacement or embedding, by which the organic…

A class of plants, commonly called brown algae, in the chlorophyll a-c phyletic line (Chromophycota). Brown algae occur almost exclusively in marine or brackish water, where they are attached to rocks, wood, sea grasses, or other algae.…

The principal food-conducting tissue in vascular plants. The long-distance transport of nutrients in plants occurs in the phloem, which is a highly specialized tissue that links all the organs of the plant. The conducting cells of phloem are known…

The first step in the long-distance transport of nutrients in plants. Phloem loading also provides the driving force for this transport. Why is long-distance transport needed? Although vascular plants produce their own organic molecules by…