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The nitrogen cycle has important implications for air quality and climate in addition to fisheries and marine ecosystems. Anthropogenic activities have perturbed the natural nitrogen cycle, with major effects on the atmosphere and marine systems.…

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from article Atmospheric chemistry

Components of the carbon cycle showing the movement of carbon between land, the atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. (NASA:…

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from article Atmospheric chemistry

Processes that determine aerosol concentrations.

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from article Anemia

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from article Anemia

Aplastic anemia. This low-power photomicrograph of the bone marrow exhibits only a small amount of hematopoiesis (the process by which the cellular elements of the blood are formed). (Copyright © McGraw-Hill Education)

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from article Anemia

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from article Forest engineering

A harvesting machine removing selected trees in a thinning operation designed to enhance the growth of the trees that remain after the operation. The “processing head” is holding a horizontal tree stem that is being delimbed and processed into logs…

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from article Forest engineering

A well-engineered forest road and timber bridge in the Tongass National Forest in southeast Alaska. The bridge, designed to support heavy logging trucks, was used rather than a culvert to reduce erosion and protect water quality. (Photograph…

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from article Pomegranate

Pomegranate (Punica granatum). (Photo courtesy of the Agricultural Research Service Germplasm Repository)

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from article Life forms of plants

Life forms of plants according to C. Raunkiaer. (After P. Dansereau, Biogeography: An Ecological Perspective, Ronald Press, 1957)

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from article Life forms of plants

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from article Echolocation

Echolocation is a physiological tool used by gray bats (Myotis grisescens). (Credit: Amareta Kelly)

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from article Echolocation

Sound spectrograms representing sonar signals of (a) Brazilian or Mexican free-tailed bats (Tadarida brasiliensis), (b) big brown bats (Eptesicus fuscus), (c) Wagner's mustached bats (Pteronotus personatus), and (d) greater horseshoe bats…

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from article Chevrotain

Water chevrotain (Hyemoschus aquaticus). (Photo by Brent Huffman; courtesy of Ultimate Ungulate)

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from article Dinosauria

Titanosaurs (based on Rapetosaurus) in the nesting ground at Auca Mahuevo, Argentina. (Credit: Mark Hallett)

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from article Dinosauria

Partly excavated dinosaur fossils. (Credit: Bert Beekmans/Getty Images)

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from article Dinosauria

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from article Dinosauria

Relationships of the ornithischian dinosaurs. (Credit: Modified from an original diagram by P. C. Sereno)

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from article Dinosauria

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from article Dinosauria

A Triceratops fossil skull. (Credit: David Herraez/Alamy Stock Photo)

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from article Dinosauria

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from article Dinosauria

Computer-generated Allosaurus dinosaur skeleton. This saurichian carnivore could attain a height of 8.5 m (28 ft). (Credit: Science Photo Library/Alamy Stock Photo)

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from article Dinosauria

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from article Dinosauria

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from article Cat

Puma (Puma concolor) and cub in forest. (Credit: Grant Faint/Stockbyte/Getty Images)

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from article Cat

A domestic cat (Felis catus) nursing her kittens. (Credit: Westend61/Getty Images)

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from article Cat

Canadian lynx (Lynx canadensis). (Credit: Erwin and Peggy Bauer/U.S. Fish and Wildlife Service)

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from article Cat

Ocelot (Leopardus pardalis). (Credit: Tom Smylie/U.S. Fish and Wildlife Service)

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from article Cat

Two cheetahs (Acinonyx jubatus). (Credit: Gary M. Stolz/U.S. Fish and Wildlife Service)

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from article Cat

Lion (Panthera leo). (Credit: Trisha M. Shears)

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from article Cat

Siberian tiger (Panthera tigris). (Credit: John and Karen Hollingsworth/U.S. Fish and Wildlife Service)

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from article Agricultural machinery

A tractor pulling a multipurpose machine used for precision tillage and planting. (Credit: Scott Bauer/U.S. Department of Agriculture)

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from article Agricultural machinery

Agricultural aircraft applying a chemical pesticide on a field. (Credit: jupiterimages/Getty Images)

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from article Agricultural machinery

A combine harvester hopper filling up with corn kernels as the machine harvests the field. (Credit: Glow Images)

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from article Agricultural machinery

A harvester machine is used to drop tomatoes into a container. (Credit: Phil Schermeister/National Geographic RF/Getty Images)

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from article Lemming

The lemming, a small burrowing rodent with short legs and stout claws adapted for digging. (Photo by Robert R. Nack, Minnesota Cooperative Fish and Wildlife Research Unit)

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from article Hydrosphere

Dissolved (a) sodium and (b) sulfate in rain (in parts per million) over the continental United States (After R. M. Garrels and F. T. Mackenzie, Evolution of Sedimentary Rocks, W. W. Norton, 1971)

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from article Hydrosphere

Accommodation of cations in aqueous solution. (After H. Blatt, G. Middleton, and R. Murray, Origin of Sedimentary Rocks, Prentice Hall, 1972)

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from article Hydrosphere

Distribution of dissolved gaseous oxygen (low values) and nutrient species (high concentrations) of orthophosphate ions at 26°22′.4N and 168°57′.5W in the Pacific Ocean. 1 m = 3.3 ft.

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from article Saffron

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from article Red Sea

Division of the Red Sea into regions representing different stages in the formation of an ocean basin. (After F. Martinez and J. R. Cochran, Structure and tectonics of the northern Red Sea: Catching a continental margin between rifting and drifting,…

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from article Deforestation

View of an area of deforestation in Central Kalimantan, Indonesia. In this case, the land is being cleared to develop areas for intensive rice production. (Credit: Australian National University)

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from article Deforestation

The relationship between forest and nonforest (other) land. Preservation of forest lands includes reforestation and natural regeneration. However, deforestation and natural events can lead to nonforest land. Afforestation (the act or process of…

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from article Deforestation

Natural forest areas (in hectares; 1 hectare = 2.47 acres) in various climatic domains (boreal/polar, temperate, subtropical, and tropical) from 1990 to 2015. The data are derived from the Global Forest Resources Assessment 2015 by the Food and…

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from article Deforestation

Former rainforest in northern Queensland, Australia, that was deforested in the early 1900s and is now used for pasture. (Credit: Rodney Keenan)

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from article Deforestation

Aerial view of deforestation in the Amazon region of Brazil. Clearing of trees in the Amazon rainforest has resulted in the elimination of natural resources and numerous species. (Credit: Luoman/Getty Images)

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from article Basalt

Thin section of upper crust of lava about 1.2 in. (3 cm) from top; black is microcrystalline groundmass. 0.5 mm = 0.02 in.

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from article Basalt

Thin section of interior of lava shown in Fig. 1, but 26 ft (7.9 m) below the top of the flow. Clear plagioclase crystals are ophitically enclosed in large pyroxene grains up to 0.04 in. (1 mm) in diameter. Crystals of black magnetite are minor. A…

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from article Basalt

Growth of crystals as a function of temperature in basalt in Hawaiian lava. °F = (°C × 1.8) + 32. (After D. L. Peck, T. L. Wright, and J. G. Moore, Crystallization of tholeiitic basalt in Alae lava lake, Hawaii, Bull. Volcanol., 29:629–656, 1966)

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from article Basalt

Diagrams illustrating the compositions of basalts in terms of normative mineral formulas. Customarily, CaAl2Si2O8 (anorthite) is grouped together with NaAlSi3O8 (albite), and Fe2SiO4, FeSiO3, and CaFeSi2O6 are grouped with the corresponding Mg…

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from article Symphyla

Scutigerella immaculata. This symphylan is a garden and greenhouse pest. (Photo by Ivo Antušek; BioLib.cz)

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from article Symphyla

Cercus (enlarged view). (Courtesy of Robert T. Allen, Academy of Natural Sciences, Philadelphia; Amanda Lawrence, Electron Microscope Center, Mississippi State University)

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from article Cestida

Structure of Cestum veneris.

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from article Ordovician

Ordovician, the second period of the Paleozoic Era.

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from article Ordovician

Early Ordovician (Arenig) paleogeography, major rock suites and potential ocean surface currents; based on remanent magnetism data supplemented by plots of the positions of shelf sea carbonates and reefs.

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from article Soil chemistry

Proper soil chemistry helps a green seedling to sprout out from the soil. Soil chemistry controls the availability of plant nutrients within the soil and thus influences plant growth. (Credit: amenic181/Shutterstock)

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from article Soil chemistry

Extensive chemical weathering can remove essential minerals and elements from soil. Typically, only the most insoluble compounds remain in the soil. For example, iron oxides may persist, imparting a reddish-brown color to the soil. (Credit: Stephen…

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from article Soil chemistry

Factors influencing the chemistry of a soil solution. (Credit: Based on an original illustration created by J. F. Hodson)

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from article Walnut

Pith of two species of Juglans: (left) butternut or white walnut (J. cinerea); (right) black walnut (J. nigra). (Photo by Bill Cook, Michigan State University; Bugwood.org)

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from article Walnut

Black walnut (Juglans nigra). (Photo by D. E. Herman, North Dakota State Soil Conservation Committee, U.S. Department of Agriculture–Natural Resources Conservation Service PLANTS Database)

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from article Walnut

Fruits of black walnut (Juglans nigra). (Photo by Rob Routledge, Sault College, Sault Ste. Marie, Ontario, Canada; Bugwood.org)

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from article Walnut

English or Persian walnut (Juglans regia). (Photo by Milan Kořínek)

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from article Walnut

Kernel of English walnut (Juglans regia). (Photo by Piero Amorati, IC Croce, Casalecchio di Reno, Italy; Bugwood.org)

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from article Walnut

Walnut leaf showing symptoms of anthracnose (caused by the fungus Gnomonia leptostyla). (Photo by Pavel Šinkyřík, Czech Republic)

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from article Walnut

Canker disease (caused by the fungus Sirococcus clavigignenti-juglandacearum) on a butternut (Juglans cinerea). (Photo by Mike Ostry, U.S. Department of Agriculture Forest Service)

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from article Thrombosis

Organized thrombus in an artery with recanalization.

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from article Thrombosis

Organized thrombus in an artery with scar formation; the artery is completely occluded by the fibrotic mass.

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from article Cephalochordata

European lancelet (Branchiostoma lanceolatum). (After J. E. Webb, On the populations of Branchiostoma lanceolatum and their relations with the West African lancelets, Proc. Zool. Soc. Lond., 127:131, 1956)

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from article Clover

Red clover (Trifolium pratense). (Photo by Tom Heutte, U.S. Department of Agriculture Forest Service)

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from article Clover

White clover (Trifolium repens). (Photo by Pavel Šinkyřík)

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from article Clover

Crimson clover (Trifolium incarnatum). (Photo by Pavel Šinkyřík)

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from article Clover

White sweet clover (Melilotus albus). (Photo by Forest and Kim Starr)

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from article Clover

Leaves of clover affected by leaf spot, which is caused by the fungus Pseudopeziza trifolii. (Photo courtesy of the U.S. Department of Agriculture Cooperative Extension Slide Series, Clemson University)

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from article Rhinoceros

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from article Ferret

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from article Siphonostomatoida

Siphonostomatoida. (a) Representative siphonostomatoid mandible; (b) brachiform maxilla (after Z. Kabata, Parasitic Copepoda of British Fishes, vol. 152, Ray Society, London, 1979). (c) Representative female caligid; (d) female Pennella (after P. A.…

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from article Holothuroidea

Cucumaria, a representative holothurian.

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from article Holothuroidea

Holothurian anatomy.

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from article Granite

Classification of granitic rocks by the IUGS scheme. All proportions are by volume percentages.

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from article South America

Map of South America.

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from article South America

Andes Mountains, Chile. The Andes, which parallel the Pacific coast, constitute one of the Earth's most majestic features. Only the Himalayas exceed them in elevation and mass. (United Press International)

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from article South America

Representative view of the hilly uplands and low mountain country of the Guiana Highlands. One of five waterfalls on the Canaima River, Gran Sabana in southern Venezuela. (Venezuelan Information Service)

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from article South America

Uplifted beaches on desert coast, east side of San Juan Bay, Departmento de Ica, Peru. The sea cliff is about 100 ft (30 m) high. Each horizontal line represents a former beach. (Photograph by F. Atchley)

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from article South America

Port of Matarani on the desert coast of southern Peru. This oblique aerial view looks coastwise southward over one of the world's driest and most barren coastal zones. (Servicio Aerofotográfico Nacional)

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from article Zosterophyllopsida

Examples of Devonian Zosterophyllopsida. (a) Reconstruction of the Early Devonian Zosterophyllum rhenanum, here depicted as an aquatic plant (after R. Kräusel and H. Weyland, Neue Pflanzenfunde im rheinischen Unterdevon, Palaeontographica B,…

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from article Sexually transmitted diseases

Acquired immune deficiency syndrome (AIDS) is a sexually transmitted disease caused by the human immunodeficiency virus (HIV). In this image, a human T cell (blue) is under attack by HIV (yellow), hampering the effectiveness of the body's immune…

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from article Sexually transmitted diseases

Prevalence of Chlamydia trachomatis (the most-common sexually transmitted bacterial pathogen) among individuals aged 14 to 39. The prevalence varies by sex, race/ethnicity, and age. Error bars indicate 95% confidence intervals. (Credit: U.S. Centers…

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from article Sexually transmitted diseases

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from article Bytownite

Bytownite, from Crystal Bay, Minnesota. (Specimen from Department of Geology, Bryn Mawr College)

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from article Calanoida

Diaptomus. (a) Lateral view of male. (b) Dorsal view of female.

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from article Ophiuroidea

Representative ophiuroids, which have a quite distinctive shape. (a) Ophiopeza cylindrica (alternatively, Pectinura cylindrica). (b) Asteroporpa australiensis (alternatively, Astroporpa wilsoni). (c) Gorgonocephalus chilensis.

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from article Ophiuroidea

External features of the ophiuroid Amphiura. (a) Aboral view. (b) Adoral view.

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from article Tetanus

Photomicrograph showing endospores forming at the ends of Clostridium tetani bacterial cells, which are shaped like small tennis rackets. Clostridium tetani is the causative agent of tetanus. (Credit: Lisa Burgess; copyright © McGraw Hill)

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from article Syphilis

Syphilis is caused by Treponema pallidum, which is a corkscrew-shaped motile bacterium (spirochete). (Credit: National Institute of Allergy and Infectious Diseases, National Institutes of Health)

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from article Malacostraca

Caridoid facies. (After E. R. Lankester, ed., A Treatise on Zoology, pt. 7, fasc. 3, A. and C. Black, 1909)

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from article Malacostraca

Representative malacostracan thoracic appendages: (a) biramous ambulatory thoracic appendage of an Anaspidacea; (b) uniramous thoracic walking leg of a Stenopodidea; (c) first maxilliped of a Paguroidea; (d) third maxilliped of a Thalassinoidea; (e)…

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from article Ornamental plants

Colorful ornamental plants in a garden. (Credit: Getty Images/iStockphoto)

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from article Pauropoda

A typical pauropod (genus Allopauropus) with bifurcate antennae, 9 pairs of legs, and a well-developed anal pygidium. The anterior part of the animal is located toward the top right-hand corner of the image. (Photo by Andy Murray)

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from article Rabies

Negri bodies (cytoplasmic inclusion bodies) in infected nerve cells in a dog brain are characteristic cellular changes that occur as a result of rabies encephalitis. (Credit: G. Heid/Centers for Disease Control and Prevention)

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from article Rabies

Schematic model of a rabies virus showing the surface glycoprotein spikes (projections) extending from the lipid envelope that surrounds the internal nucleocapsid and the matrix protein lining the envelope. The nucleocapsid comprises the single RNA…

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