R

r selection Selection favoring rapid rates of reproduction and growth, especially among species that specialize in colonizing short-lived habitats.

r-strategist Species of organism that uses a survival and reproductive "strategy" characterized by high fecundity, high mortality, short longevity. Populations are controlled by density-independent factors.

raceme A vertical inflorescence with stalked flowers.

rachis Central, longitudinal, supporting structure in the ovary of some nematodes.

radial canals. Canals along the ambulacra radiating from the ring canal of echinoderms; also choanocyte-lined canals in syconoid sponges.

radial cleavage. Embryonic development in which early cleavage planes are symmetrical to the polar axis, each blastomere of one tier lying directly above the corresponding blastomere of the next layer; indeterminate cleavage.

radial cut A longitudinal section cut along a radius.

radial micellation Reinforcement of the cell wall of guard cells in specific regions such that the cells curve outward when fully turgid.

radial symmetry. A morphological condition in which the parts of an animal are arranged concentrically around an oral-aboral axis, and more than one imaginary plane through this axis yields halves that are mirror images of each other.

radial A spoke or ray.

radiation A form of energy that includes visible light, ultraviolet light, and X rays; means by which body heat is lost in the form of infrared rays.

radicle The primary root of a germinating seed.

radioactive An unstable isotope that decays spontaneously and releases subatomic particles or units of energy.

radioactive isotope An unstable form of an element that spontaneously emits either high- energy electromagnetic radiation or subatomic particles (or both). The decay rate describes the time necessary for a radioactive isotope to change to a stable element.

radioactive decay A change in the nuclei of radioactive isotopes that spontaneously emit high-energy electromagnetic radiation and/or subatomic particles while gradually changing into another isotope or different element.

radioactivity The emission of radiation by unstable atoms in the form of particles and rays.

radioisotope An isotope that is radioactive, i.e., one having an unstable nucleus, which gives it the property of decay by one or more of several processes. Also called a radioactive isotope.

radioisotope dating A technique for determining the age of fossil materials based on the rate of disintegration of the radioisotopes.

radiolarian ooze A type of biogenous sediment that consists mostly of the silica shells of radiolarians.

radiolarian Members of the classes Acantharea, Phaeodarea, and Polycystinea (phylum Sarcomastigophora) with actinopodia and beautiful tests.

radioles Featherlike processes from the head of many tubicolous polychaete worms (phylum Annelida), used primarily for feeding.

radionucleides Isotopes that exhibit radioactive decay.

radula The rasping, tonguelike structure of most molluscs that is used for scraping food; composed of minute chitinous teeth that move over a cartilaginous odontophore.

rain forest A forest with high humidity, constant temperature, and abundant rainfall (generally over 380 cm [150 in] per year); can be tropical or temperate.

rain shadow Dry area on the downwind side of a mountain.

ram ventilation The movement of water across gills as a fish swims through the water with its mouth open.

random motion Molecular movement that is always present but nondirectional.

range of optimum The range of values for a condition in the environment that is best able to support survival and reproduction of an organism.

rangeland Grasslands and open woodlands suitable for livestock grazing.

raphe a groove in the frustules of pennate diatoms through which cytoplasm extends for locomotion

raprenox A process to reduce nitrogen oxide emissions by mixing combustion gases with isocyanic acid, which converts nitrogen oxides to dimolecular nitrogen gas.

Ras protein. A protein that initiates a cascade of reactions leading to cell division when a growth factor is bound to the cell surface. The gene encoding Ras becomes an oncogene when a mutation produces a form of Ras protein that initiates the cascade even in the absence of the growth factor.

ratite Referring to birds having an unkeeled sternum; compares with carinate.

rays Parenchymal cells found in secondary xylem and phloem in both woody angiosperms and conifers, which provide for lateral (radial) transport.

reaction wood Wood produced in response to a tree that has lost its vertical position, causing the tree to straighten.

realized heritability See heritability in the narrow sense.

realized niche The multidimensional space that an animal actually occupies in the presence of competitors, predators, pathogens, and limited food. Contrast with fundamental niche.

recapitulation. Summing up or repeating; hypothesis that an individual repeats its phylogenetic history in its development.

receptacle The expanded tip of a pedicel or peduncle to which the floral organs are attached.

receptive field The area of a group of sensory cells that, when stimulated, influences the activity of a given neuron.

receptor-mediated endocytosis The type of endocytosis that involves a specific receptor on the plasma membrane that recognizes an extracellular molecule and binds with it.

recessive gene Gene that is masked by its dominant allelic partner, having the recessive phenotype expressed only when both alleles for a given trait are recessive.

recessive trait A trait caused by an allele, the phenotype of which is suppressed when it occurs with the dominant allele. Hence, two recessive alleles are necessary for the recessive phenotype to be displayed.

recessive A gene whose expression is masked when it is present in combination with a dominant allele.

recharge zone Area where water infiltrates into an aquifer.

reciprocal altruism The trading of altruistic acts by individuals at different times, i.e., the payback to the altruist occurs some time after the receipt of the act.

recognition surface The three-dimensional structure of a biological membrane surface that gives specificity due to macromolecules of various sizes and shapes that extend above the lipid bilayer.

recolonization Revegetation; the reestablishment of a natural community following a natural or unnatural event that removed the existing community.

recombinant DNA The incorporation of DNA from one organism into that of another organism (usually a bacteria) so that the second organism produces a desired protein.

recombination The formation of new combinations of alleles through exchange of sections of homologous chromosomes during the process of meiosis; sometimes refers to the combinations of new alleles that result from fertilization.

rectal gland The excretory organ of elasmobranchs and the coelacanth located near the rectum; it excretes a hyperosmotic salt solution.

recycling Reprocessing of discarded materials into new, useful products; not the same as reuse of materials for their original purpose, but the terms are often used interchangeably.

red algae Seaweeds that have a predominance of red pigments.

red blood cell The type of blood cell that contains hemoglobin and no nucleus. During their formation in mammals, erythrocytes lose their nuclei; those of other vertebrates retain the nuclei.

red clay A fine sediment that is the most common type of lithogenous sediment in the open ocean floor.

red tide A population explosion or bloom of minute, single-celled marine organisms called dinoflagellates. Billions of these cells can accumulate in protected bays where the toxins they contain can poison other marine life.

Red Queen hypothesis The hypothesis that sexual reproduction has evolved because the genetic variation that results is adaptive in the evolutionary "arms race" between hosts and their pathogens and parasites as well as between predators and prey.

redia A larval stage in the life cycle of flukes; it is produced by a sporocyst larva, and in turn gives rise to many cercariae.

redox reactions Oxidation-reduction reaction; a chemical reaction involving the transfer of electrons from one molecule to another.

reduced tillage systems Systems, such as minimum till, conserve-till, and no-till, that preserve soil, save energy and water, and increase crop yields.

reduction division The first nuclear division of meiosis during which the paired homologues migrate to opposite poles, resulting in cells with a reduction from a diploid to a haploid number of chromosomes.

reduction. In chemistry, the gain of an electron by an atom or molecule of a substance; also the addition of hydrogen to, or the removal of oxygen from, a substance.

reef crest The shallow outer edge of the reef slope of a coral reef .

reef flat The wide and shallow upper surface of a coral reef.

reef slope The outer, steep margin of a coral reef. Also see fore reef.

reef massive near-shore deposits of coral skeletal material

refraction The change in the direction of a wave as it moves into shallow water.

refractory period In neurobiology, the period during which voltage-gated sodium channels in a neuron cannot be depolarized to threshold polarity by the influx of sodium in adjacent channels due to the hyperpolarizing effects of potassium exiting through potassium voltage-gated channels in that region.

refuse-derived fuel Processing of solid waste to remove metal, glass, and other unburnable materials; organic residue is shredded, formed into pellets, and dried to make fuel for power plants.

regeneration The ability of an organism to grow a body part that has been lost.

regenerative farming Farming techniques and land stewardship that restore the health and productivity of the soil by rotating crops, planting ground cover, protecting the surface with crop residue, and reducing synthetic chemical inputs and mechanical compaction.

regular symmetry Radial symmetry; having two or more planes through which the structure (flower) could be cut to result in mirror-image halves; actinomorphy.

regulations Rules established by administrative agencies; regulations can be more important than statutory law in the day-to-day management of resources.

regulative development. Progressive determination and restriction of initially totipotent embryonic material.

regulator genes Genes that control the activity of other genes that code for enzymes or structural proteins.

rehabilitate land A utilitarian program to make an area useful to humans.

reinforcement In learning, anything that alters the probability of behavior.

relative humidity At any given temperature, a comparison of the actual water content of the air with the amount of water that could be held at saturation.

relativists Those who believe moral principles are always dependent on the particular situation.

releaser gland A gland in turbellarians that secretes a chemical that dissolves the attachment of the organism from a substrate.

releaser Simple stimulus that elicits an innate behavior pattern.

releasing factors Hormones or neurosecretions from the hypothalamus that travel either via the hypothalamic-pituitary portal system or along axons between the hypothalamus and the pituitary where they exert their effect in terms of production and release of hormones.

Remipedia A class of crustaceans whose members possess about 30 body segments and uniform, biramous appendages. This class contains a single species of cave-dwelling crustaceans from the Bahamas.

Renal ren = kidney.

renette An excretory structure found in some worms.

renewable resources Resources normally replaced or replenished by natural processes; resources not depleted by moderate use; examples include solar energy, biological resources such as forests and fisheries, biological organisms, and some biogeochemical cycles.

rennin A milk-clotting endopeptidase secreted by the stomach of some young mammals, including bovine calves and human infants.

replication (1) The sample size or number of animals in each particular treatment in an experiment. (2) The ability of other investigators to use your description of the methods that you have employed in order to repeat exactly what you have done in your experiment.

replication In genetics, the duplication of one or more DNA molecules from the preexisting molecule.

Repolarization Return of a neuron to a resting potential.

repolarized The reestablishment of polarity, especially the return of plasma membrane potential to resting potential after depolarization.

repressible enzymes Enzymes whose production can be inhibited by a corepressor.

repressor A compound that binds to and controls the regulator in gene regulation.

reproductive barrier The factors that prevent one sexually propagating population from interbreeding and exchanging genes with another population.

reproductive effort The energy expended and risk taken to reproduce, measured in terms of the decrease in ability of the organism to reproduce at a later time.

reproductive isolation Occurs when individuals are prevented from mating, even though they may occupy overlapping ranges. See premating and postmating isolation.

reproductive strategy The reproductive patterns that are followed by a particular species.

reproductive success The number of progeny born, or surviving progeny produced by an organism.

reptiles Vertebrates with scales on their skin and leathery eggs that are laid on land. Marine reptiles include sea turtles, sea snakes, the marine iguana, and the saltwater crocodile.

Reptilia The class of vertebrates whose members have dry skin with epidermal scales and amniotic eggs that develop in terrestrial environments. Snakes, lizards, and alligators.

repugnatorial glands Glands secreting a noxious substance for defense or offense, for example, as in the millipedes.

reservoir Living or (rarely) nonliving means of maintaining an infectious agent in nature that can serve as source of infection for humans or domestic animals.

residence time The length of time a component, such as an individual water molecule, spends in a particular compartment or location before it moves on through a particular process or cycle.

resilience The ability of a community or ecosystem to recover from disturbances.

resilin The elastic protein found in some arthropod joints that stores energy and functions in jumping.

resin canal Long intercellular spaces present in the longitudinal system of vascular cells of pine, spruce, larch, and Douglas fir. Adjacent parenchymal cells secrete the resin into the canals.

resins Complex carbohydrates synthesized by certain plants in glands, canals, or ducts; insoluble in water. Used in various industrial products, including paints and varnishes. They may aid wood in resistance of decay.

resistance The ability of a community to resist being changed by potentially disruptive events.

resonance A vibration of electrons from one atom to another within chlorophyll.

resource defense polygyny Males control access to females indirectly by monopolizing critical resources.

resource partitioning In a biological community, various populations sharing environmental resources through specialization, thereby reducing direct competition. See also ecological niche.

resource scarcity A shortage or deficit in some resource.

respiration Gaseous interchange between an organism and its surrounding medium. In the cell, the release of energy by the oxidation of food molecules.

respiratory exchange See gas exchange.

respiratory fibrotic agents Special class of irritants, including chemical reagents and particulate materials, that damages the lungs, causing scar tissue formation that lowers respiratory capacity.

respiratory pigment Organic compounds that have either metallic copper or iron with which oxygen combines.

respiratory tree A pair of tubules attached to the rectum of a sea cucumber that branch through the body cavity and function in gas exchange.

response to selection (R) The difference in the mean phenotypic value between a group of progeny and the mean value for the parents of those progeny.

resting membrane potential The potential difference that results from the separation of charges along the plasma membrane of a neuron.

resting potential The unequal distribution of charges on either side of a neuron membrane that is not undergoing an action potential.

restoration ecology Seeks to repair or reconstruct ecosystems damaged by human actions.

restore ecosystems To reinstate an entire community of organisms to naturally occurring association.

restriction endonuclease. An enzyme that cleaves a DNA molecule at a particular base sequence.

rete mirabile A network of small blood vessels so arranged that the incoming blood runs countercurrent to the outgoing blood and thus makes possible efficient exchange between the two bloodstreams. Such a mechanism serves to maintain the high concentration of gases in the fish swim bladder.

rete system Highly branched system of tubules in acanthocephalans, lying on longitudinal muscles or between longitudinal and circular muscles; thought to assist in contraction stimuli.

reticular Resembling a net in appearance or structure.

reticulate venation Netted venation; the arrangement of veins in a leaf that resembles a net; characteristic of dicot leaves.

reticuloendothelial (RE) system Total complement of fixed macrophages in the body, especially reticular connective tissue and the lining epithelium of the blood vascular system. Some authorities also include the phagocytic white blood cells.

reticulopodia Pseudopodia that branch and rejoin extensively.

reticulopodium A pseudopodium that forms a threadlike branched mesh and contains axial microtubules.

retina The posterior sensory membrane of the eye that receives images.

retrofection Process of reinfection, whereby juvenile nematodes hatch on the skin and reenter the body before molting to third-stage larvae.

retroperitoneal External to the peritoneum.

retting The process that frees flax fibers by allowing microbial decomposition to break down the outer part of the stem.

revegetate The natural or induced replacement of plants into a cleared area; the recurrence of the same plant community that existed prior to clearing.

reversing thermometer A thermometer that permits the measurement of temperature at depth, since the mercury column breaks when the thermometer is turned upside down.

rhabdite A rodlike structure in the cells of the epidermis or underlying parenchyma in certain tubellarians that are discharged in mucous secretions.

rheoreceptor A sensory organ of aquatic animals that responds to water current.

rhinencephalon The part of the vertebrate brain that comprises the septum, cingulate gyrus, hypothalamus, amygdala, hippocampus, and the fornix; also called the limbic system.

rhinophore Chemoreceptive tentacles in some molluscs (opisthobranch gastropods).

rhizoid Rootlike absorptive structures on the underside of certain gametophytes.

rhizome A fleshy, horizontal underground stem.

Rhodophyta The division of protists commonly called the red algae.

rhodopsin Light-sensitive substance that occurs in the rods of the retina; visual purple.

rhombogen Stage in the life cycle of a dicyemid mesozoan.

rhopalium A sensory structure at the margin of the scyphozoan medusa. It consists of a statocyst and a photoreceptor (phylum Cnidaria).

rhopalium One of the marginal, club-shaped sense organs of certain jellyfishes; tentaculocyst.

rhoptries Club-shaped bodies in Apicomplexa composing one of the structures of the apical complex; open at anterior and apparently functioning in penetration of host cell.

rhynchocoel In nemerteans, the dorsal tubular cavity that contains the inverted proboscis. It has no opening to the outside.

Rhyniophyta An extinct division of early, vascular land plants characterized by their small stature, dichotomous branching, lack of leaves, roots, and possession of terminal sporangia (e.g., Rhynia, Cooksonia).

ribbon (or nemertean) worms (phylum Nemertea)Invertebrates with a complete digestive tract, a true circulatory system, and a long proboscis to capture prey.

ribonucleic acid (RNA) The nucleic acid formed from DNA and involved in protein synthesis; nucleotide of chain of phosphates, ribose sugars, and purine and pyrimidines.

ribose A 5-carbon sugar important in RNA and many other compounds.

ribosomal RNA A form of ribonucleic acid that makes up a portion of ribosomes.

ribosome Subcellular structure composed of protein and ribonucleic acid. May be free in the cytoplasm or attached to the membranes of the endoplasmic reticulum; functions in protein synthesis.

rickettsia A very small and specialized pathogenic bacterium transmitted by ticks, mites, and fleas.

ridge and rise system the interconnecting chain of seafloor mountains that trace the edges of crustal plates and the sites of new oceanic crust production

rift A crack in the earth's crust formed as pieces of the crust separate.

rill erosion The removing of thin layers of soil as little rivulets of running water gather and cut small channels in the soil.

ring-porous wood Secondary xylem characterized by larger vessels and tracheids being produced early in the season (or following favorable growing conditions). Thus each term of growth activity is seen as a ring.

risk assessment Evaluation of the short-term and long-term risks associated with a particular activity or hazard; usually compared to benefits in a cost-benefit analysis.

risk Probability that something undesirable will happen as a consequence of exposure to a hazard.

risk-sensitive foraging A model that incorporates variation in prey encounter rates to predict where predators forage. Depending on their internal state and other factors, predators may be risk-prone, foraging where encounter rates are variable, or risk-averse, foraging where encounter rates are relatively constant.

ritualization. In ethology, the evolutionary modification, usually intensification, of a behavior pattern to serve communication.

RNA polymerase The enzyme responsible for forming mRNA during transcription.

RNA world. Hypothetical stage in the evolution of life on earth in which both catalysis and replication were performed by RNA, not protein enzymes and DNA.

RNA Ribonucleic acid; nucleic acid used for transcription and translation of the genetic code found on DNA molecules.

rock cycle The process whereby rocks are broken down by chemical and physical forces; sediments are moved by wind, water, and gravity, sedimented and reformed into rock, and then crushed, folded, melted, and recrystallized into new forms.

rockweeds Brown algae such as Fucus that are inhabitants of rocky shores in temperate zones.

rod cell A type of light receptor that is responsible for color vision.

rod A class of vertebrate photoreceptors that, unlike cone cells, is usually insensitive to color.

Romana's sign Symptoms of recent infection by Trypanosoma cruzi, consisting of edema of the orbit and swelling of the preauricular lymph node.

Romanovsky stain Complex stain, based on methylene blue and eosin, used to stain blood cells and hemoparasites. Wright's and Giemsa's stains are two common examples.

root cap A thimble-shaped group of cells found at the tip of roots; it functions to protect the meristem.

root hair A root epidermal cell that functions in water absorption.

root nodule Gall-like structures on the roots of legumes that contain symbiotic nitrogen-fixing bacteria.

root pressure A positive pressure in the xylem due to a negative solute potential and closed stomata.

root system The portion of the plant formed from the growth of the radicle and its subsequent branching; generally the underground portion of the plant.

rootbound The condition of a restricted root system caused by excess growth in too small a volume of soil; sometimes called potbound.

rorquals Blue, fin, and the other baleen whales that display long grooves on the underside.

rostellum Projecting structure on scolex of tapeworm, often with hooks.

rostrum Dorsal part of capitulum projecting over chelicerae in acarines.

Rotifera The phylum of aschelminths that has members with a ciliated corona surrounding a mouth; muscular pharynx (mastax) present with jawlike features; nonchitinous cuticle; parthenogenesis common; both freshwater and marine species.

rough ER A portion of the endoplasmic reticulum containing ribosomes.

roundworms See nematodes.

RUBP carboxylase The enzyme that fixes CO2 in the Calvin cycle of photosynthesis.

RUBPRibulose 1, 5-bisphosphate, the 5-carbon compound that combines with CO2 in the Calvin cycle.

ruffles Slender projections of the exterior surface of a dicyemid mesozoan.

rumen The large first compartment of the stomach of ruminant mammals.

ruminant Cud-chewing artiodactyl mammals with a complex four-chambered stomach.

ruminant animals Cud-chewing animals, such as cattle, sheep, goats, and buffalo, with multichambered stomachs in which cellulose is digested with the aid of bacteria.

run-of-the-river flow Ordinary river flow not accelerated by dams, flumes, etc. Some small, modern, high-efficiency turbines can generate useful power with run-of-the-river flow or with a current of only a few kilometers per hour.

runaway selection Selection for male ornaments that happens due to the genetic correlation, and resulting positive feedback relationship, between the trait and the preference for the trait.

runner A horizontal stem that grows along the surface of the ground; also known as a stolon.

runoff The excess of precipitation over evaporation; the main source of surface water and, in broad terms, the water available for human use.

rural area An area in which most residents depend on agriculture or the harvesting of natural resources for their livelihood.