G

G2 Part of interphase after the synthesis of DNA and before the start of nuclear division and known as Gap 2.

Gaia hypothesis A theory that the living organisms of the biosphere form a single, complex interacting system that creates and maintains a habitable Earth; named after Gaia, the Greek Earth mother goddess.

galactose A 6-carbon sugar.

Galapagos Islands An archipelago on the equator in the Pacific Ocean about 1,000 km west of Ecuador. Charles Darwin's observations of the plant and animal life of these islands were important in the formulation of the theory of evolution by natural selection.

gallbladder The pear-shaped reservoir for bile.

gametangia Any cell or organ in which gametes are formed.

gametangium (pl. gametangia)Structure in which gametes are produced.

gamete A haploid reproductive cell that develops into a new individual after its union with another gamete.

gametes Mature haploid cells (sperm and ova) that fuse to form a zygote.

gametic meiosis. Meiosis that occurs during formation of the gametes, as in humans and other metazoa.

gametocyst Cyst produced by some apicomplexan parasites. Sexual reproduction and spore formation occurs within this cyst.

gametocyte The mother cell of a gamete, that is, immature gamete.

gametogenesis The formation of gametes by way of meiosis.

gametogony Multiple fission that forms gametes that fuse to form a zygote. Also called gamogony. Occurs in the class Sporozoea.

gametophyte The haploid, gamete-producing plant in the alternation of generations; undergoes mitosis to produce the haploid gametes, which fuse to form the diploid zygote of the sporophyte.

gamma rays That portion of the sun's total range of radiation in which rays are shorter than x rays; below 0.1 nm in length.

gamont Apicomplexan life cycle stage that is committed to undergoing gametogenesis.

ganglion An aggregation of nerve tissue containing nerve cells.

ganoid scales Thick, bony, rhombic scales of some primitive bony fishes; not overlapping.

gap analysis A biogeographical technique of mapping biological diversity and endemic species to find gaps between protected areas that leave endangered habitats vulnerable to disruption.

gap junction An area of tiny canals communicating the cytoplasm between two cells.

garden city A new town with special emphasis on landscaping and rural ambience.

gas (respiratory) exchange The movement of oxygen and other gases between the atmosphere and the ocean or between the water or atmosphere and living organisms.

gasohol A mixture of gasoline and ethanol.

gastric shield A chitinized plate in the stomach of a bivalve (phylum Mollusca) on which the crystalline style is rotated.

gastrodermis Lining of the digestive cavity of cnidarians.

gastrolith Calcareous body in the wall of the cardiac stomach of crayfish and other Malacostraca, preceding the molt.

Gastropoda The class of molluscs characterized by torsion. A shell, when present, is usually coiled. Snails.

gastropods Snails and other molluscs that typically possess a coiled dorsal shell and a ventral creeping foot.

Gastrotricha A small phylum of marine and freshwater species of gastrotrichs that inhabit the spaces between bottom sediments.306

gastrovascular canals Fluid-filled canals opening at the mouth of cnidarians and ctenophores that function in gas exchange and in the distribution of nutrients.

gastrovascular cavity Body cavity in certain lower invertebrates that functions in both digestion and circulation and has a single opening serving as both mouth and anus.

gastrozooid The feeding polyp of a hydroid, a hydranth.

gastrula Embryonic stage, usually cap or sac shaped, with walls of two layers of cells surrounding a cavity (archenteron) with one opening (blastopore).

gastrulation The embryological process that results in the formation of the gastrula; results in the formation of the embryonic gut, ectoderm, and endoderm.138

gel That state of a colloidal system in which the solid particles form the continuous phase and the fluid medium the discontinuous phase.

gemmae cup A specialized structure on the gametophyte of certain liverworts that produces asexual plantlets (gemmae) capable of starting new gametophytes.

gemmule Resistant, overwintering capsule formed by freshwater, and some marine, sponges that contains masses of mesenchyme cells; amoeboid mesenchyme cells are released and organize themselves into a sponge. 268

gena Anterioventral portion of an insect head. For example, genal ctenidium is a row of heavy spines on the gena of a flea.

gene A unit of heredity; a segment of DNA nucleus of the cell that contains

gene activators Hormones that can turn a given gene on or off.

gene banks Storage for seed varieties for future breeding experiments.

gene flow An exchange of genes between two populations of a species, or in extreme cases, between populations of two species (hybridization).

gene insertion The process by which one or more genes from one organism are incorporated into the genetic makeup of a second organism.

gene pool A collection of all of the alleles of all of the genes in a population.

gene regulation The process by which genes are turned on and off to regulate growth and development of an organism.

gene synthesizer A machine that can produce a functional gene segment in a short period of time.

general fertility rate Representation of population age structure and fecundity; crude birth rate multiplied by the percentage of fecund women (between approximately fifteen and forty-four years of age) by 1000.

generalists Animals that are capable of eating a variety of foods or living in a variety of places.Contrast with specialists.

generalization In learning, a principle that states if an animal has been conditioned to respond to a certain stimulus, the response will usually also occur to stimuli used during the original acquisition trials.

generalized niche Niche that has broad requirements; tolerates a range of conditions.

generation time The length of time between when an organism is born and when it first reproduces.

generative nucleus Produced by the haploid microspore nucleus of a pollen grain, the generative nucleus divides mitotically to form two sperm nuclei.

generator potential A graded potential that travels only a short distance along the plasma membrane of a sensory cell.

genetic assimilation The disappearance of a species as its genes are diluted through crossbreeding with a closely related species.

genetic drift. Random change in allelic frequencies in a population occurring by chance. In small populations, genetic variation at a locus may be lost by chance fixation or a single allelic variant.

genetic engineering Modifying the genetic structure of one organism by splicing in selected genetic information from another organism.

genetic erosion Irreversible loss of genetic diversity due to extinction of traditional varieties and wild ancestors of crop plants.

genetic recombination Crossing-over; a major source of genetic variation in a population or a given species.

genetic screening The use of a number of mutant forms of a species to test for a specific type of change or deficit, as with respect to biological clocks.

genetic variability Produced by sexual reproduction, all organisms are genetically different from each other.

genetically engineered microorganisms (GEMs) Bacteria that have been genetically engineered by the insertion or deletion of DNA segments.

genetics The study of the mechanisms of transmission of genes from parents to offspring.

genital atrium Cavity in the body wall of a flatworm into which male and female genital ducts open.

genital slit The genital opening of cetaceans.

genitointestinal canal Duct connecting the oviduct and intestine of some polyopisthocotylean Monogenea.

genome All the DNA in a haploid set of chromosomes (nuclear genome), organelle (mitochondrial genome, chloroplast genome) or virus (viral genome, which in some viruses consists of RNA rather than DNA).

genotype The genetic constitution, expressed and latent, of an organism; the total set of genes present in the cells of an organism; contrasts with phenotype.

genotypic ratio The relative numbers of progeny in each genotypic category produced by a genetic cross.

genu Knee.

genus A group of related species with taxonomic rank between family and species.

geometric growth Growth that follows a geometric pattern of increase, such as 2, 4, 8, 16, etc. See exponential growth.

georeceptor A specialized nerve ending that responds to the force of gravity.

geothermal energy Energy drawn from the internal heat of the earth, either through geysers, fumaroles, hot springs, or other natural geothermal features, or through deep wells that pump heated groundwater.

geotropism The bending responses of a plant to the forces

germ layer. In the animal embryo, one of three basic layers (ectoderm, endoderm, mesoderm) from which the various organs and tissues arise in the multicellular animal.

germ plasm Genetic material that may be preserved for future agricultural, commercial, and ecological values (plant seeds or parts or animal eggs, sperm, and embryos).

germ Embryo of a cereal grain.

germinate To resume growth and increase metabolic activity,

germplasm Entire genetic makeup of an organism.

gerontology The scientific problems of aging in all their aspects, including clinical, biological, and sociological.

gestation Period of development of the young in viviparous animals, from the time of fertilization of the ovum until birth.

gestation period The portion of the reproductive cycle in a female mammal extending from fertilization to birth of its offspring.

giardiasis A common infection of the lumen of the small intestine with the flagellated protozoan Giardia lamblia, and spread via contaminated food and water and by direct person-to-person contact.

gibberellin A group of related compounds which cause single-gene dwarf mutants of corn and peas to elongate normally.

gid (GID)Disorientation caused by cysticercia in the brain; usually manifested by staggering or whirling.

gill A structure specialized for gas exchange in aquatic animals.

gill arch A supporting structure of fish gills.

gill arches Bony or cartilaginous gill supports of some vertebrates; also called visceral arches.

gill cover (or operculum)The flap of bony plates that covers the gills of bony fishes.

gill filaments A thin-walled, fleshy extension of a gill arch that contains vessels carrying blood to and from gas exchange surfaces.

gill lamellae Thin plates of tissue on gill filaments that contain the capillary beds across which gases are exchanged.

gill raker Each of the projections along the inner surface of fish gills.

gill slit One of several openings in the pharyngeal region of chordates. Gill slits allow water to pass from the pharynx to the outside of the body. In the process, water passes over gills or suspended food is removed in a filter-feeding mechanism.

gill An aquatic respiratory organ for obtaining oxygen and getting rid of carbon dioxide.

Ginkgo A decidious gymnosperm tree (Ginkgo biloba) with distinctive fan-shaped leaves, native to eastern China.

girth Circumference, as of a tree trunk.

glass sponges Deep-water sponges with a skeleton of fused silica spicules.

glial cells Supportive cells that are closely associated with neurons.

global environmental citizenship A shift in our attention from pollution in a specific place to a concern about the life-support systems of the whole planet.

global warming Some experts contend our earth's annual average temperature is gradually increasing, partly due to the greenhouse effect.

globular protein A coiled polymer of amino acids forming a compact macromolecule.

globulins A large group of compact proteins with high molecular weight; includes immunoglobulins (antibodies).

glochidium A larval stage of freshwater bivalves in the family Unionidae; it lives as a parasite on the gills or fins of fishes.

glomerulus A tuft of capillaries projecting into a renal corpuscle in a kidney. Also, a small spongy mass of tissue in the proboscis of hemichordates, presumed to have an excretory function. Also, a concentration of nerve fibers situated in the olfactory bulb.

glossa Tonguelike mouthpart in Hymenoptera (considered a hypopharynx by some authors).

gluconeogenesis Synthesis of glucose from protein or lipid precursors.

glucose Six-carbon monosaccharide; one of the most abundant simple sugars. It is the building block of both cellulose and starch and is important to several metabolic pathways.

glumes Pair of bracts at the base of a spikelet in a grass flower.

gluon Subatomic particle that binds quarks together in the nucleus of an atom.16

Gluteal The rump.

gluten Protein complex in endosperm of wheat and some other cereals that is essential in making a leavened bread.

glycerol An organic molecule to which fatty acids are attached to form a fat.

glycocalyx Finely filamentous layer containing carbohydrate, found on the outer surface of many cells, from 7.5 to 200.0 nm thick.

glycogen A polysaccharide constituting the principal form in which carbohydrate is stored in animals; animal starch.

glycolysis The conversion of glucose to pyruvic acid (pyruvate) with the release of some energy in the form of ATP. Occurs in the cytosol; literally "sugar splitting."

glycoprotein A protein with attached sugars.

glycoside Physiologically active compound in plants that always contains a sugar group, although the active part of the molecule may differ.

glycosomes Organelles found in Trypanosoma that contain enzymes of glycolysis and for oxidizing reduced NAD.

glyoxylate cycle Metabolic pathway that functions to convert fatty acids or acetate to carbohydrate.

glyoxysomes A subcellular microbody present in the cytoplasm of many oil seeds. Enzymes packaged in the glyoxysome convert lipids to carbohydrates during the germination process.

gnathobase A median basic process on certain appendages in some arthropods, usually for biting or crushing food.

gnathopod Prehensile appendages of some Crustacea, such as the second and third thoracic legs of Amphipoda and the first thoracic legs of some Isopoda.

gnathosoma Anterior of two basic regions of the body of a mite or tick. Also called a capitulum.

Gnathostomata A superclass of vertebrates whose members possess hinged jaws and paired appendages. Notochord may be replaced by the vertebral column.

gnathostomes Vertebrates with jaws.

gnetophytes Taxa of the division Gnetophyta, a group of gymnosperms.

goblets Markings on the stigmatal plates of certain hard ticks.

Golgi body (apparatus) Organelle of membranous, hollow sacs arranged in a stack; functions in modification, storage, and packaging of secretion materials; may be called dictyosome in plants.

Golgi complex An arrangement of sacs and membranes in many eukaryotic cells that is involved in collecting and transporting molecules.

gonad An organ that produces gametes (ovary in the female and testis in the male).

gonadotropin A hormone that stimulates activity in the gonads.

gonads Glands responsible for the production of gametes and where certain gonadal hormones are produced. These consist of the ovaries in females and the testes in males.

gonangium Reproductive zooid of hydroid colony (Cnidaria).

Gondwana One of two large continents, the southern one, that formed when the supercontinent Pangaea broke up about 180 million years ago. Also see Laurasia.

gonochoristic Having separate sexes; that is, an individual is either male or female, but never both. "Dioecious" means the same thing.

gonoduct Duct leading from a gonad to the exterior.

gonopore A genital pore found in many invertebrates.

gonotyl Muscular sucker or other perigenital specialization surrounding or associated with the genital atrium of a digenetic trematode.

gonozooid A polyp of a hydrozoan cnidarian that produces medusae.

Gordian worm See horsehair worms.

gorgonians Colonial anthozoans that secrete a skeleton made of protein.

grade A level of organismal complexity or adaptive zone characteristic of a group of evolutionarily related organisms.

graded signal A form of communication that is analog, or varying in intensity or frequency, providing quantitative information about the strength of the stimulus.

gradualism A component of Darwin's evolutionary theory postulating that evolution occurs by the temporal accumulation of small, incremental changes, usually across very long periods of geological time; it opposes claims that evolution can occur by large, discontinuous or macromutational changes.

grafting A method of asexual propagation characterized by placing a shoot (the scion) onto the rootstock (the stock) of another plant.

grain Single, dry indehiscent fruit of a single seed that is fused to the ovary wall.

gram-negative Denoting bacterial cells that do not stain with crystal violet and iodine.

gram-positive Denoting bacterial cells that are readily stained with crystal violet and iodine.

grana flattened saclike structures inside chloroplasts containing chlorophyll and other photosynthetic enzymes

granite An igneous rock overlying most of the landmasses

granulocytes White blood cells (neutrophils, eosinophils, and basophils) bearing "granules" (vacuoles) in their cytoplasm that stain deeply.

granuloma, granulomatous tissue Repaired area of a body marked by fibrous connective tissue (fibrosis). Also, fibrous connective tissue surrounding an antigen source.

granum Structures within chloroplasts, seen as green granules with the light microscope and as a series of stacked disk-shaped membranes with an electron microscope; the grana contain the chlorophylls and carotenoids and are the sites of the light reactions of photosynthesis.

grasslands A biome characterized by grasses and a few scattered trees. Grassland animals include numerous insects and other herbivores such as bison.

gravel Inorganic soil particles larger than 2.0 mm in diameter.

gray crescent A dark arching band that forms on the surface of the amphibian zygote opposite the point of sperm penetration; forms in the region where gastrulation will occur.

grazer An organism that feeds primarily on plants.

great ocean conveyor A global circulation pattern in which water cycles throughout the ocean basins.

great whales Large whales: the sperm and the baleen whales.

green algae Seaweeds in which chlorophyll is not green beard effectA mechanism for recognizing kin that does not require previous experience. Recognition genes enable an organism to identify and behave altruistically towards other organisms bearing that same gene.

green gland Excretory gland of certain Crustacea; the antennal gland.

green plans Integrated national environmental plans for reducing pollution and resource consumption while achieving sustainable development and environmental restoration.

green political parties Political organizations based on environmental protection, participatory democracy, grassroots organization, and sustainable development.

green revolution Dramatically increased agricultural production brought about by "miracle" strains of grain; usually requires high inputs of water, plant nutrients, and pesticides.

greenhouse effect The warming of the Earth due to the atmospheric accumulation of carbon dioxide and other gases, which trap heat and reradiate it back to the Earth's surface.

gregarious Living in groups or flocks.

gross national product (GNP) The sum total of all goods and services produced in a national economy. Gross domestic product (GDP) is used to distinguish economic activity within a country from that of off-shore corporations.

gross primary production The total amount of photosynthesis accomplished in a given period of time.

ground itch Skin rash caused by bacteria introduced by invasive hookworm larvae.

ground meristem The basic or fundamental tissue of the apical meristem; dermal tissues surround the ground meristem, and the provascular strands are embedded in it.

ground parenchyma The basic ground tissue consisting of living parenchymal cells.

ground tissue Includes primary tissues of parenchyma, sclerenchyma, and collenchyma that make up much of the bulk of the primary plant body; function in support, photosynthesis, and storage; also known as fundamental tissue.

groundwater Water held in gravel deposits or porous rock below the earth's surface; does not include water or crystallization held by chemical bonds in rocks or moisture in upper soil layers.

group selection Selection that operates on two or more genetic lineages (groups) as units; broadly defined, this includes kin and interdemic selection.

growth ring A concentric layer of wood (secondary xylem) formed in one growing season through cell divisions of the vascular cambium. Also termed annual ring.

growth An irreversible increase in size or volume of a cell, tissue, organ, or organism.

guanine A white crystalline purine base, C5H5N5O, occurring in various animal tissues and in guano and other animal excrements.

guano The accumulation of the excrement of seabirds.

guard cell One of a pair of specialized cells in the epidermis that regulates the opening and closing of a stoma.

guild In ecology, a group of species that exploit the same class of environment in a similar way.

gular flutter The type of breathing experienced by some birds. Rapid movement of the throat region promotes evaporative water loss.

gully erosion Removal of layers of soil, creating channels or ravines too large to be removed by normal tillage operations.

gustation The act of tasting or the sense of taste.

gustatory Having to do with the sensation of taste, the chemoreception of molecules in solution by specialized epithelial receptor cells.

guttation Root pressure forcing water out of the leaves, usually overnight when stomates are closed.

guyot A flat-topped seamount.

gymnosperm Seed-producing plants in which the seed is not enclosed in an ovary as in angiospems. Ginkgos, cycads, and conifers are examples.

gynandromorph An abnormal individual exhibiting characteristics of both sexes in different parts of the body; for example the left side of a bilateral organism may show characteristics of one sex and the right side those of the other sex.

gynandry In a hermaphroditic organism, maturation first of the female gonads and then of the male organs. Also called protogyny.

gynocophoral canal Longitudinal groove in the ventral surface of a male schistosome fluke.

gynocophoric canal Groove in male schistosomes (certain trematodes) that carries the female.

gynoecium The collective term referring to the carpels or ovule-bearing structures, within a flower.

gyre A large, nearly circular system of wind-driven surface currents that center around latitude 30in both hemispheres.