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Glossary of Terms

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The heavy spike of spike-like inflorescence, such as the ear of wheat.

The median effective concentration (ppm or ppb) of the toxicant in the environment (usually water) that produces a designated effect in 50% of the test organisms exposed.

EC50; EC90
1. The concentrations at which 50 or 90% of a population reacts. 2. The concentration at which the reaction of an organism is 50% or 90% less than the control value.

Pertaining to ecchymosis.

An effusion of blood into the tissues causing irregular patches visible on the surface of an organ or tissue.

Moulting or discarding of in-expansible portions of cuticle in arthropods, after which there is an increase in size of the individual's body before the newly secreted cuticle hardens.

Hormone secreted by insects that is essential to the process of moulting.

Ecesis (= Oikesis)
The establishment of an organism in a new habitat; accidental dispersal and establishment in a new area.

Eclipsed antigen
Antigen borne by the parasite that is common to both the host and the parasite but which genetically is of parasite origin.

Eclipse period
In the developmental cycle of viruses, a phase or period, occurring immediately after infection (i.e. immediately after a virus enters the host cell), in which infective particles cannot be detected.

The process of hatching from the egg.

Climate within a plant community, e.g. crop.

Ecological longevity
Average length of life of individuals of a population under stated conditions.

Pertaining to the relationship between an organism or organisms and features of the environment.

Derived from the Greek oikos, 'house or place to live', the science dealing with living organisms and their relation to the environment. (Applied to plant pathology) The study that deals with the effect of environmental factors, such as soil, climate and culture etc. on the occurrence, severity and distribution of plant diseases or plant pathogens.

The application of statistical methods to the estimation of economic models.

Econometric model
An econometric model is a probabilistic model consisting of a system of one or more equations that describe the relationship among a number of economic and time series variables.

Economic damage
The amount of injury which will justify the cost of artificial control measures.

Economic injury level
The degree of crop damage or pest density at which economic losses become significant. The lowest population density that will cause economic damage.

Economic pest
A pest that can cause a crop loss of between 5-10%.

Economic poison
The term 'economic poison' means any substance or mixture of substances intended for preventing, destroying, repelling or mitigating any insects, rodents, nematodes, fungi or weeds or any of the other forms of life declared to be pests; and any substance or mixture of substances intended for use as a plant regulator, defoliant or desiccant.

Economic threshold
The pest population level at which control measures should be started to prevent the pest population from reaching economic-injury level (Hill, 1983). The density of pest population at which a control action becomes profitable (Stern, 1966) (see action threshold).

Economies of scale
Reductions in cost which arise from the utilisation of larger sets of machinery and equipment.

Economies of scope
Reductions in cost which arise from producing a number of different products together.

The interacting system of all the biotic community and its abiotic environment.

Transition zone between two diverse communities (e.g. the tundra-boreal forest ecotone).

The study of toxic effects of chemical and physical agents in living organisms, especially on populations and communities within defined ecosystems; it includes transfer pathways of these agents and their interaction with the environment.

Subspecies of race that is especially adapted to a particular set of environmental conditions. Individuals are infertile with each other and members of other eco-types of the same species.

A Greek-derived prefix meaning external. Here used only to qualify parasite, to distinguish, e.g. fleas and ticks from endo- (internal) parasites such as disease-producing micro-organisms.

Pertaining to the outer surface of the heart.

Commensal symbiont that lives on the outer surface of its host.

Strands of protoplasm linking cells.

An organism that lives parasitically on the outside of it host. Insects which live on the skin of cattle, e.g. ticks, lice, fleas.

Infection in a location other than normal or expected.

Symbiosis in which one member (micro-symbiote) develops on the outside of the other member.

Refers to a mycorrhiza in which the mycelium forms an external covering on the root (e.g. in pine).

The median effective dose of a pesticide, expressed as mg/kg of body weight, which produces a designated effect in 50% of the test organism exposed.

Of or pertaining to soil or, more generally, substrate, especially as it affects living organisms.

Accumulation of more than normal amounts of tissue fluid or lymph in the intercellular spaces, resulting in localised swelling of the area.

Not toxic; nutritious, as a worm to a chicken, a chicken to a man, and a man to a worm.

Nemas, nematodes.

Effective concentration
EC Refers to a concentration of a chemical substance effective in producing some specific result such as death, paralysis, increase in oxygen consumption etc.

Pertaining to the degree to which a dispersal unit or population of dispersal units leads to establishment of new infections.

Effects matrix
A matrix having coefficients that represent both the sub-lethal and lethal effects of toxicants on ecosystem biota.

Capacity to produce the desired effect.

A ratio of output (or performance or success) to the input(s) (or costs) involved, over a specified time and in a specified context.

Efficiency of a pest control measure
The more or less fixed reduction of a pest population regardless of the number of pests involved.

A biological change in an organism, organ or tissue.

Efficiency ratio
Used in standard costing to measure efficiency by expressing the standard hours equivalent of the work produced as a percentage of the actual hours taken.

The process of passing indigestible material out of the gut. Also termed defecation.

A female gamete. In nematodes, the first stage of the life cycle containing a zygote or a larva.

Egg cell
The female germ cell. On being fertilised by the male sperm it develops into the embryo.

The finding of eigenvalues and eigenvectors of a square matrix.

Coefficients associated with a matrix A that satisfy the equation |A - I| = 0. Used in the solution to linear compartment models and Leslie matrices.

Vectors vi associated with eigenvalues i of a matrix A satisfying the relationship Avi = ivi.

Elateriform larva
A larva resembling a wire-worm with a slender body, heavily sclerotinised, with short thoracic legs and only a few body bristles.

Electron acceptor
A substance that accepts electrons during an oxidation-reduction reaction. An electron acceptor is an oxidant.

Electron donor
A compound that donates electrons in an oxidation-reduction reaction. An electron donor is a reductant.

Electron-transport phosphorylation
Synthesis of ATP involving a membrane-associated electron transport chain and the creation of a proton-motive force. Previously termed oxidative phosphorylation.

A technique used to separate chemical substances according to the electrical charge on their molecules which migrate in response to the application of an electric field.

Electrophoretic mobility
The rate of movement of virus per unit potential gradient. Mobility may be towards the cathode or the anode depending on whether the virus has a net positive or negative charge at the pH used.

Electrostatic application
A pesticide spray application technique by which droplets are given an electrostatic charge.

A response in which an electromotive force constitutes the energy of activation.

(Krummholz) scrubby, stunted growth form of trees, often forming a characteristic zone at the limit of tree growth in mountains.

A physical, chemical or biological stimulus that triggers defence responses in plants.

Elimination (in metabolism)
The expelling of a substance or other material from the body (or a defined part thereof), usually be a process of extrusion or exclusion, but sometimes through metabolic transformation.

Enzyme linked immunosorbant assay; a very sensitive diagnostic test that employs antibodies linked to an enzyme whose activity can be determined for the quantitative determination of the antigen which reacts with that antibody.

The hard upper wings of beetles.

Elytron/femur ratio (E/F)
Ratio of the length of the elytron (outer wing) of locust to the length of the femur (part of hind leg); used to determine morphological phase status of locusts.

Reduced in flesh; thin.

To remove the anthers (male organs) from a bud or flower before the pollen is shed.

In plant pathology a term used to describe the disruption of water conduction, usually by an air-lock.

An organism in its earliest stage of development usually surrounded by protective tissue e.g. a plant still enclosed in the seed which arises from the zygote.

Any toxic effect on the developing foetus resulting from prenatal exposure, including structural or functional abnormalities or post-natal manifestation of these.

Containing an embryo, i.e. the early developing stage of life after fertilisation of the egg by the sperm.

The simultaneous initiation of root and shoot meristems in component cells so that bipolar structures develop into embryos and later into whole plants.

Embryonic callus
Callus cultures that under suitable conditions are capable of producing embryos (i.e. young plants). Conversely callus cultures that have lost this capability are termed 'non-embryonic'.

Embryo rescue
When cross-pollination occurs between genetically widely different plants, the resulting embryo may be aborted because of parental mutual incompatibility. Such embryos may be excised and grown on a congenial medium such as nutrient agar. This process is called embryo rescue.

(i) The adult insect leaving the pupal case. (ii) Germination of a seed and the appearance of the shoot. The time when the first leaves of the crop plant come through the ground.

The movement of individuals out of a population.

Emission standard
A quantitative limit on the emission or discharge of a potentially toxic substance from a particular source. The simplest system is uniform emission standard where the same limit is placed on all emissions of a particular contaminant.

Empirical distribution
The cumulative distribution function of a sample of size n, denoted Fn(x). It is 1/n x {the number of values in the sample less than or equal to x}.

The presence of air in the alveolar tissue of the lungs. It is caused when damage to the alveoli allows air to penetrate the interstitial tissue which then inflates.

Emulsifiable concentrate (= Miscible oil)
A liquid pesticide formulation that when added to water spontaneously disperses as fine droplets to form a stable emulsion. The more water that is added the thinner the emulsion becomes.

Emulsifiable liquid
A liquid that will form an emulsion when it is mixed with water.

Surfactant used to stabilise suspensions of one liquid in another, e.g. oil based pesticide formulations in water or water based pesticide formulations in oil.

Very fine droplets of one liquid dispersed in another, e.g. oil in water.

An abnormal outgrowth (often on the leaf) often induced by virus infection, caused by an increase in cell numbers (hyperplasia) e.g. gall formation

The enclosure of a virus's nucleic acid genome with a protein shell.

Encapsulation (or encapsulated formulation)
Or micro-encapsulation: the encapsulation of a pesticide in a non-volatile envelope of gelatine, polyvinyl or other material, usually of minute size, for controlled release of the chemical to extend the period of diffusion.

Encephalitic syndrome
The syndrome exhibited by an animal due to a disease condition of the brain.

Inflammation of the brain.

To form a cyst.

Surrounded by a hard shell (cyst).

Endangered species
A species of animal or plant threatened with extinction.

Refers to an organism restricted to a particular area, that is constantly present in low numbers in a population.

A chemical reaction requiring input of energy to proceed.

Medial process from the protopod.

Opposite of ecto-

Commensal symbiont that lives inside its host.

Ingestion of particulate matter, fluid or virus by phagocytosis or pinocytosis; i.e., bringing material into a cell by invagination of its surface membrane, then pinching off the invaginated portion as a vacule.

A layer of cells with thick walls and no intercellular spaces that surrounds the vascular tissues of the roots.

Same as endopolyogeny except that only two daughter cells are formed.

Arising from within the generating structure (e.g. endoconidia of Thielaviopsis basicola).

Endolytic insecticides
Systemic insecticides that remain in their original form until decomposed by the biological system in which they are taken up.

Endometatoxic insecticides
Systemic insecticides which are transformed or metabolised by the biological system in which they are taken up to secondary toxicants.

Nucleases are enzymes that break down nucleic acids into strands of DNA. 'Endo' or inside nucleases act at points along the strand and thus break DNA into short pieces. Endonucleases recognise a particular base sequence in DNA and cut the DNA. Some endonucleases cut the DNA at a specific point while others randomly split the DNA sequences. Specific endonucleases are used by genetic engineers who excise strands of DNA coded for a desirable genetic character. Endonucleases are classed as 'Restriction enzymes' since they are employed e.g., by bacteria to restrict infection by viruses (bacteriophages). The bacterial restriction enzyme attacks the DNA of the infecting organisms.

Parasites which live inside the host and feeds internally on its tissues, e.g. tapeworm, liver fluke, lung-worm.

Endoplasmic reticulum
A complex pattern of internal membranes that permeates the cytoplasmic matrix of eucaryotic cells.

(Endopodite) Medial branch of a biramous appendage.

Formation of daughter cells, each surrounded by its own membrane, while still in the mother cell.

Condition of internal wing bud development in an insect; also, an insect secondarily wingless but derived from such an ancestor; associated with holometabolous insects.

The bony and cartilaginous structure which is inside the body.

Nucleous-like organelle that does not disappear during mitosis.

The starchy or oily nutritive material that surrounds the embryo in some seeds (sometimes referred to as albumen); it is triploid, having arisen from the triple fusion of a sperm nucleus and the two polar nuclei of the embryo sac.

A structure produced inside some bacteria, highly resistant to heat and chemicals; can germinate to form a new cell.

Symbiosis in which one member (micro-symbiote) lives within the other. The hypothesis that mitochondria and chloroplasts are the descendants of ancient procaryotic organisms.

Endothermic reaction
A chemical reaction that requires input of energy to proceed.

Substances produced by micro-organisms which are not secreted into the surrounding medium but are confined within the microbial cell; they are released after auto-lysis. A toxin not released from the cell; bound to the cell surface or intra-cellular.

Endothelial cells
The flat thin cells which line blood and lymph vessels and the body cavities.

Refers to a mycorrhiza in which the mycelium grows within the cortical cells of the root (e.g. in orchids).

The study of the energy transferred to or expended in life processes.

Energy flow
The rate of energy transfer between elements of an ecological system.

Engel curve
A curve showing the relationship between a household's income and its consumption of a commodity.

The sudden and destructive development of a plant disease, usually over large but restricted areas.

Enrichment culture method
A technique for isolating organisms from nature using media and growth conditions highly selective for a particular type.


Curing succulent animal feed in the silo.

The process of being tangled in net fragments or other flotsam.


A toxin affecting the intestine.

A virus which causes infections of the gastrointestinal tract; sometimes this term is used to indicate a group of picornaviruses which cause such infections as polio and infectious hepatitis.

The production of a particular commodity or group of related commodities.

Entner-Doudoroff pathway
A reaction sequence used by some bacteria to catabolise glucose.

An animal (or plant) which feeds on insects.

The consumption of insects by other animals.

Used to cover the associations between insects and plant micro-organisms, insects and protozoa, and insects and nematodes; meaning 'insect loving'.


A plant living within or on the body of an insect.

Refers to almost any relationship between plant micro-organisms (bacteria and fungi) and insects.

The process of being captured in cooling water of a power plant. This usually applied to small organisms such as fish larvae and zooplankton.

The degree of randomness or disorder in a system.

The outermost layers of a cell or the membrane of a virus.

Enveloped virus
Plant viruses of the reovirus and rhabdovirus groups which have an outer lipid-protein membrane surrounding the protein shell of the virus.

The external conditions (biotic and abiotic) that may act on soils or organisms to influence their development, which includes sunlight, temperature, moisture and other organisms.

Environmental biotypes
Biotypes distinguished by their degree of sensitivity to environmental factors (e.g. moisture, temperature) rather that to host resistance factors.

Environmental pollution
Undesirable levels of pesticides in air, soil, water, vegetation and animal life. This encompasses man, his food and his habitat.

Environmental quality objective
The quality to be aimed for in a particular aspect of the environment, e.g. ' the quality of water in a river such that coarse fish can maintain healthy populations'. Un-like an environmental quality standard, the EQO is not usually expressed in quantitative terms.

Environmental quality standard
The concentration of a potentially toxic substance which can be allowed in an environmental component, usually air (air quality standard, or water) over a defined period.

Regularly occurring animal disease of restricted range.

Enzootic disease
A disease (usually in low prevalence) which is constantly present in a population.

Specific proteins which act as biological catalysts to regulate the rate of biochemical reactions in all living organisms by lowering the energy of activation. Enzymes may be biologically synthesised, extracted and employed to catalyse laboratory or industrial biochemical reactions.

Enzyme-linked immunosorbant assay (ELISA)
A serological test in which the sensitivity of the antibody-antigen reaction is increased by attaching an enzyme to one of the two reactants.

Enzymic process
(Enzymatic) A chemical reaction or series of reactions catalysed by an enzyme or enzymes. An enzyme is a protein which acts as a highly selective catalyst permitting reactions to take place rapidly in living cells under physiological conditions.

Elevated eosinophil count in the circulating blood. Commonly associated with chronic parasite infections.

Plant with a short life-cycle, producing several generations in a year; flowers persisting for one day only.

First larval stage of the isopod suborder Epicaridea; attaches to a free-living copepod.

Thin, outermost layer of arthropod cuticle; contains sclerotin but not chitin.

A widespread and severe, temporary increase in the incidence of an infectious disease, particularly within a season.

Epidemic ephiphytotic
A progressive but temporary increase in the incidence of a particular disease within a defined host population.

Epidemic hemorrhagic
Virus disease transmitted by mosquitoes. Also called denuge fever.

The science of disease in populations (Vanderplank, 1963); the study of the development and spread of disease and of the factors affecting these processes.

Epidemiology, comparative
Studies in which comparisons are made of the influence of host, pathogen and environment on disease development and spread particularly insofar as these affect initial inoculum or disease, the length of the latent period, the length of the infectious period, the numbers of dispersal units formed and the effectiveness of these units.

Epidemiology, descriptive
Studies, usually of a non-quantitative nature, that depict the behaviour of disease in populations.

Epidemiology, quantitative
Studies of disease behaviour in populations which utilise measurement or quantification of states and rates of disease processes.

Epidermal-strip test
An electron microscope technique by which a virus can be quickly examined in crude-sap extracts. The quick leaf-dip and quick dip techniques are similar procedures.

Outermost layer of cells on plants or animals. The outermost, non-vascular layer of the skin.

An oblong body attached to the upper part of each testicle, consisting of an convoluted tube along which spermatozoa pass from the testis to the penis and within which they are stored.

Epigenetic changes
Changes in an organism brought about by alterations in the action of genes. Epigenetic transformation refers to those processes which cause normal cells to become tumour cells without any mutations having occurred.

A lid-like structure which covers the entrance to the larynx during respiration and closes it during deglutition.

Epilepitform fit
A condition in which the animal loses consciousness and has convulsions, often violent and usually intermittent.

The upper region of a lake, subject to wind mixing. Above the thermocline.

Like a promastigote but with a short undulating membrane, such as in Blastocrithidea.

Arthropod development in which all specifically larval forms are suppressed or passed before hatching; the juvenile that hatches has the adult body form.

Downward curling of a leaf blade due to cell growth on the upper side of a petiole being more rapid than that on the lower side; often a hyperplastic symptom of plant disease (e.g. the disease caused by potato virus Y in Physalis floridana).

Epipelic algae
Algae living in or on the sediments of a water body.

Non-parasitic plant that is attached to another plant for mechanical support only, examples include orchids, lichens and mosses.

Existing on the surface of a plant or plant organ without causing infection.

The study of ephiphytotics with special reference to occurrence and severity of a plant disease as influenced by such environmental factors as moisture, temperature, wind, the nature of the soil, and so on. Analogous to 'epidemiology'.

An epidemic in a plant (as opposed to epizootic, an epidemic disease in an animal) the study of which is sometimes known as epiphytotiology.

(Epipodite) Lateral process, from the protopod, usually with one or more joints; may be called an exite.

An element which carries genetic material, is able to transmit inheritable traits and to alternate between autonomous and integrated states in the host cell; in the integrated state an episome is physically associated with a chromosome, while in the autonomous state it is extrachromosomal.

The suppression of the effect of a gene by a non-allelic gene.

Bleeding from the nose.

The outer half wall of a diatom cell.

The layers of cells that make up the surfaces of the body, such as skin and linings of the interior openings of the body (mouth, upper respiratory tract, gastrointestinal tract, genitourinary tract).

The epitope is the location or locations where the antibody binds to an antigen.

The widespread or simultaneous occurrence of a disease in a large proportion of an animal population.

The study of diseases of animals on the basis of mass widespread phenomena.

Equilibrium moisture
The moisture content of grain after it has been exposed content to a particular environment for an indefinitely long period of time It is dependent on the humidity and temperature conditions of the environment.

Evenness of distribution of species abundance patterns; maximum equitability occurs when all species are represented by the same number of individuals.

Assets minus liabilities.

Equity per cent
Equity capital as a percentage of total capital,
i.e. (assets - liabilities)/assets x 100

A chemical or treatment applied to eliminate a pest or pathogen from its host or environment.

Control of a pest or disease by eliminating the organism after it is established or by eliminating the plants that are its host.

A major fungal sterol.

Members of the grass family may become infected with a fungus, ergot Claviceps purpurea, which attacks the immature seed head. Ergot may cause pregnant animals to abort because of uterine contractions. Animals may develop skin problems due to contraction of the blood vessels. At lower levels of ingestion in empty stock, sore teats and mouths only may be seen. Severe effects may require longer periods of ingestion.

A type of poisoning caused by a mycotoxin produced by the fungus Claviceps purpurea; the toxin is found in contaminated grain and may cause dramatic neurologic effects as well as decreased blood flow to the extremities.

A growth of hairs in dense patches on plant leaves resulting from the attack of certain Eriophyidae.

The wearing away of the land surface by water and wind.

Erosive lesion
A lesion which causes wearing or wasting away of the tissue.

Error of estimation
The difference between an estimated value and the true value.

Red blood cell.

Eruciform larva
Caterpillar; a larva with a cylindrical body, well-developed head, and with both thoracic legs and some abdominal prolegs.

Breaking through (e.g. growth of sporodochia through the epidermis of susceptible plant organs).

A redness of the skin; a rash.

The red corpuscles of the blood.

A plant growing away from cultivation, but not well naturalised. Failure of inherently susceptible plants to show infestation and damage even though disease or pest attack is prevalent.

An infestation, in which the host plant is attacked by a parasite that originated on the plant itself. Opposite of exodemic.

That portion of the alimentary canal lying between the stoma and cardia; also known as the pharynx.

Disease caused by Leishmania braxiliensis; also called chiclero ulcer, uta, pian bois, or mucocutaneous leishmaniasis.

Sowing or planting a new pasture. Successful plant (tree) reproduction on a site.

Establishment of natural enemy
The permanent occurrence of an imported natural enemy in a new environment after introduction as part of a biological control programme.

A form in which some types of weed-killers may be prepared, notably 2,4-D and 2,4-5T. Weed-killers prepared as esters are not soluble in water. They are dissolved in oil to which an emulsifier is added so that it may be diluted with water.

The period of suspended animation in many tropical insects and in some animals in response to hot, dry conditions.

Ether extract
Substances such as fats, oils and waxes that are extracted from plants with ether.

Excessive spindliness in plants, owing to insufficient light or to disease. A phenomenon exhibited by plants grown in the dark; etiolated plants are pale yellow and have long internodes and small leaves.

The study of cause of disease; that phase of plant pathology which deals with the causal agent and its relations with the susceptible plant.

One of the three major evolutionary lines of cellular life forms, primarily procaryotic micro-organisms which possess a peptidoglycan cell wall (mycoplasma are the only exception).

A cell or organism having a true nucleus. One of the three major evolutionary lines cellular life forms, including plants, animals, fungi, protozoa, and algae, all of which possess a nucleus and organelles.

Refers to cells of all organisms except bacteria and a blue-green algae. The nucleus of such cells is separated from the cytoplasm by a nuclear membrane.

Organism with membrane-bound nuclei in its cells.

Euler technique
A numerical approximation solution for systems of differential equations on which the solution is projected forward through time in steps of size in linear segments of slope f(x,t) given by the differential equation.

Euphotic zone
The upper part of the water column for which there is positive net photosynthesis by the phytoplankton.

European foulbrood
A disease of larval honey bees caused by the bacteria Streptococcus pluton.

Growing over a wide temperature range.

Cell or nuclear constancy; the adult has the same number of nuclei or cells as the first-stage juvenile; eutely may exist in tissues, organs or entire animals.

Aquatic plant growth is stimulated to produce excessive choking growth. The process involves a complex series of inter-related changes in the chemical and biological status of a water body most often due to a depletion of the oxygen content caused by decay of organic matter resulting from a high level of primary productivity and typically caused by enhanced nutrient input.

Out-pouching; the bulging out, usually under pressure, of a membrane normally within a body.

In meteorology, the change of liquid water or ice to water vapour. In certain usages the term signifies only the liquid to vapour phase change, as distinct from sublimation which signifies the solid to vapour change. The rate of evaporation is controlled by the water and energy (mainly solar radiation) supplies and the ability to take up more water.

Loss of water by evaporation and transpiration from the above-ground parts of the plant, dominated by meteorological conditions, especially when water supply is not limiting.

A collection of sample points.

An eruption or breaking out on the skin; a rash.

Exarate pupa
A pupa in which the appendages are free and not glued to the insect body.

Control of plant disease by excluding the pathogen or infected plant material from disease-free areas e.g. by seed disinfestation or quarantine.

Loss of substance from for example the skin, by scratching.

The process of removal of a compound or its metabolites from the body, normally via the bile or urine, but also via the lungs for volatile substances and by either minor routes such as skin, saliva or intestinal mucosa.

Exergonic reaction
A chemical reaction that proceeds with the liberation of heat.

Rapid formation of micro-gametes from a micro-gamete of Plasmodium and related genera.

Lateral process or joint from the protopod, sometimes referred to as an epipod.

An infestation, or phase thereof, in which the host plant is attacked by a parasite originating on another plant.

Originating externally, outside the organism. Arising on the outside of the generating structure.

A segment of eucaroytic DNA which codes for protein. Contrasted with introns, the intervening non-coding regions.

Nucleases that attack the nucleic acid strand starting from the ends and thus gradually shortening it are termed 'exo' or outside nucleases.

(Exopodite) Lateral branch of a biramous appendage.

Condition of external wing bud development in an insect; also, an insect in which the wing buds develop externally; associated with hemimetabolous insects.

The hard structure developed on the outside of, and giving support to, a body, such as the chitinous covering of an insect.

Exothermic reaction
A chemical reaction that proceeds with the liberation of heat.

Alien, introduced from another country.

Exotic species
An organism that evolved in one part of the world and that now occurs either accidentally or intentionally in a new region i.e. not native.

Poisonous substances produced by the microbial cell and liberated into the surrounding environment, without destruction of the cell.

Expected value
Let x be a discrete variable with probability distribution p(x) and let E(x) represent the expected value of x. Then E(x) = xp(x)/x

A small piece of a living tissue taken for the purpose of establishing an in vitro culture. Cell cultures are often identified by the source of the initial explant: e.g. meristem tip cultures, anther cultures.

Exponential distribution
A probability distribution often used to describe the time between events in which the probability is highest for events occurring close to each other. Sometimes referred to as the negative exponential distribution.

Exponential probability distribution
A distribution used to model lifetimes or waiting times in which the right tail of the distribution decreases expotentially. The probability function is given as f(x) = e-x x _ 0

Exponential smoothing
In time series analysis a computational method that averages the first t time series values by increasingly weighting out the contribution of remote values. The expotentially smoothed value at time t is t-2 St - _ _ (1 - _)iyt-i + (1 - _)t-1y1 i=0 where y1, y2, .... yt are the response values and _ is the smoothing constant (0 _ _ _ 1).

Exposure assessment
The process of measuring or estimating intensity, frequency and duration of human exposures to an agent currently present in the environment or of estimating hypothetical exposures that might arise from the release of new chemicals into the environment.

Expotential growth
The phase of most rapid growth, in which the progressive doubling of cell number results in a continually increasing rate of growth.

Expotential phase
A period during the growth cycle of a population in which growth increases at an expotential rate.

The synthesis of a protein or proteins encoded by a particular gene.

Complete extinction of a species over a large continuous area such as an island or a continent.

Outside the cells.

Extra (marginal) capital
Additional capital invested with a view to increasing profit e.g. clearing land.

Extra (marginal) return
The extra return (revenue) which results from adding one more unit of input.

Extractable acid
Total acid extractable from a formulation based on an organic acid. Salts and esters are converted to the parent acid before the determination. This will also include any free mineral acid.

Extraction buffer
The buffer used in grinding infected leaves during the initial stages of virus purification.

A substance issuing from a tissue either as a normal process or because of disease in the tissue - usually in plants.

Heteroecious aphids on their secondary host plant (= alienocolae).

The cast skin of arthropods after moulting.

Eye gnat
Fly of the family Chloropidae.