06413_09.pdf

IAA (indoleacetic acid) See auxin.

ice See water.

ice age A period in the earth’s history

during which ice advanced towards the

equator and a general lowering of tem-

peratures occurred. The last major ice

age, that of the Pleistocene period (some-

times known as the Ice Age ), ended about

10 000 years ago. At least four major ice

advances (glacials) occurred during the

Pleistocene period; these were separated

by interglacials during which the ice re-

treated and temperatures rose. At present

it is not known if the earth is between ice

ages or is in an interglacial of the Pleis-

tocene Ice Age. It has been established

that ice ages also occurred during the Pre-

cambrian (over 500 million years ago) and

during the Permo-Carboniferous (about

250 million years ago).

ice point The temperature at which

there is equilibrium between ice and

water at standard atmospheric pressure

(i.e. the freezing or melting point under

standard conditions). It was used as a

Ü xed point (0 ° ) on the Celsius scale, but

the kelvin and the International Practical

Temperature Scale are based on the

* triple point of water.

iconoscope A form of television cam-

era tube ( see camera) in which the beam

of light from the scene is focused on to a

thin mica plate. One side of the plate is

faced with a thin metallic electrode, the

other side being coated with a mosaic of

small globules of a photoemissive ma-

terial. The light beam falling on the mo-

saic causes photoemission of electrons,

creating a pattern of positive charges in

what is effectively an array of tiny capaci-

tors. A high-velocity electron beam scans

the mosaic, discharging each capacitor in

turn through the metallic electrode. The

resulting current is fed to ampli

mination it has received. In this way the

optical information in the light beam is

converted into an electrical signal.

ICSH See luteinizing hormone.

ideal crystal A single crystal with a

perfectly regular lattice that contains no

impurities, imperfections, or other de-

fects.

ideal gas (perfect gas) A hypothetical

gas that obeys the * gas laws exactly. An

ideal gas would consist of molecules that

occupy negligible space and have negligi-

ble forces between them. All collisions

made between molecules and the walls of

the container or between molecules and

other gas molecules would be perfectly

elastic, because the molecules would

have no means of storing energy except

as translational kinetic energy.

ideal solution See raoult’s law.

identical twins (monozygotic twins)

Two individuals that develop from a sin-

gle fertilized egg cell by its division into

two genetically identical parts. Each part

eventually gives rise to a separate individ-

ual and these twins are therefore identi-

cal in every respect. Compare fraternal

twins.

identity Symbol

cation

circuits, the current from a particular sec-

tion of the mosaic depending on the illu-

. A statement of

equality that applies for all values of the

unknown quantity. For example, 5 y ≡ 2 y +

3 y .

idiogram See karyogram.

idiosyncrasy An abnormal reaction to

a drug or other foreign substance shown

by an individual, which is usually geneti-

cally determined. An individual that

shows immunological idiosyncrasy is said

to be hypersensitive to a particular sub-

stance, agent, etc.

IE Ionization energy. See ionization po-

tential.

IGF (insulin-like growth factor) See

growth factor.

≡

igneous rocks

414

es, igneous

rocks may be classi Ü ed as plutonic, hypa-

byssal, or volcanic. Plutonic rocks solid-

ify slowly at great depth, typically

forming large intrusive masses (e.g.

batholiths and stocks), and generally have

the coarsest texture. Examples of plutonic

rocks are granite, syenite, diorite, and

gabbro. Volcanic (extrusive) rocks are

formed from magma that has been

poured out onto the earth’s surface; these

rocks (e.g. basalt, andesite) are characteris-

tically Ü ne-grained or glassy as a result of

their rapid cooling. Hypabyssal rocks (e.g.

diorite), which cool at shallower depths

than the plutonic, are intermediate in

character and medium-grained. They com-

monly occur in the form of small intru-

sions, such as dykes and sills. Igneous

rocks may also be classi

image A representation of a physical

object formed by a lens, mirror, or other

optical instrument. If the rays of light

actually pass through the image, it is

called a real image . If a screen is placed in

the plane of a real image it will generally

become visible. If the image is seen at a

point from which the rays appear to come

to the observer, but do not actually do so,

the image is called a virtual image . No

image will be formed on a screen placed

at this point. Images may be upright or in-

verted and they may be magni

ed or di-

minished .

image converter An electronic device

in which an image formed by invisible ra-

diation (usually gamma rays, X-rays, ultra-

violet, or infrared) is converted into a

visible image. Commonly the invisible ra-

diation is focused on to a photocathode,

which emits electrons when it is exposed

to the radiation. These electrons fall on a

uorescent anode screen, after accelera-

tion and focusing by a system of electron

lenses. The Û uorescent screen produces a

visible image. The device is used in

ed chemically ac-

cording to their silica content as acidic

(over 66% silica), intermediate (55–66%),

basic (45–55%), or ultrabasic (under 45%).

ignition temperature 1. The tempera-

ture to which a substance must be heated

before it will burn in air. 2. The tempera-

ture to which a * plasma has to be raised

in order that nuclear fusion will occur.

ileum The portion of the mammalian

* small intestine that follows the * je-

junum and precedes the * large intestine.

It is a site of digestion and absorption. The

internal lining of the ileum bears numer-

ous small outgrowths ( see villus), which

increase its absorptive surface area.

ilium The largest of the three bones that

make up each half of the * pelvic girdle.

The ilium bears a

uoro-

scopes, infrared telescopes, ultraviolet

microscopes, and other devices.

imaginary number A number that is a

multiple of √ –1, which is denoted by i; for

example

√

–3 = i

√

3. See also complex num-

attened wing of bone

that is attached by ligaments to the

sacrum ( see sacral vertebrae). See also is-

chium; pubis.

illuminance (illumination) Symbol E .

The energy in the form of visible radia-

tion reaching a surface per unit area in

unit time; i.e. the luminous Û ux per unit

time. It is measured in * lux (lumens per

square metre).

ber.

imago The adult sexually mature stage

in the life cycle of an insect after meta-

morphosis.

imbibition The uptake of water by sub-

stances that do not dissolve in water, so

that the process results in swelling of the

substance. Imbibition is a property of

many biological substances, including cel-

lulose (and other constituents of plant cell

walls), starch, and some proteins. It occurs

in dry seeds before they germinate and –

together with osmosis – is responsible for

the uptake of water by growing plant

cells.

imides Organic compounds containing

the group –CO.NH.CO.– (the imido group ).

imido group See imides.

imines Compounds containing the

group –NH– in which the nitrogen atom is

part of a ring structure, or the group =NH,

igneous rocks A group of rocks formed

from the crystallization of magma

(molten silicate liquid). Igneous rocks

form one of the three major rock cate-

gories ( see also metamorphic rocks; sedi-

mentary rocks). According to the depth

at which the magma solidi

415

impedance

in which the nitrogen atom is linked to a

carbon atom by a double bond. In either

case, the group is referred to as an imino

group .

imino group See imines.

immersion objective An optical

microscope objective in which the front

surface of the lens is immersed in a liquid

on the cover slip of the microscope speci-

men slide. Cedar-wood oil (for an oil-

immersion lens ) or sugar solution is fre-

quently used. It has the same refractive

index as the glass of the cover slip, so that

the object is effectively immersed in it.

The presence of the liquid increases the

effective aperture of the objective, thus

increasing the resolution.

immune response The reaction of the

body to foreign or potentially dangerous

substances ( * antigens), particularly dis-

ease-producing microorganisms. The re-

sponse involves the production by

specialized white blood cells ( * lympho-

cytes) of proteins known as * antibodies,

which react with the antigens to render

them harmless. The antibody–antigen re-

action is highly speci

cial

means. Active immunization ( vaccination )

involves the introduction, either orally or

by injection ( inoculation ), of specially

treated bacteria, viruses, or their toxins to

stimulate the production of * antibodies

( see vaccine). Passive immunization is in-

duced by the injection of preformed anti-

bodies.

immunoassay Any of various tech-

niques that measure the amount of a par-

ticular substance by virtue of its binding

antigenically to a speci Ü c antibody. In

solid-phase immunoassay the speci

c anti-

body is attached to a solid supporting

medium, such as a PVC sheet. The sample

is added and any test antigens will bind to

the antibody. A second antibody, speci

for a different site on the antigen, is

added. This carries a radioactive or

uorescent label, enabling its concentra-

tion, and thus that of the test antigen, to

be determined by comparison with

known standards. Variations on this tech-

nique include * ELISA and * Western blot-

ting.

immunoglobulin One of a group of

proteins ( * globulins) in the body that act

as * antibodies. They are produced by spe-

cialized white blood cells called * B cells

and are present in blood serum and other

body Û uids. There are several classes (e.g.

IgE, IgG, and IgM) having different func-

tions.

immunosuppression The suppression

of an * immune response. Immunosup-

pression is necessary following organ

transplants in order to prevent the host

rejecting the grafted organ ( see graft); it

is arti

c. See also anaphy-

laxis; immunity.

immunity The state of relative insuscep-

tibility of an animal to infection by dis-

ease-producing organisms or to the

harmful effects of their poisons (toxins).

Immunity depends on the presence in the

blood of * antibodies and white blood cells

( * lymphocytes), which produce an * im-

mune response. Inherited natural immu-

nity is that with which an individual is

born. Acquired immunity is of two types,

active and passive. Active immunity arises

when the body produces antibodies

against an invading foreign substance

( * antigen), either through infection or

* immunization; this type of immunity

may be humoral , in which B lymphocytes

produce free antibodies that circulate in

the bloodstream ( see b cell), or cell-

mediated , caused by the action of T lym-

phocytes ( see t cell). Passive immunity is

induced by injection of serum taken from

an individual already immune to a partic-

ular antigen; it can also be acquired by

the transfer of maternal antibodies to off-

spring via the placenta or breast milk ( see

colostrum). Active immunity tends to be

cially induced by radiation or

chemical agents that inhibit cell division

of * lymphocytes. Immunosuppression oc-

curs naturally in certain diseases, notably

* AIDS.

impact printer See printer.

impedance Symbol Z. The quantity that

measures the opposition of a circuit to the

passage of a current and therefore deter-

mines the amplitude of the current. In a

d.c. circuit this is the resistance ( R ) alone.

long-lasting; passive immunity is short-

lived. See also autoimmunity.

immunization The production of * im-

munity in an individual by arti

imperfect fungi

416

In an a.c. circuit, however, the * reactance

( X ) also has to be taken into account, ac-

cording to the equation: Z 2 = R 2 + X 2 ,

where Z is the impedance. The complex

impedance is given by Z = R + i X , where

bre and is the means

by which information is transmitted

through the nervous system. It is marked

by the

ow of ions across the membrane

of the * axon caused by changes in the

permeability of the membrane, producing

a reduction in potential difference that

can be detected as the * action potential.

The strength of the impulse produced in

any nerve Ü bre is constant ( see all-or-

none response). 2. (in physics) Symbol J .

The product of a force F and the time t for

which it acts. If the force is variable, the

impulse is the integral of F d t from t 0 to t 1 .

The impulse of a force acting for a given

time interval is equal to the change in

momentum produced over that interval,

i.e. J = m ( v 1 – v 0 ), assuming that the mass

( m ) remains constant while the velocity

changes from v 0 to v 1 .

inbreeding Mating between closely re-

lated individuals, the extreme condition

being self-fertilization, which occurs in

many plants and some primitive animals.

A population of inbreeding individuals

generally shows less variation than an

* outbreeding population. Continued in-

breeding among a normally outbreeding

population leads to inbreeding depression

(the opposite of * hybrid vigour) and an in-

creased incidence of harmful characteris-

tics. For example, in humans, certain

mental and other defects tend to occur

more often in families with a history of

cousin marriages.

incandescence The emission of light by

a substance as a result of raising it to a

high temperature. An incandescent lamp

is one in which light is emitted by an elec-

trically heated

–1. The real part of the complex

impedance, the resistance, represents the

loss of power according to * Joule’s law.

The ratio of the imaginary part, the reac-

tance, to the real part is an indication of

the difference in phase between the volt-

age and the current.

imperfect fungi See deuteromycota.

Imperial units The British system of

units based on the pound and the yard.

The former f.p.s. system was used in engi-

neering and was loosely based on Imperial

units; for all scienti Ü c purposes * SI units

are now used. Imperial units are also

being replaced for general purposes by

metric units.

implant Any substance, device, or tissue

that is inserted into the body. For exam-

ple, drug implants and heart pacemakers

are typically inserted under the skin.

implantation (nidation) (in embryol-

ogy) The embedding of a fertilized mam-

malian egg into the wall of the uterus

(womb) where it will continue its develop-

ment. After fertilization in the fallopian

tube the egg passes into the womb in the

form of a ball of cells ( blastocyst ). Its outer

cells destroy cells of the uterine wall,

forming a cavity into which the blastocyst

sinks.

implosion An inward collapse of a ves-

sel, especially as a result of evacuation.

imprinting 1. (in behaviour) A special-

ized form of learning in which young ani-

mals, during a particularly sensitive

period in their early development, learn

to recognize and approach some large

moving object nearby. In nature this is

usually the mother, though simple mod-

els or individuals of a different species (in-

cluding humans) may suf

lament. See electric

lighting.

incisor A sharp

attened chisel-shaped

* tooth in mammals that is adapted for

biting food and – in rodents – for gnaw-

ing. In humans there are normally two

pairs of incisors (central and lateral) in

each jaw. See permanent teeth.

inclination 1. See geomagnetism.

2. The angle between the orbital plane of

a planet, satellite, or comet and the plane

of the earth’s * ecliptic.

inclusive

ce. Imprinting

rst described by Konrad * Lorenz,

working with young ducks and geese. See

learning (Feature). 2. (in genetics) See

molecular imprinting.

impulse 1. (in physiology) (nerve im-

pulse) The signal that travels along the

tness The quality that or-

ganisms attempt (unconsciously) to maxi-

length of a * nerve

√

was

417

indicator

mize as the result of natural selection act-

ing on genes that are in Û uential in con-

trolling their behaviour and physiology. It

includes the individual’s own reproduc-

tive success (usually taken as the number

of its offspring that survive to adulthood)

and also the effects of the individual’s ac-

tions on the reproductive success of its

relatives, because relatives have a higher

probability of sharing some identical

genes with the individual than do other

members of the population. When inter-

actions between relatives are likely to

occur (which happens during the lives of

many animals and plants) * kin selection

will operate.

incoherent scattering See coherent

scattering.

incompatibility 1. The condition that

exists when foreign grafts or blood trans-

fusions evoke a marked * immune re-

sponse and are rejected. 2. The

phenomenon in which pollen from one

Û ower fails to fertilize other Û owers on

the same plant ( self-incompatibility ) or on

other genetically similar plants. This ge-

netically determined mechanism prevents

self-fertilization (breeding between likes)

and promotes cross-fertilization (breeding

between individuals with different ge-

netic compositions). See also allogamy;

fertilization; pollination.

incomplete dominance The condition

that arises when neither * allele control-

ling a characteristic is dominant and the

aspect displayed by the organism results

from the partial in

fection and the appearance of the

rst

symptoms.

incus (anvil) The middle of the three

* ear ossicles of the mammalian middle

ear.

inde

nite in

orescence See racemose

inflorescence.

inde

nite integral See integration.

indehiscent Describing a fruit or fruit-

ing body that does not open to release its

seeds or spores when ripe. Instead, release

occurs when the fruit wall decays or, if

eaten by an animal, is digested. Compare

dehiscence.

indene A colourless Û ammable hydro-

carbon, C 9 H 8 ; r.d. 0.996; m.p. –1.8

C; b.p.

182.6 ° C. Indene is an aromatic hydrocar-

bon with a

ve-membered ring fused to a

benzene ring. It is present in coal tar and

is used as a solvent and raw material for

making other organic compounds.

independent assortment The separa-

tion of the alleles of one gene into the re-

productive cells (gametes) independently

of the way in which the alleles of other

genes have segregated. By this process all

possible combinations of alleles should

occur equally frequently in the gametes.

In practice this does not happen because

alleles situated on the same chromosome

tend to be inherited together. However, if

the allele pairs Aa and Bb are on different

chromosomes, the combinations AB , Ab ,

aB , and ab will normally be equally likely

to occur in the gametes. See meiosis;

mendel’s laws.

indeterminacy See uncertainty prin-

ciple.

index fossil (zone fossil) An animal

* fossil of a group that existed continu-

ously during a particular span of geologi-

cal time and can therefore be used to date

the rock in which it is found. Index fossils

are found chie Û y in sedimentary rocks.

They are an essential tool in stratigraphy

for comparing the geological ages of sedi-

mentary rock formations. Examples are

* ammonites and * graptolites.

indicator A substance used to show the

presence of a chemical substance or ion

by its colour. Acid–base indicators are

uence of both alleles.

For example, a snapdragon plant with

alleles for red and for white Û owers pro-

duces pink

owers. Compare codomi-

nance.

incubation 1. The process of maintain-

ing the fertilized eggs of birds and of

some reptiles and egg-laying mammals at

the optimum temperature for the success-

ful development of the embryos. A period

of incubation follows the laying of the

eggs and precedes their hatching. 2. The

process of maintaining a * culture of bac-

teria or other microorganisms at the opti-

mum temperature for growth of the

culture. 3. The phase in the development

of an infectious disease between initial in-

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