vacuole A
membrane-bounded, fluid-filled space in a cell.
vagabond's disease Darkened, thickened skin caused
by years of infestation with body lice, Pediculus humanus humanus.
vagina Tubular organ that leads from the uterus to the vestibule of the female reproductive tract.
valence Degree of combining
power of an element as expressed by the number of atoms of hydrogen
(or its equivalent) that the element can hold (if negative) or
displace in a reaction (if positive). The oxidation state of an
element in a compound. The number of electrons gained, shared,
or lost by an atom when forming a bond with one or more other
atoms.
valve One of
the two shells of a typical bivalve mollusc or brachiopod.
valvifers Basal portions of the ovipositor in Hymenoptera,
derived from coxae of segmental appendages.
valvulae Processes from the valvifers to form the
body of the ovipositor (terebra) and the ovipositor sheath (third
valvulae) in Hymenoptera.
variability Generally refers to genetic differences
between all organisms that results in uniqueness.
variance A statistical measure of the amount of
variation in a series of measurements.
variant antigen type (VAT) Applied to certain trypanosomes,
any one of numerous antigenic types expressed on the surface of
the organisms and "seen" by the immune system of the
host. See variant-specific surface glycoprotein.
variant-specific surface glycoprotein (VSG)
Glycoprotein on the surface of certain trypanosomes recognized
by the host's immune system. Each VSG is responsible for one VAT.
variation Differences among
individuals of a group or species that cannot be ascribed to age,
sex, or position in the life cycle.
Vas L. Duct or vessel.
vascular bundle A strand of tissue containing primary
xylem and primary phloem, often surrounded by a bundle sheath.
vascular cambium Meristematic tissue that gives
rise to secondary xylem and secondary phloem.
vascular cylinder The stele; vascular tissue making
up the central cylinder of roots.
vascular plant A general name for any plant that
has xylem and phloem.
vascular ray Sheet of parenchyma that extends radially
through the wood, across the cambium and into the secondary phloem;
rays are produced by the vascular cambium and function in lateral
transport.
vascular system The total of the tissues that function
in the conduction of water and food by means of xylem and phloem
in the plant body.
vascular tissue Tissue that is specialized for
the long-distance transport of water or photosynthetic products;
xylem and phloem.
vector Any agent, such as water, wind, or insect,
that transmits a disease organism.
vegan A pure vegetarian consuming no animal products
at all.
vegetal pole The lower pole of an egg; usually more
dense than the animal pole because it contains more yolk.
vegetarian A person who does not consume animal
flesh; some consume dairy products and eggs, while others are
vegans.
vegetative reproduction See asexual reproduction.
vein A vascular bundle that forms part of the conducting
and supporting tissue of a leaf.
veins Blood vessels conducting blood toward the
heart in any animal. Also more heavily sclerotized portions of
wings of insects, which are remains of lacunae.
velarium Shelf-like
extension of the subumbrella edge in cubozoans (phylum Cnidaria).
veliger larva The second free-swimming larval
stage of many molluscs; develops from the trochophore and forms
rudiments of the shell, visceral mass, and head-foot before
settling to the substrate and undergoing metamorphosis.
veliger Larval form
of certain molluscs; develops from the trochophore and has the
beginning of a foot, mantle, shell, and so on.
velum A membrane on the subumbrella
surface of jellyfish of class Hydrozoa. Also, a ciliated swimming
organ of the veliger larva.
vena cava the major vein returning blood to the
heart of vertebrates
venation The pattern of vein development.
veneer A thin sheet of wood, often with attractive
grain, used to cover less expensive wood.
Ventral venter = the belly or towards the belly.
In human anatomy, anterior.
Ventriculi ventricul = belly.
venule A small blood vessel that collects blood
from a capillary bed and joins a vein.
vermicle Infective stage of Babesia in a tick.
vermiculite Expanded mica used as a sterile medium
for the rooting of cuttings; high water-holding capacity
and relatively inert.
vermiform Adjective
to describe any wormlike organism; an adult (nematogen) rhombozoan
(phylum Mesozoa).
verruga peruana Clinical form of Carrion's disease,
caused by the bacterium Bartonella bacilliformis and transmitted
by sand flies.
vertebra (pl. vertebrae)Each of the bones that
make up the backbone.
Vertebrata The subphylum of chordates whose members
are characterized by cartilaginous or bony vertebrae surrounding
a nerve cord. The skeleton is modified anteriorly into a skull
for protection of the brain.
vertebrates (subphylum Vertebrata)Chordates with
a backbone.
vertical city See technopolis.
vertical migration daily or seasonal movement of
small marine animals between the photic zone and midwater depths
vertical stratification The vertical distribution
of specific subcommunities within a community.
vesicles Membrane-bound particles pinched
off by constriction of a membrane, as in the Golgi apparatus.
vesicular disease Any disease of the urinary bladder,
such as vesicular schistosomiasis.
vessel element One of the cells forming a vessel
and characterized by a perforation plate.
vessel member One of the cells comprising a vessel,
characterized by the possession of a lumen and perforation plate
shared with other vessel members above and below in the series.
Also termed vessel elements.
vessel A tubelike column of vessel elements that
are connected by open end walls and are specialized for the conduction
of water and minerals.
vestibulum Cavity leading into another cavity or
passage, such as in the ciliate order Vestibulifera.
vestige A rudimentary
organ that may have been well developed in some ancestor or in
the embryo.
vestigial structures Visible evidence of a structure
that was present in an earlier stage in the evolution of an organism.
One of the sources of evidence for evolution.
vestimentiferans Pogonophorans that are common at hydrothermal vents.
viability The period of time an organism remains
alive; often used to describe the length of time before a seed
will fail to germinate.
vibrissa pl. vibrissae
Stiff hairs that grow from the nostrils or other parts of the
face of many mammals and that serve as tactile organs; "whiskers."
vicariance Geographical
separation of populations, especially as imposed by discontinuities
in the physical environment that fragmented populations that were
formerly geographically continuous.
village A collection of rural households linked
by culture, custom, and association with the land.
villus pl. villi A small fingerlike,
vascular process on the wall of the small intestine. Also one
of the branching, vascular processes on the embryonic portion
of the placenta.
viroid An infectious agent consisting of a single-stranded
RNA molecule with no protein coat; produces diseases in plants.
virulence Degree of pathogenicity of an agent;
how much damage the agent can cause.
virus A submicroscopic
noncellular particle composed of a nucleoprotein core and a protein
shell; parasitic; will grow and reproduce in a host cell.
viscera Internal
organs in the body cavity.
visceral arches See gill arches.
visceral mass The region of a mollusc's body that
contains visceral organs.
Visceral organs of the thoracic, abdominal,
and pelvic cavities.
viscosity the resistance of water molecules to
external forces that would separate them
viscous Thick and highly dense.
visible light A portion of the electromagnetic
spectrum that includes the wavelengths used for photosynthesis.
visible spectrum That portion of all the sun's
radiation that can be perceived as light by humans; between approximately
380 and 750 nm in wavelength.
vitalism The view that natural
processes are controlled by supernatural forces and cannot be
explained through the laws of physics and chemistry alone, as
opposed to mechanism.
vitamin An organic substance other than a carbohydrate,
lipid, or protein that is needed for normal metabolism but cannot
be synthesized in adequate amounts by the body.
vitellaria Structures
in many flatworms that produce vitelline cells, that is, cells
that provide eggshell material and nutrient for the embryo.
vitelline gland .See vitellaria.
vitelline membrane
The noncellular membrane that encloses the egg cell.
viviparity Reproduction in which eggs develop within the female body,
with nutritional aid of maternal parent as in therian mammals,
many reptiles, and some fishes; offspring are born as juveniles.
Adj., viviparous.
viviparity a condition describing the act of giving
birth to live young
viviparous An animal whose eggs develop inside
the female while the embryo derives nutrition from the mother.
viviporous Characterized by beginning embryo growth
while still attached to the mother plant.
volatile oils Terpenes composed of two to four
isoprene units; also known as essential oils, such as lemon and
peppermint.
volatile organic compounds Organic chemicals that
evaporate readily and exist as gases in the air.
volatilization The process of evaporation; to pass
into the atmosphere as a gas.
voltage-gated channels Ion-specific channels
that open only in response to a specific polarity across the cell
membrane.
voluntary (somatic) nervous system That part of
the nervous system that relays commands to skeletal muscles.
Vomer Plough.
vomeronasal organ See Jacobson's organ.
vulnerable species Naturally rare organisms or
species whose numbers have been so reduced by human activities
that they are susceptible to actions that could push them into
threatened or endangered status.
vulva The external genital organs in the female.