lesion, or blister containing a clear watery fluid; a cavi-
ty or sac, especially one filled with fluid. Anatomy
refers to the umbilical vesicle, while zoology defines a
vesicle as a small, convex, hollow prominence on the
surface of a shell or a coral. In geological terms, a vesi-
cle is a small cavity, nearly spherical in form and the
size of a pea or smaller, common in some volcanic
rocks and produced by the liberation of watery vapor
in the molten mass.
vessel element Individual, short, wide, or fat cells
arranged end to end, forming a system of tubes in the
xylem. The cell walls are pitted and contain lignin,
which gives them strength. They function to carry
water and minerals upward in the stem and root.
vestigial organ Nonfunctional remains of organs
that were previously functional and served a purpose in
ancestral species, that no longer serve that purpose, but
that remain part of the body and may still be function-
al in related species; e.g., the dewclaws of dogs, tails in
human embryos, wisdom teeth in adults, wings of the
ostrich, rudimentary legs in snakes, and whales with
hip bones.
vetch(Vicia sativa) An annual forage legume, 40 to
80 cm in height. Chiefly pollinated by bees because it
produces much nectar. Also, the maximum surface of a
lake exposed to prevailing winds.
vicariance The separation of a group of organisms
caused by a geographic barrier (e.g., mountain, lake,
sea, etc.), resulting in the original group differentiating
into new varieties or species.
viremia The presence of virus in the bloodstream.
Often associated with malaise, fever, and aching in the
back and extremities.
viroid (satellite RNA) Once thought to be a virus, a
viroid is an infectious, pathogenic entity similar to a
virus but having only one strand of nucleic acid without
the protein coat that defines a virus; a naked unencap-
sulated strand of RNA. Viroids are known to cause
plant diseases. For example, potato spindle tuber viroid
(PSTV) can cause a destructive disease of potatoes.virus Asmall microorganism that contains RNA or
DNA and is surrounded by a protein coat. Viruses
infect cells and then replicate new viruses after invasion
using the protein of the infected cells to reproduce.
Viruses can cause many human diseases, including
chicken pox, measles, mumps, rubella, pertussis, and
hepatitis, and they are not affected by common drugs
such as antibiotics, which are effective against bacteria-
based disease. Instead, vaccines are used to prevent or
fight off virus attacks.
See alsoBACTERIA.visceral(visceral muscle) Pertaining to the internal
organs of the body, especially those within the abdomi-
nal cavity such as the digestive tract, bladder, and the
heart; smooth muscle, as opposed to the two other
muscle types, skeletal and cardiac.visible light The portion of the electromagnetic spec-
trum that humans perceive. This excludes radio waves,
microwaves, infrared light, ultraviolet light, X rays,
and gamma rays. The human eye perceives the visible-
light spectrum as a continuum of colors (red, orange,
yellow, green, blue, indigo, and violet, along with vari-
ous combinations and shades of these colors), with the
perceived color depending on the wavelength. The
spectrum of visible light ranges in wavelength from
about 400 nm to about 700 nm. Visible light travels at
the same speed as all other radiation, i.e., at 186,000
miles per second, and its wavelength is longer than
ultraviolet light but shorter than X rays. Violet has the
shortest wavelength, while red has the longest.
See alsoENERGY.vitalism Attributed to the teachings of Aristotle, the
concept of vitalism, in its many forms, is the belief that
life forms and natural phenomena cannot be explained
by simple explanations of matter and processes. Vital-
ism posits that there must be another force, perhaps avitalism 341