Glossary ❮ 337vertebrate Animal with a backbone.
vessel elements Xylem cells in charge of water
transport in angiosperms. More efficient than tra-
cheid cells.
vestigial characters Characters that are no longer
useful, although they once were.
viral envelope Protective barrier that surrounds
some viruses but also helps them attach to cells.
viroidsPlant viruses that are only a few hundred
nucleotides in length.
virus A parasitic infectious agent that is unable to sur-
vive outside a host organism. Viruses do not contain
enzymes for metabolism or ribosomes for protein
synthesis.
visual communication Communication through the
use of the visual senses.
water biomesBoth freshwater and marine biomes,
which occupy the majority of the surface of the earth.
water cycle The earth is covered in water. A lot of
this water evaporates each day and returns to the
clouds. This water is then returned to the earth in
the form of precipitation.
water potential The force that drives water to move
in a given direction. Combination of solute poten-
tial and pressure potential.
water vascular system Series of tubes and canals
within echinoderms that play a role in ingestion of
food, movement, and gas exchange.
wild-type phenotype The normal phenotype for a
characteristic in fruit flies and other organisms.
within-sex competition Competition for mates
between members of the same sex.
wobbleNucleotides in the third position of an anti-
codon are able to pair with many nucleotides
instead of just their normal partner.
X-inactivation During the development of the
female embryo, one of the two X chromosomes in
each cell remains coiled as a Barr body whose genes
are not expressed. A cell expresses the alleles of the
active X chromosome only.
xylem The “superhighway,” or important part of the
vascular tissue in plants, through which water and
nutrients travel throughout the plant. Also func-
tions as a support structure that strengthens the
plant.
yolk sac Derived from the hypoblast, this is the
site of early blood cell formation in humans and
the source of nutrients for bird and reptile
embryos.
zone of cell division Region at the tip of a root
formed by the actively dividing cells of the apical
meristem.
zone of elongationCells of this region elongate
tremendously during plant growth.
zone of maturationRegion in the plant where cells
differentiate into their final forms.http://www.ebook3000.com