Vaccination • Injection of virus or bacteria or their proteins into the body, to induce immunization. The injected material is usually attenuated (weakened) before injection.
Vacuole (vac´ yew ole) [Fr.: small vacuum] • A liquid-filled cavity in a cell, enclosed within a single membrane. Vacuoles play a wide variety of roles in cellular metabolism, some being digestive chambers, some storage chambers, some waste bins, and so forth.
Vagina (vuh jine´ uh) [L.: sheath] • In female mammals, the passage leading from the external genital orifice to the uterus; receives the copulatory organ of the male in mating.
van der Waals interaction • A weak attraction between atoms resulting from the interaction of the electrons of one atom with the nucleus of the other atom. This attraction is about one-fourth as strong as a hydrogen bond.
Variable regions • The part of an immunoglobulin molecule or T-cell receptor that includes the antigen-binding site.
Vascular (vas´ kew lar) • Pertaining to organs and tissues that conduct fluid, such as blood vessels in animals and phloem and xylem in plants.
Vascular bundle • In vascular plants, a strand of vascular tissue, including conducting cells of xylem and phloem as well as thick-walled fibers.
Vascular ray • In vascular plants, radially oriented sheets of cells produced by the vascular cambium, carrying materials laterally between the wood and the phloem.
Vascular tissue system • The conductive system of the plant, consisting primarily of xylem and phloem. (Contrast with dermal tissue system, ground tissue system.)
Vasopressin • See antidiuretic hormone.
Vector • (1) An agent, such as an insect, that carries a pathogen affecting another species. (2) A plasmid or virus that carries an inserted piece of DNA into a bacterium for cloning purposes in recombinant DNA technology.
Vegetal hemisphere • The lower portion of some animal eggs, zygotes, and embryos, in which the dense nutrient yolk settles. The vegetal pole refers to the very bottom of the egg or embyro. (Contrast with animal hemisphere.)
Vegetative • Nonreproductive, or nonflowering, or asexual.
Vein [L. vena: channel] • A blood vessel that returns blood to the heart. (Contrast with artery.)
Ventral [L. venter: belly, womb] • Toward or pertaining to the belly or lower side. (Contrast with dorsal.)
Ventricle • A muscular heart chamber that pumps blood through the body.
Vernalization [L. vernalis: belonging to spring] • Events occurring during a required chilling period, leading eventually to flowering.
Vertebral column • The jointed, dorsal column that is the primary support structure of vertebrates.
Vertebrate • An animal whose nerve cord is enclosed in a backbone of bony segments, called vertebrae. The principal groups of vertebrate animals are the fishes, amphibians, reptiles, birds, and mammals.
Vessel [L. vasculum: a small vessel] • In botany, a tube-shaped portion of the xylem consisting of hollow cells (vessel elements) placed end to end and connected by perforations. Together with tracheids, vessel elements conduct water and minerals in the plant.
Vestibular apparatus (ves tib´ yew lar) [L. vestibulum: an enclosed passage] • Structures associated with the vertebrate ear; these structures sense changes in position or momentum of the head, affecing balance and motor skills.
Vestigial (ves tij´ ee al) [L. vestigium: footprint, track] • The remains of body structures that are no longer of adaptive value to the organism and therefore are not maintained by selection.
Vicariance (vye care´ ee unce) [L. vicus: change] • The splitting of the range of a taxon by the imposition of some barrier to dispersal of its members.
Vicariant distribution • A distribution resulting from the disruption of a formerly continuous range by a vicariant event.
Villus (vil´ lus) (plural: villi) [L.: shaggy hair] • A hairlike projection from a membrane; for example, from many gut walls.
Virion (veer´ e on) • The virus particle, the minimum unit capable of infecting a cell.
Viroid (vye´ roid) • An infectious agent consisting of a single-stranded RNA molecule with no protein coat; produces diseases in plants.
Virus [L.: poison, slimy liquid] • Any of a group of ultramicroscopic infectious particles constructed of nucleic acid and protein (and, sometimes, lipid) that can reproduce only in living cells.
Visceral mass • The major internal organs of a mollusk.
Vitamin [L. vita: life] • Any one of several structurally unrelated organic compounds that an organism cannot synthesize itself, but nevertheless requires in small quantity for normal growth and metabolism.
Viviparous (vye vip´ uh rus) [L. vivus: alive] • Reproduction in which fertilization of the egg and development of the embryo occur inside the mother’s body. (Contrast with oviparous.)