V

vaccine vak-SEEN A killed or weakened form of, or part of, an infectious agent that initiates an immune response so that when the real agent is encountered, antibodies are already available to deactivate it. 803

vacuole VAK-yew-ole A storage sac in a cell. 75

vagina va-GINE-a The birth canal in a female in many species. 197

valence electron VAY-lense e-LEC-tron Electron in the outermost shell. 35

van der Waals attractions VAN dur walls a-TRAC-shuns Dynamic attractions within or between molecules when oppositely charged regions attract. 39

variable A changeable factor in an experiment. 21, 25

variable regions Amino acid sequences that form the upper parts of heavy and light antibody chains, which vary greatly in different antibody types. 795

variably expressive VAIR-ee-ah-blee ex-PRESS-ive A phenotype that varies in intensity in different individuals. 275

vascular bundle VAS-ku-ler BUN-del Organized group of vascular tissues in stem. 541

vascular cambium VAS-ku-ler KAM-bee-um A thin cylinder of meristematic tissue in roots and stems that produces most of the secondary plant body. 550

vascular tissue VAS-ku-ler TISH-ew Specialized conducting tissue that forms leaf veins. 534

vas deferens VAS DEF-er-enz In the human male, a tube from the epididymis that joins the urethra in the penis. 197

vein In plants, strand of vascular tissue in leaves. In some animals, a large blood vessel arising from venules that returns blood to the heart. 708

venous valves VEEN-is VALVES Flaplike structures in veins that keep blood flowing in one direction. 725

ventral VEN-tral The belly or underside. 512

ventricles VEN-treh-kelz Spaces in the brain into which cerebrospinal fluid is secreted. Also, the two muscular heart chambers beneath the atria. 710

venule VEN-yule Vessel that arises from capillaries and drains into veins. 708

vertebral column ver-TE-bral COL-um Bones that protect the spinal cord. 685

vertebrate VER-te-brayt An animal that has a backbone. 508

vertical stratification ver-ti-kal strat-if-i-KAY-shun Layering of different plant species in the forest canopy. 872

vesicle VES-i-cal A membrane-bounded sac in a cell. 71

vessel element VES-el EL-eh-ment More specialized conducting cell in plants that is elongated, dead at maturity, and has thick walls. 538

vestibule VES-teh-bule A structure in the inner ear that provides information on the position of the head with respect to gravity and changes in velocity. 653

vestigial organ ves-TEEG-e-el OR-gan A structure that seems not to have a function in an organism but that resembles a functional organ in another species. 395

villi VILL-i Tiny projections on the inner lining of the small intestine that greatly increase surface area for nutrient absorption. 762

viroid VIE-royd Infectious genetic material. 77

virus An infectious particle consisting of a nucleic acid (DNA or RNA) wrapped in protein. 75

visceral mass vis-er-al MASS Part of the molluscan body that contains the digestive and reproductive systems. 519

vitreous humor VIT-ree-us HU-mer A jellylike substance behind the lens that makes up most of the volume of the eye. 649

vocal cords Two elastic tissue bands that cover the glottis. They vibrate as air passes, producing sounds. 736

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