V
vaccination The process of protecting against infectious disease by introducing into the body a vaccine that stimulates a primary immune response and the production of memory cells against the disease-causing agent.
vaccine A preparation containing dead or weakened pathogens that when injected into the body elicit an immune response.
vacuoles Membrane-bound þuid-Þlled spaces in plant and animal cells that remove waste products and store ingested food.
vascular bundle Groups of xylem, phloem and cambium cells in stems of plants descended from the procambium embryonic tissue layer.
vascular cambium A layer of lateral meristematic tissue between the xylem and phloem in the stems of woody plants. Lateral meristem tissue in plants that produces secondary growth.
vascular cylinder A central column formed by the vascular tissue of a plant root; surrounded by parenchymal ground tissue.
vascular parenchyma Specialized parenchyma cells in the phloem of plants.
vascular plants Group of plants having lignified conducting tissue (xylem vessels or tracheids).
vascular system Specialized tissues for transporting þuids and nutrients in plants; also plays a role in supporting the plant; one of the four main tissue systems in plants.
vas deferens The duct that carries sperm from the epididymis to the ejaculatory duct and urethra.The tube connecting the testes with the urethra.
vectors Self-replicating DNA molecules that can be joined with DNA fragments to form recombinant DNA molecules.
veins Thin-walled vessels that carry blood to the heart. Units of the circulatory system that carry blood to the heart.
ventilation The mechanics of breathing in and out through the use of the diaphragm and muscles in the wall of the thoracic cavity.
ventral Term applied to the lower side of a fish, or to the chest of a land vertebrate.
ventricle The chamber of the heart that pumps the blood into the blood vessels that carry it away from the heart. The lower chamber of the heart through which blood leaves the heart.
venules The smallest veins. Blood þows into them from the capillary beds. Small veins that connect a vein with capillaries.
vernalization ArtiÞcial exposure of seeds or seedlings to cold to enable the plant to þower.
vertebrae The segments of the spinal column; separated by disks made of connective tissue (sing.: vertebra).
vertebrate Any animal having a segmented vertebral column; members of the subphylum Vertebrata; include reptiles, Þshes, mammals, and birds.
vesicles Small membrane-bound spaces in most plant and animal cells that transport macromolecules into and out of the cell and carry materials between organelles in the cell.
vessel elements Short, wide cells arranged end to end, forming a system of tubes in the xylem that moves water and solutes from the roots to the rest of the plant. Large diameter cells of the xylem that are extremely specialized and efficient at conduction. An evolutionary advance over tracheids. Most angiosperms have vessels.
vestigial structures Nonfunctional remains of organs that were functional in ancestral species and may still be functional in related species; e.g., the dewclaws of dogs.
villi Finger-like projections of the lining of the small intestine that increase the surface area available for absorption. Also, projections of the chorion that extend into cavities Þlled with maternal blood and allow the exchange of nutrients between the maternal and embryonic circulations. Projections of the inner layer of the small intestine that increase the surface area for absorbtion of food.
viroids Infective forms of nucleic acid without a protective coat of protein; unencapsulated single-stranded RNA molecules. Naked RNA, possibly of degenerated virus, that infects plants.
virus Infectious chemical agent composed of a nucleic acid (DNA or RNA) inside a protein coat.
vitamins A diverse group of organic molecules that are required for metabolic reactions and generally cannot be synthesized in the body.