W
	water cycle	Worldwide circulation of water molecules, powered by the sun. Water evaporates from oceans, lakes, rivers, and, in smaller amounts, soil surfaces and bodies of organisms; water returns to the Earth in the form of rain and snow. Of the water falling on land, some flows into rivers that pour water back into the oceans and some percolates down through the soil until it reaches a zone where all pores and cracks in the rock are filled with water (groundwater); the deep groundwater eventually reaches the oceans, completing the cycle.
	water potential	The physical property predicting the direction in which water will flow, governed by solute concentration and applied pressure.
	water vascular system	A network of hydraulic canals unique to echinoderms that branches into extensions called tube feet, which function in locomotion, feeding, and gas exchange.
	wavelength	The distance between crests of waves, such as those of the electromagnetic spectrum.
	wild type	An individual with the normal phenotype.
	wobble	A violation of the base-pairing rules in that third nucleotide (5' end) of a tRNA anticodon can form hydrogen bonds with more than one kind of base in the third position (3' end) of a codon.
	worker	A member of the nonreproductive laboring caste of social insects.
		X
	xylem [Gk. xylon, wood]	The tube-shaped, nonliving portion of the vascular system in plants that carries water and minerals from the roots to the rest of the plant.
		Y
	yeast	A unicellular fungus that lives in liquid or moist habitats, primarily reproducing asexually by simple cell division or by budding of a parent cell.
	yolk	The stored food in egg cells that nourishes the embryo.
	yolk sac	One of four extraembryonic membranes that supports embryonic development; the first site of blood cells and circulatory system function.
		Z
	zoned reserve systems	Habitat areas that are protected from human alteration and surrounded by lands that are used and more extensively altered by human activity.
	zoology [Gk. zoe, life + logos, a discourse]	The study of animals.
	zooplankton [Gk. zoe, life + plankton, wanderer]	A collective term for the nonphotosynthetic organisms present in plankton.
	zygote (zi-goat) [Gk. zygon,yolk, pair]	The diploid product of the union of haploid gametes in conception; a fertilized egg.
		A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
		Glossary from Biology, Fifth Edition By Campbell, Reece, Mitchell © 1998, Benjamin Cummings. Reprinted with Permission. All Rights Reserved