white blood cell Component of the blood that functions in the immune system. Also known as a leukocyte.
wood The inner layer of the stems of woody plants; composed of xylem.
X-chromosome One of the sex chromosomes.
xerophytic leaves The leaves of plants that grow under arid conditions with low levels of soil and water. Usually characterized by water-conserving features such as thick cuticle and sunken stomatal pits.
x-ray diffraction Technique utilized to study atomic structure of crystalline substances by noting the patterns produced by x-rays shot through the crystal.
xylem Tissue in the vascular system of plants that moves water and dissolved nutrients from the roots to the leaves; composed of various cell types including tracheids and vessel elements. Plant tissue type that conducts water and nutrients from the roots to the leaves.
zebroid A hybrid animal that results from breeding zebras and horses.
Z lines Dense areas in myoÞbrils that mark the beginning of the sarcomeres. The actin Þlaments of the sarcomeres are anchored in the Z lines.
zone of differentiation Area in plant roots where recently produced cells develop into different cell types.
zone of elongation Area in plant roots where recently produced cells grow and elongate prior to differentiation.
zone of intolerance The area outside the geographic range where a population is absent; grades into the zone of physiological stress.
zone of physiological stress The area in a population's geographic range where members of population are rare due to physical and biological limiting factors.
zygomycetes One of the division of the fungi, characterized by the production of zygospores; includes the bread molds.
zygospore In fungi, a structure that forms from the diploid zygote created by the fusion of haploid hyphae of different mating types. After a period of dormancy, the zygospore forms sporangia, where meiosis occurs and spores form.
zygote A fertilized egg. A diploid cell resulting from fertilization of an egg by a sperm cell.
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