Dictionary of Physics Chemistry Biology Mathematics

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organisms that are reproductively isolated in nature from all other organisms.
Populations of individuals capable of interbreeding and producing viable,
fertile offspring. The least inclusive taxonomic category commonly used.
species diversity The number of living species on Earth.
species packing The phenomenon in which present-day communities
generally contain more species than earlier communities because organisms
have evolved more adaptations over time.
species richness The number of species present in a community.
sperm The male gamete.
spermatogenesis The development of sperm cells from spermatocytes to
mature sperm, including meiosis.
spicules Needle-shaped skeletal elements in sponges that occur in the matrix
between the epidermal and collar cells.
spinal cord A cylinder of nerve tissue extending from the brain stem; receives
sensory information and sends output motor signals; with the brain, forms the
central nervous system. Nerve cell collections extending from the base of the
brain to just below the last rib vertebrae.
spindle apparatus Microtubule construction that aligns and segregates
chromosomes during eukaryotic cell division.
spleen An organ that produces lymphocytes and stores erythrocytes.
spongy bone The inner layer of bone; found at the ends of long bones and is
less dense than compact bone. Some spongy bone contains red marrow.
spongy mesophyll Parenchyma cells found in plant leaves that are irregularly
shaped and have large intracellular spaces. |
sporangia The structures in which spores are produced (sing.: sporangium).
spores Impervious structures formed by some cells that encapsulate the cells
and protect them from the environment; haploid cells that can survive
unfavorable conditions and germinate into new haploid individuals or act as
gametes in fertilization.
sporophyte The diploid stage of a plant exhibiting alternation of generations. The
diploid, spore producing phase of the plant life cycle.
Sporozoans Members of the protists that are referred to as slime molds; may
include organisms resembling the ancestors of fungi.
stability One of the phases of a population's life cycle. The population's size
remains roughly constant, �uctuating around some average density. Also, the
ability of a community to persist unchanged.
stabilizing selection A process of natural selection that tends to favor genotypic
combinations that produce an intermediate phenotype; selection against the
extremes in variation.
stalk A leaf's petiole; the slender stem that supports the blade of a leaf and
attaches it to a larger stem of the plant.
stamens The male reproductive structures of a �ower; usually consist of
slender, thread-like filaments topped by anthers. The male reproductive
structures in the flower, composed of a filament and anther.
stapes One of the three bones that function in hearing.
start codon The codon (AUG) on a messenger RNA molecule where protein
synthesis begins.
stem cells Cells in bone marrow that produce lymphocytes by mitotic division.
sternum The breastbone.
steroids Compounds with a skeleton of four rings of carbon to which various
side groups are attached; one of the three main classes of hormones.
sticky ends Term applied to DNA sequences cut with restriction enzymes where
the cuts will bond with each other or with another sequence cut with the same
enzyme.
stigma Part of the female reproductive structure of the carpel of a �ower; the
sticky surface at the tip of the style to which pollen grains attach. The receptive
surface of the pistil (of the flower) on which pollen is placed by a pollinator. |
stimulus A physical or chemical change in the environment that leads to a
response controlled by the nervous system.
stolons Stems that grow along the surface of the ground; a method of plant
vegetaive propagation.
stomach The muscular organ between the esophagus and small intestine that
stores, mixes, and digests food and controls the passage of food into the small
intestine.
stomata Pores on the underside of leaves that can be opened or closed to
control gas exchange and water loss. Openings in the epidermis (usually of the
leaf) that allow gas exchange. |
stomatal apparatus The stomata and guard cells that control the size of the
stoma. |
stop codon The codon on a messenger RNA molecule where protein synthesis
stops.
stratification The division of water in lakes and ponds into layers with
different temperatures and oxygen content. Oxygen content declines with
depth, while the uppermost layer is warmest in summer and coolest in winter.
stressed community A community that is disturbed by human activity, such
as road building or pollution, and is inadvertently simpli�ed. Some species
become superabundant while others disappear.
stroma The matrix surrounding the grana in the inner membrane of chloroplasts.
The area between membranes (thylakoids, grana) inside the chloroplast.
style Part of the female reproductive structure in the carpel of a �ower; formed
from the ovary wall. The tip of the style carries the stigma to which pollen grains [ Pobierz całość w formacie PDF ]

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