grassland dominates. A midlatitude variant, the park-
land, is located in the drier portions of the humid con-
tinental climate, such as the eastern United States.
Associated with climates having seasonal precipitation,
tropical wet and dry climate type, accompanied with a
seasonal drought. Found close to the equator and trop-
ical areas of Africa, Australia, and India.
scavenger A substance that reacts with (or otherwise
removes) a trace component or traps a reactive reaction
intermediate (as in the scavenging of RADICALs or free
electrons in radiation chemistry). Also an animal that
feeds on dead animals.
See alsoTRAPPING.
Schiff bases Imines bearing a hydrocarbyl group on
the nitrogen atom: R 2 C=NR’ (R’ ≠H).
schizophrenia A form of mental illness with associ-
ated problems such as psychotic disorder characterized
by severe problems with a person’s thoughts, feelings,
or behavior; often includes delusions and/or hallucina-
tions. Men usually are affected in late teens or early
20s, while females are affected between their 20s and
30s. Mental health disorders account for four of the
top 10 causes of disability worldwide, and 2 million
American adults are affected by schizophrenia each
year.
school A large group of similar fish swimming
together.
Schwann cells Schwann cells are the supporting cells
of the peripheral nervous system (PNS) that wrap
themselves around nerve axons to form a myelin
sheath. A single Schwann cell makes up a single seg-
ment of an axon’s myelin sheath. They also assist in
cleaning up debris in the peripheral nervous system and
guide regrowth of PNS axons.
scientific method The steps taken by a scientist after
he or she develops a hypothesis, tests its predictions
through experimentation, and then changes or discards
the hypothesis if the predictions are not supported by
the results.sclereid A type of sclerenchyma cell. Sclereids are
found throughout the plant body and are short, cubical
cells that can be living or dead at maturity and form part
of the shell and pits of fruits. Also called stone cells.sclerenchyma cell Cells that have a rigid secondary
cell wall and serve to support and protect plants. Often
dead at maturity and composed of lignin, they provide
structural support. Sclerenchyma tissue can be composed
of two different kinds of cells: sclereids and fibers.screwworm There are two, the Old World screw-
worm (Chrysomia bezziana) and the New World
screwworm (Cochliomyia hominivorax). Larvae of
screwworm flies are obligate parasites of mammals
(including humans). The larvae of the flies cause lesions
known as myiasis that can be fatal and causes serious
livestock production losses. The New World species has
been eliminated from the United States and several
Central American countries.scrub typhus A formof typhus caused by a microor-
ganism collectively called rickettsia (Orientia tsutsuga-
mushiwith multiple serologically distinct strains) and
transmitted via chigger mites, by which they are trans-
mitted to humans and other animals. Fatality is as high
as 60 percent if not treated. Found in central, eastern,
and southeastern Asia; from southeastern Siberia and
northern Japan to northern Australia and Vanuatu, as
far west as Pakistan, to as high as 10,000 feet above
sea level in the Himalayan Mountains, and particularly
prevalent in northern Thailand. Restricted mainly to
adult workers who frequent scrub overgrown terrain or
other mite-infested areas, such as forest clearings, refor-
ested areas, new settlements, or even newly irrigated
desert regions.secondary compound Compounds that are not
directly a function in the process of growth and devel-302 scavenger