haploid spores (asexual) are produced by meiosis in
fungi and other plants.
spore The reproductive organ in cryptogams (ferns,
mosses, fungi, and algae), which functions like a seed
but has no embryo. In alternation of generations, it
germinates and produces a gametophyte in ferns and
mosses. It is the reproductive unit encased in a protec-
tive coat of a fungus. Usually environmentally resistant
and can lay dormant for many years.
See alsoSPOROPHYTE.
sporophyte The phrase alternation of generations
refers to the way in which the diploid and haploid
stages in a plant’s life cycle take different and distinct
forms. The diploid stage is called the sporophyte and
the haploid stage the gametophyte. The diploid sporo-
phyte phase results from a union of gametes that mei-
otically produces haploid spores. The haploid
gametophyte phase develops following meiosis and
develops mechanisms for fertilization by producing
and accepting gametes. The haploid, gamete-produc-
ing vegetative phase is the gametophyte, and the
diploid, spore-producing vegetative phase is the
sporophyte.
See alsoGAMETOPHYTE.
sporopollenin An acetolysis resistant biopolymer
that makes up most of the material of the exine, an
outer layer of the wall of pollen grains and spores that
is highly resistant to acids and bases. A granule of
sporopollenin is called an orbicule.
spotted fever(Rocky Mountain spotted fever) A
disease caused by Rickettsia rickettsiiand transmitted
by ticks. Confined to the Western Hemisphere and
found in all states in the United States with the excep-
tion of Maine, Hawaii, and Alaska. Dermacentor
andersoni(the wood tick) is the usual vector in the
western United States, while D. variabilis(dog tick) is
the vector in the eastern and southern United States.
Without treatment, fatality is about 20 percent, but
antibiotics reduces the rate if treated early.
See alsoRICKETTSIA.
square plane SeeCOORDINATION.stability constant An equilibrium constant that
expresses the propensity of a species to form from its
component parts. The larger the stability constant, the
more STABLEthe species. The stability constant (forma-
tion constant) is the reciprocal of the instability con-
stant (dissociation constant).stabilizing selection A form of natural selection
that favors maintaining existing character traits in a
population and eliminating those individuals who
exhibit extreme variations of a trait. While it main-
tains the existing state of adaptation and exhibits the
highest fitness, it results in an overall decrease in
genetic diversity.stable A term describing a system in a state of equi-
librium corresponding to a local minimum of the
appropriate thermodynamic potential for the specified
constraints on the system. Stability cannot be defined
in an absolute sense, but if several states are in princi-
ple accessible to the system under given conditions,
then that with the lowest potential is called the stable
state, while the other states are described as metastable.
Unstable states are not at a local minimum. Transitions
between metastable and stable states occur at rates that
depend on the magnitude of the appropriate activation
energy barriers separating them.stamen The male organ of a pollen-bearing flower.
See alsoOVULE.starch A polysaccharide containing glucose (long-
chain polymer of amylose and amylopectin) that is the
energy storage reserve in plants.stasipatric speciation A formof speciation where
new species are created as a result of chromosomal
rearrangements or mutations (translocations, inver-
sions, changes in chromosome numbers). Reproductive
isolation occurs, and new species develop.stasipatric speciation 313