self-incompatibility The process in which plants
avoid inbreeding. It is the capacity for plants to block
fertilization by pollen from members of its own species
or closely related ones.
self-pollination The deposit of pollen from the
anther of a flower to the stigma of the same flower or
to different flowers on the same plant; must be com-
plete flowers. Self-pollination is frequently prevented in
plants because it would result in inbreeding, and some
mechanisms have evolved to prevent it, such as imper-
fect flowers and dioecious plants. African violets and
orchids are often self-pollinators.
semelparity The production of offspring where
organisms produce all of their offspring over a short
time in one single reproductive event.
semen The fluid that contains sperm and glandular
secretions that is ejaculated from a male during
orgasm.
semicircular canals Three small tubes in the inner
ear that maintain equilibrium and allow an organism to
detect changes in movement (acceleration) in a front-to-
back, up-and-down, and side-to-side motion by using
hair cells that detect movements of the fluid in the
canals. The canals are connected to the auditory nerve.
semilunar valve Two valves, or connective tissue,
around the origin of the aorta (aortic valve) and begin-
ning of the pulmonary artery (pulmonary valve). Each
valve is composed of three flaps of tissue that maintain
the unidirectional blood flow and prevent blood from
reentering the ventricles during diastole.
seminiferous tubules Highly convoluted channels in
the testes, where male sperm is produced. The sper-
matogonia, stem cells for spermatogenesis, are nour-
ished by Sertoli cells in the tubules, which also serve to
phagocytose damaged germ cells; provide a barrier
between the testis and blood; and produce mullerian
inhibition factor (MIF), inhibin, and androgen-binding
protein (ABP).
Interstitial cells, the Leydig cells between the
tubules, produce testosterone, the hormone responsible
for secondary sexual characteristics (e.g., facial and
pubic hair, male voice lowering at puberty). As the
sperm cells undergo meiosis and age, they migrate to
the center of the tubule and into the epididymus, locat-
ed on top of each testicle in the scrotum. The scrotum
keeps the testes about 2°C cooler than body tempera-
ture, increasing sperm production.
Inhibin, a polypeptide hormone produced in the
ovary and testes, regulates the release of follicle-stimu-
lating hormone (FSH) from the anterior pituitary and is
released by the Sertoli cells in the testes and granulosa
cells in the ovary. Androgens stimulate the release of
inhibin.
Total length of a seminiferous tubule is around 250
meters in one testis.semipermeable A membrane where some substances
will pass through but others will not.sensation The results of an impulse that is sent to the
brain when receptors and sensory neurons are activated.sensory neuron A specialized neuron that sends
messages that it receives from external or internal stim-
uli such as light, sound, smell, and chemicals to the
central nervous system.sensory receptor A cell or organ that converts a
stimulus from a formof sound, light, or thermal, chem-
ical, or mechanical stimulation into a signal, or action
potential, that can be transmitted through the organ-
ism. Sensory receptors are specific to the stimulus to
which they are responding and fall into specific types
such as chemoreceptors, photoreceptors, thermorecep-
tors, mechanoreceptors, and pain receptors, to name a
few. Each changes the polarization of the cell that may
eventually cause an action potential. Phasic receptors
send action potentials quickly when first stimulated
and then soon reduce the frequency of action poten-
tials, even if the stimulus continues, e.g., odor or pres-304 self-incompatibility