andovaries/testes) that secrete their hormones to the
blood. The pituitary gland stimulates the secretion of
these hormones, but the pineal gland stops it via mela-
tonin. If there is too much hormone, the pineal releases
melatonin to counteract; too much stress in the body,
serotonin triggers the release of adrenaline. The pineal
is also sensitive to light, and it releases melatonin at
night and is inhibited during the day.
New studies regarding the effect of electromagnetic
frequency (EMF) on the body (from cell phones, high
voltage lines, etc.) also relates to the pineal gland,
which is sensitive to EMF and seems to suppress the
activity of the pineal gland by reducing melatonin pro-
duction.
The pineal gland is suspected to play a role in a
number of problems including cancer, sexual dysfunc-
tion, hypertension, and the decline in melatonin has
been suggested to be a trigger for the aging process.
pinocytosis An active transport process by which
liquids or very small particles are ingested into the cell
by endocytosis. The cytoplasmic membrane invaginates
(forms “pockets”), fills with liquid or material, and
pinches off into a pinocytic vesicle or vacuole that can
then be transported.
pith The core of plant stems; location of vascular sys-
tems containing parenchyma cells and ground tissue.
Also to kill by severing the spinal cord.
pituitary gland Atwo-lobed, pea-size gland at the
base of the brain and attached to the hypothalamus
that controls the endocrine system. The major divisions
of the gland are the anterior lobe, or adenohypophysis,
and the posterior lobe, the neurohypophysis. Each lobe
produces hormones. The anterior lobe produces
growth hormone, thyroid-stimulating hormone,
adrenocorticotropic hormone, prolactin, luteinizing
hormone (LH), and follicle-stimulating hormone
(FSH), the last two called gonadotropins because they
stimulate the gonads. The posterior lobe releases anti-
diuretic hormone (vasopressin) and oxytocin. These
hormones regulate many body functions from growth
to birth contractions.
placebo Aninert substance or dosage form that is
identical in appearance, flavor, and odor to the active
substance or dosage form. It is used as a negative con-
trol in a BIOASSAYor in a clinical study.placenta A structure that develops in the uterus dur-
ing pregnancy that provides a blood supply and nutri-
ents for the fetus and eliminates waste; formed from
the uterine lining and embryonic membranes. In
humans, it is also referred to as the afterbirth because it
is ejected after the baby in a normal vaginal birth. If the
placenta is abnormally low in the uterus and covering
the uterus, a pregnancy-related condition called placen-
ta praevia occurs and usually necessitates delivery by a
cesarean section.placental mammal Any mammals that bear their
young live and are nourished before birth in the moth-
er’s uterus through a placenta. There are about 4,000
species, from bats to cats to humans.placoderm Amember of an extinct class of early
hinged-jaw fishlike vertebrates coveredwith a tough
outer body armor of bony plates with paired fins; lived
during the Silurian and Devonian periods (438 to 360
million years ago). Examples include the antiarchs and
dinichthyids.plankton Plankton includes mostly small-sized
plants called phytoplankton (e.g., diatoms) and ani-
mals called zooplankton (e.g., radiolarians) that drift
and float along with the tides and currents of water
bodies. Their name comes from the Greek meaning
“drifter” or “wanderer.” Phytoplankton produce their
own food by photosynthesis and are primary produc-
ers and food supply for a host of other organisms.
Plankton are also an oxygen producer, generating
according to some estimates as much as 80 percent of
the Earth’s oxygen supply.planula The free-swimming, flat, ciliated larvae of
the coelenterates.268 pinocytosis